Jahnsen LA-2A Tube Compressor Handbook

5/12/2018 Jahnsen LA-2A Tube Compressor Handbook

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HOW TO BUILD ATUBECOMPRESSOR

LIMITER

VOLUME 1an electro-optical leveling amplifier

Dave Jahnsen


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b o O


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. - - - -

CautionThe author and/or publisher assumes no liability or responsibility for the safety ofthe reader or the reader's property. This book is for reference purposes only. Aknowledge of electronic safety is required before implementing the proceeduresoutlines within this book. The author recommends that a qualified technician bepresent while assembling and testing the project.And if you're not an attorney, that means ...This is a high voltage tube project. High voltages on the order of 300 to 500 voltsAC are present within the circuit.

These voltages can kill yOU!! !Please make absolutely sure the project is not connected to an AC line whileworking in or around the circuit. That means do not touch,jiggle, solder, or in anyway disturb the wires or components of your project while the unit is plugged in.Note also that even after the unit is unplugged, the capacitors, particularly thelarge electrolytic capacitors, can continue to hold their charge, sometimes for days.Be sure to discharge all capacitors before working on the circuit.

Lastly, have a qualified audio technician look at your project before you plug it in.This may save your time, the Joss of components to fire and other natura] disastersand possibly your life. It is money very well spent!


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ContentsSECTION 1 Basic Skills

CHI Components Primer 9CH2 Tools 15CH3 Soldering 19

SECTION 2 Circuit and ComponentsCH4 The LA2 Story 25

Photo Gallery 29CH5 Circuit Hardware 33CH6 Ordering Information 41

Pearless Electronics Inc. 41Mouser 42New Sensor Corporation 43Allied Electronics/Empire West Audio 44Additional Suppliers 46

SECTION 3 Mechanical DrawingsCH7 Component Chart 51

Schematic 55Mounted Components and Placement 56Twisted Pair Connections 60Power Connections 60Buss Wire Connections 60Star Ground Connections 60Filament Wire Connections 62Shielded Wire Connections 62Resistors And Other Components Placement 64Capacitor Placement 66

CH8 Building The Compressor 69APPENDICES

Specifications 81Block Diagram &Gain Reduction Curves 82Differences Between LA2 Revisions 83Resistor Color Code 84Console Wire Color Code 85Jumper Plug and Input Pad 86Voltage Chart 87Matching Resistor 88

VI.4


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SECTION 1BASIC SKILLS


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8 HOW TO BUILD A TUBE COMPRESSOR I LIMITER


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Resistors

CHAPTER 1COMPONENTS PRIMER

A resistor resists the flow of electrons across it causing a "voltagedrop". This resistance is measured in Ohms (Q). Resistor values areidentified by a code consisting of 4 colored bands painted around theresistor body. (Occasionally, five bands on precision resistors.)Thecolor code is: black (0), brown (1), red (2), orange (3), yellow (4),green (5), blue (6), violet (7), gray (8), white (9). The first two bandsare significant digits, the third is a multiplier representing a power often, and the forth is the tolerance range. For example, if you have aresistor that is yellow, violet, red, and gold, it's value is 4700 or4.7KQ5% ( 47 x 102). A color code chart is included in the appendix.There are three resistor types we may consider for the circuit: carboncomposite, carbon film and metal film. The LA2 used carbon compos-ite resistors throughout it's production life with the exception of thematching resistor (R25) supplied with the T4b. These are generallyregarded as the least preferred resistors for use in high quality audioproducts as they are prone to value changes with age, the value driftswith temperature changes, and some have high inductance. However,they are the most cost effective so they are used quite often in tubeequipment. For most applications, metal is better than carbon andfilm is better than composite. Resistor types to avoid include wire-wound, cement, and aluminum housed as these are optimized for highpower applications not superior audio performance.Note: The Hi-End Audio market has introduced a number of hi-firesistor options including a wirewound construction designed forsuperior audio. These are generally more to much more expensivethan the three options listed above, but if you have the dough and wishto experiment, go for it. The bragging rights alone may be worth theadded cost.


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10 HOW TO BUILD A TUBE COMPRESSOR / LIMITERThis introduces us to our first dilemma. You must choose between the"vintage" carbon composite resistors and the better sounding metal orcarbon film. Ifit is any help, the carbon composite "camp" describesmetal film as harsh or edgy and the metal filmers (my term) use termslike "fuzzy", and "lack of detail" to describe carbon composite. (Ineither case, the effect can be very subtle.) The parts ordering recom-mendations later in the book lists metal film resistors but the decisionis entirely yours.

Potentiometers (pots)Pots are adjustable resistors. There are 4 common materials used:carbon composite, conductive plastic, wirewound, and cermet. Car-bon composite construction is most common it production audio gear,conductive plastic is generally regarded as superior to carbon compos-ite, wirewound is used for high power applications and cermet soundsgood but has a lower turns before failure count than the others (it wearsoutfaster). Allen Bradley J series carbon composite pots were used byTektronix.

CapacitorsCapacitors store an electrical charge which has the effect of opposingchanges in voltage. A capacitor's capacitance is measured in faradsand its maximum working voltage is measured in volts. Caps arefurther classified by the type of dielectric used in their construction.Large capacitor values in audio, those in excess of IpF (micro farad)at 450 volts, are usually handled by aluminum electrolytic capacitors.A good choice for these are Sprague "Atom" although there are manygood manufacturers available.Note: electrolytics are one way devices, there is a + and a - lead.Always insure they are installed in the Circuit in the correct direction.When installed backwards in a circuit they may explode.Electrolytic capacitors are built into 3 common package styles: axial,radial, and can. Axial lead capacitors have the leads (wires) mounted


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COMPONENTS PRIMER 11on opposite ends of the capacitor body whereas radial lead capacitorsmount the wires on only one side of the body. Axial and radial capsare designed to solder mound on a printed circuit board or to besoldered point to point in a circuit within an enclosure. C5 and CIOare axial lead electrolytics. Radial lead electrolytics weren't used inthe LA2."Can" style caps are designed to mount outside the chassis usingspecial mounting clamps, mounting plates and twist mounts to securethe cap in place. Additionally, cans often contain 2 or more indi vidualcapacitors in a single package which can reduce the parts count andsimplify construction. The drawbacks of cans are that they are gener-ally larger and more expensive than a radial or axial design of similarvalue, they require mounting hardware, and there are fewer manufac-turers. See C7a-d in the LA2 photo gallery for an example of a canstyle cap.There are many more choices in dielectric with decreased capacitorvalues. For most tube applications the best considerations are polypro-pylene and polycarbonate with the former being ever so slightly better.Sprague "Orange Drops" (polypropylene) are highly regarded fortheir sonic properties. Their only drawbacks are price and size. A.OOIJlF Orange drop runs about 50 ea. to a typical ceramic disk's 15ea. and is much larger than it's ceramic counterpart ..Polyester (or its DuPont trademark name, Mylar) film capacitorswere usedforCI-3 and C9. Polyester caps are often used in tube ampsbecause they are cost effective but they are relatively poor performers.Polystyrene are large and difficult to solder without damage - micacaps are a better choice where polystyrene may have been considered.Mica caps come in two varieties: metal foil, referred to simply as micaand silvered mica capacitors which contain a thin layer of silver in thedielectric. Mica capacitors perform similarly and cost less than theirceramic counterpart. Cl2 and Cl4 are mica caps and C8 and CII areZ5U class ceramic disks. Z5U or class 3 ceramic caps are generalpurpose (the lowest grade) ceramics use in non critical applications.There are many other material used in the manufacturing of capacitorsamong them: Teflon, glass, paper and foil, air core. These mayormay not be good choices for tube audio because of their voltage rating,cost, or performance. Ifyou're capacitor knowledge is limited, stick


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12 HOW TO BU ILD A TUBE CO MPRESSOR / LIM ITERto the suggested styles. Ifyou're more experienced, you're probablynot reading this section.

Variab le Capac ito rsVariable capacitors are adjustable capacitors. Two common materialused in variable capacitors are air and ceramic. Air variable capacitorsare usually used in RF applications ( radio tuners). The LA2 used aceramic variable capacitor ( ARCO 465 ).

DiodesDiodes are one way devices that allow current to flow in only onedirection. There are many types of diodes including germanium,silicone, zener (usually used as a voltage reference), light emitting(LED's) and others. Germanium and silicone diodes are used asrectifiers that change AC power into DC power in power supplycircuits. Most modern equipment uses silicon diodes for this purpose.

TransformersA transformer is a device that consists of 2 or more wire coils(windings) wound such that when a current is passed through onewinding ( the primary) the magnetic field created by that currentinduces a current flow in the other windings (secondaries). Thetransformer physically isolates the circuits from on another whilesimultaneously "coupling" theAC signal between the circuits throughmutual inductance. A transformer may also "transform" the voltage,current, or impedance present at the primary to a different value at thesecondary.Our circuit utilizes 3 transformers: the input and output transformers(A-lO and A24 respectively) are used to isolate the circuit from theoutside world as well as impedance matching with other equipment;the power transformer is used to change the line voltage of 115 to 500volts with a center tap.


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COMPONENTS PRIM ER 13Tubes

A tube, or valve as the British call it, is an amplifier. It consists of aheated cathode that emits electrons, a plate that attracts these electronsand in most cases 1 or more grids to control this electron flow. Toaccomplish this, tubes require two separate voltages. The platerequires a high positive often called B+. A second low voltage is usedto heat the filaments and is called, appropriately, the heater voltage.Often two separate tubes are contained within a single glass envelopeas is the case with the 12AX7a and 12BH7a in our compressor circuit.These operate essentially as if they were two separate tubes exceptthey share a single heater and heater voltage.Tubes are identified by a number and/or letter combination painted onthe side of the glass or metal (yes, some are metal) envelope. Thereare three common tube numbering systems: the North American,European, and industrial.The North American standard consists of 4 parts: a 1 or 2 numberfilament voltage, a one or two letter function or type code, the numberof active elements in the tube, and an optional 1 or more letterconstruction or performance designator code. For example the 12AX7aoperates from a 12.6 volt filament voltage (12), is a duel triode (AX),has 7 active elements (7), and is a type "a" revision (a). Most of thecodes are listed in various tube manuals if you want further informa-tion.The European system consists of a 2 to 4 letter generic tube designa-tion, followed by a 1-4 digit function identifier. The only thing youreally need to know is that an ECC83 is the same as a 12AX7a.European equivalents aren't available for the other tubes in thecompressor circuit.The last system is the Industrial numbering system which consists ofa 4 digit number that gives no coded information about a specific tube,just an id number. Industrial types used to be high performancevariations of standard tubes but more and more tube are made withboth the North American and Industrial numbers. For instance, theRCA 6AQ5a has "6AQ5a / 6005" printed on it.


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14 HOW TO BU ILD A TU BE CO MPRESSO R I LIMITERThe following is a chart listing the North American tubes used in thecompressor and the Industrial and European equivalents.

NO S and JAN Tubes

Tube Equivalency Chart12AX7a*12BH7a6AQ5a

6057,6681,7025,7729, ECC8369136008

Two other terms you may come across are NOS and JAN. NOS standsfor New Old Stock which are tubes that were manufactured 2 or moredecades ago but went unsold. American tubes are generally regardedas some of the best ever produced but currently there is only 1American tube in production (the WE300b). All the other new Ameri-can manufactured tubes for sale today are NOS or relabeled foreignimports.

* Note: many people list the 5751 as an equivalentto the 12AX7a. Although is has many similarities,there are enough differences to possibly cause pro-blems in the circuit.

JAN tubes were originally purchased through a Joint Army Navygovernment purchasing program. JAN tubes are American tubes(though, occasionally, European tubes were remarked as Americantubes) built to military specifications, that were warehoused then soldas surplus in government auctions. These are generally consideredfine tubes.


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CHAPTER 2TOOLS

To do the job right you need the right tools. I believe in owning andusing quality tools. I've never heard anyone complain that their tools,instrument or car for that matter, were too high a quality for theirneeds. But cost is also a concern (otherwise you would have alreadybought all the LA2's you thought you needed.) For this reason, I'veprovided recommendations for both very inexpensive and sometimesdisposable tools and better quality tools, the kind you'll cherish, or atleast use, for a lifetime.

Soldering IronThe importance of a good soldering iron cannot be overstated - yoursuccess as a builder depends on the quality of your solder joints. Youshould obtain the best soldering iron you can afford. A soldering ironwith a power rating between 25 and 40 watts will do nicely and atemperature control is not mandatory but certainly desirable. The twoprimary names in the business are Weller and Ungar and are availablethrough most electronics mail order suppliers or at your local electron-ics store. I bought both my "good" irons from local electronics storeswhen they went on sale. For those on a really tight budget Radio Shacksells an iron that will do in a pinch but believe me, you will be muchhappier with the "good" iron.

Soldering SuppliesYou'll want to get a roll of 60/40 rosin core solder. I suggestMulticore SN60 in a 20 gauge. If you want the really good stuff useMulticore 2% silver solder SN62 available at Pacific Radio in Califor-nia. SN62 is about twice the price but works very well.Add to this a Vacuum Desolder Tool or "solder sucker" (not a


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16 HOW TO BUILD A TUBE COMPRESSOR / LIMITERDesolder Station, although they are great if you can afford it). This isused to suck molten solder from connections when you are desolderingand to remove excessive solder fromjoints. Desoldering Braid Wireor "solder wick" is a wire mesh that when heated and pressed againsta solder "glob" removes what the solder sucker misses.

Metal Working ToolsTo do the metalwork on the enclosure, you'll need a few metalworking tools. As with everything, there is the cheap (and dirty) wayor the expensive (and right) way. If you plan on building a lot ofequipment, you'll want to invest in the "right" (expensive) metalworking tools. These would include punches for punching large andsmall holes in the aluminum, a drill press with assorted drills, and forthe really serious, metal brakes and other metal working tools. Greenleemakes the best punches (at $25- $100+ apiece), Sears and Home Depothas fine lines of reasonably priced medium use drill presses, and if youare looking at brakes, you are beyond the scope of this book.And now for the way you are probably going to do it ...

I do most of my metal work with a 3/8" drill motor with drill,grinding and sanding bits, a hand-held jig saw motor with a metalblade, and a few hand files. I'm not going to detail the use of thesetools here as these are skills best acquired through personal instruc-tion. If you are uncertain how to use any of these tool consult anexperienced user or have a metal shop do the work for you.The key to all this metal work is to disguise your cuts. Use tubesockets, barrier strips and connectors that mount over, not in, the metaledges and your imperfect metal edges will be disguised to the outside.

Hand ToolsThe basic hand tools needed are needle nose pliers, diagonal cuttingpliers (dikes), a wire stripper, and assorted flat head and phillipsscrewdrivers including a set of miniatures. The recognized leader inelectronics hand tools is Xcelite. Radio Shack tools will work fine (thewire cutters work particularly well and, Sears sells some good elec-tronics pliers and screwdriver sets.


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Multimeter

TOOLS 17Several other tools I find invaluable are a flashlight (or several ofvarious sizes), some Sharpies (permanent markers), and hemostats.Hemostats are locking pliers that doctors use to grab and hold tissue.They are available at swap meets, electronics mail order houses andRadio Shack. Hemostats serve a dual purpose in that they can hold twoor more components together while you solder and they act as heatsinks to protect the components being soldered. The last of therequired tools is a bleeder resistor. You can make one of these bysoldering 2 alligator clips with wire leads, one on either end of a lKQ10 watt resistor. This is used to discharge the filter capacitors beforeworking inside your enclosure.

To discharge a capacitor: Unplug the AC power. Clip the two alligator clips on either side of the capacitor. Remove after a few seconds.

Although you can survive without one, you'll want to get a Multim-eter. New Sensor always seems to have a good one on sale. With amultimeter you can measure AC and DC voltages; current; resistance,continuity (zero resistance); and in some cases diode and semiconduc-tor junctions. Multimeters are a valuable diagnostic tools and easy tooperate.


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18 HOW TO BUILD A TUBE COMPRESSOR / LIMITER


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CHAPTER 3SOLDERING

We need a process to connect the various components into a usablecircuit. The most practical method is soldering. Soldering requiresheating the metal ends of the components to be connected (with asoldering iron), melting a small amount of solder (a tin and lead alloy)onto these wires, and allowing this junction to form a bond betweenthe metals through cooling. Soldering only works on the metal to metaljunctions between components and will not bear a load.

What Soldering W ill Not DoSoldering will not: hold the enclosure box together hold the transformers to the chassis (enclosure box) work in any manner with plastics

Also note that special "acid core" solder is required to solder alumi-num, chrome, iron and a few other metals. Most (all) electroniccomponents we are working with are designed for use with rosin coresolders. Just don't try to solder a ground lug to an aluminum chassis.

P reparing a Soldering IronWhen you use a soldering iron for the first time it is necessary to "tinthe tip."


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20

How to Solder

HOW TO BU ILD A TUBE COMPRESSOR / LIM ITER

To tin the tip: plug in the iron and heat for 1 minute. apply solder to the iron - enough to cover the tip wipe tip with a damp sponge or cloth

To make a good solder joint: Tin each component of the solder joint. Do this by heating each

individually and melting a small amount of solder on it. join together the parts to be soldered touch the iron tip to the junction if necessary, melt a small amount of solder onto the junction (not on

the iron itself)When the junction is coated with melted solder remove the iron. Donot move the junction until the solder cools, slightly changing color.The connection should be bright and shiny. If the solder is dull,crystalline or forms a glob on one or more of the metal parts of thejunction it is a "cold solder" and will not properly conduct electricity.If a cold solder is made, simply reheat the junction, apply a smallamount of solder as necessary, remove the iron and let cool.Occasionally, you may try to solder a junction that just won't take.Usually this is because not enough heat is getting to one or more of themetal parts or the junction. This can be dealt with in several ways.

If the solder won't take:1. You may have to first apply heat specifically to the offending metal

part(s) before heating the entire junction.2. If the parts are new, you may have to apply a small amount of solder

onto the junction before it is hot enough in order to evenly applyheat to the junction.


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Solder Bridges

SOLDERING 213. You may be trying to heat too large an area. This usually happenswhen soldering a ground wire to the chassis or to a large ground

lug or binding post. In this case you may have to apply so muchheat that the insulation on the wire starts to melt. This is not good(you probably figured that out already) Use a smaller lug or bind-ing post or if you are soldering to the chassis, use a lug and screw(don't solder) the lug to the chassis.

4. If you are reusing used parts, like vintage transformers for example,there may be gunk built up on the lugs. Clean the lugs first with agood residue free (non lubricated) cleaner like Radio ShackCleaner / Degreaser or any of the new and improved non CFCcontaining environmentally safe commercial cleaners availablefrom Mouser, DigiKey et al.

Solder bridges are more of a problem with printed circuit boards thanwith point to point soldering (using wires for all the connections) butstill may be a problem on tube sockets and transformers. This is whereexcess solder flows to a nearby junction and fuses the two into singleconnection. Just keep plenty of room between junctions and keep aneye out for solder bridges and all should be fine.And even though this is readily obvious to anyone whose plugged ina patch cable, electricity will follow the path of least resistance, whichin practice means that if there are any loose or frayed wires touchingmetal parts they shouldn't be touching, the circuit won't work asdesigned. Just keep everything neat and trimmed and well soldered.It's a good idea to have someone with electronics repair or buildingexperience look at your work before testing.

For Those With Limited Soldering ExperienceSuccessful electronics project building depend on good solderingskills. Fortunately, the difference between good and poor solderingtechnique is practice. Ifyou haven't soldered at least a couple thou-


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22 HOW TO BUILD A TUBE COMPRESSOR / LIMITERsand joints, you could probably use some practice. A great way to getthis practice is to repair any cable you can get your hands on. If youhang out with musician (guitar, bass, and keyboard player in particu-lar) volunteer to fix their broken cables. Most will have been stressedfrom being "yanked on" andjusthave to be re-soldered. If you boughtan old used piece of radio gear for the chassis, desolder the guts ratherthan cutting them out. You'll get a real feel for soldering after you'vedesoldered a decommissioned army radio.


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SECTION 2CIRCUIT AND COMPONENTS


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Introduction

History

CHAPTER 4THE LA2 STORY

The Teletronix Leveling Amplifier is a compressor/limiter used inbroadcast or recording applications. Gain reduction is accomplishedby the use of an electro-optical variable attenuator, placed ahead of thefirst amplifier stage. The attenuation is controlled by the amplitude ofthe input signal just after the input transformer.

The Teletronix LA2a is the successor to the very rare LA1 and LA1bprogram limiters. There were only a handful of LA 1's built before theLA2 was introduced in the mid 60's and the LA1's were discontinued.The LA2's were produced by Teletronix in Pasadena, California(although they were labeled Los Angeles). They are identified by theirgray face paint, octal jumper socket located before theHA-lOOX inputtransformer and limit only function. In 1965 Teletronix became adivision of Babcock Electronics Corp. Costa Mesa, California Manu-facturing moved to Studio Electronics Corp. of North Hollywood,California in 1967 when they acquired Babcock's assets, including theT4(b) patent. These units (post-serial #383) featured a brushedaluminum face, the addition of a compress/limit switch on the backand the designation LA2a. Late in the LA2a's production life, UnitedTransformer Company (UTC) replaced the HA-lOOX with the A-lO,which was incorporated into the LA2a from that time forward. TheLA2a was discontinued shortly after the introduction of the LA3asolid state (discrete) leveling amplifier in 1969.Studio Electronics Corp. became United Recording Electronics In-dustries (URE!) and continued production of the LA3a and later


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26 HOW TO BUILD A TUBE COMPRESSOR I LIMITERreintroduced the LA2a in 79 producing it into the early 80's. Thesewere identical to the later Hollywood brushed aluminum units with theaddition of an second power switch (normally open) behind the folddown front panel that powers the unit down when the front panel isopen. This was to keep those operators not used to working with the200-300 volts de from learning ohm's law by the touch method. UREIhas since been acquired by JBL who is a division of Harman Interna-tional.

Circuit DescriptionA typical gain reduction curve for this system is illustrated in theappendix (Typical Gain Reduction Plot on page 78.). With the com-press/limit switch (SW3) set to limit, compressor action occurs fromthe breakaway point at -30 dB input up to -20 dB, where the curvebecomes horizontal exhibiting limiting action. The input increases anadditional 20 dB, but the output increases less than 1 dB. The LA2athus combines the characteristics of a compressor and a limiter. Whenthe switch is set for compressor operation, after the breakaway point,the output will increase with increasing input level at a rate ofapproximately 3: 1.Compression and Limiting are a function of the electro-optical attenu-ator, T4b, placed between the input transformer and the first amplifierstage. The optical attenuator consists of a photo-conductive cell,which is optically coupled to an electroluminescent light panel con-sisting of a plastic plate coated with a clear conducting material on oneside and a metallic plate sandwiching a thin layer of phosphor on theother. The phosphors are excited as alternating current is applied tothe conducting plates, producing light in proportion to the appliedvoltage and frequency. Because the panel has an inherent rms conver-sion property, the side chain processing, filtering and rectificationrequired by other compressor designs are not needed.The cadmium sulfide photoconductive cell is the level controllingelement. The resistance of the cell decreases proportionally to anincrease in light intensity Photocell selection criteria require aminimum attack time, and a release time of 60 milliseconds for 50%release, then a gradual complete release over 1to 15 seconds (programdependent) .


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THE LA2 STORY 27The input signal is applied directly to the optical attenuator from theinput transformer's secondary winding. Gain reduction is controlledby the audio voltage applied to the luminescent driver amplifier(6AQ5a) A second matched photoconductive cell is used to controlthe metering section.Voltage amplifier (12AX7a #1) is controlled by the manual gaincontrol (Rl). The voltage amplifier stage provides a gain of 40 dB.Overall amplifier feedback of approximately 20 dB provides lowdistortion, flat response, and gain stability. The output stage (12BH7a)utilizes a double cathode follower for low distortion and flat fre-quency response.

OperationThe LA2a is designed to work as either a compressor or limiter. Theknee is determined by the setting of the "Peak Reduction" control.Compression occurs and gradually increases over the first 10 dB ofinput level rise. At this point, the curve changes to 3: 1 i f SW3 is set tocompress, or becomes essentially 00 : 1 if SW3 is set to limit.

MeteringThe VU Meter indicates output level as well as gain reduction in dBby way of SW2. When the meter selector switch is placed in the"output" position, the meter will indicate output level across the 600ohm terminals. The meter is calibrated to read 0 VU at +10 dBM or +4dBM, depending upon the switch position.The "Gain Reduction" position indicates amount of gain reduction indB on the VU meter. The meter adjust pot (R4) adjusts the zero pointof the meter .

Stereo OperationStereo compression and Limiting can be accomplished using two


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28 HOW TO BUILD A TUBE COMPRESSOR / LIMITERLA2a's by interconnecting the stereo port (terminal 6) and ground(terminal 7). A shielded twisted wire pair of less than 2 feet in lengthshould be used and the shield should be grounded at one end toterminal 7 (ground).The limiter response pot (R37) was implemented to deal with overmodulation problems related to pre-emphasis used in FM broadcast-ing and TV aural transmissions. For normal operation, set R37 to fullclockwise. Other settings reduce the low frequency content of thevoltage applied to the Gain Reduction pot (R2). High frequencycomponents are not affected


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PHOTO GALLERY 29

LA2

LA2aAn LA2 and a late model LA2a. Differences between the LA2 and LA2a include: the Simpson meter was replacedwith a Beede unit; a compress / limit switch over the input transformer (an A-lO in this example) replaces thejumper plug (early LA2a's retained the jumper plug); and the LA2a has an additional pot (stereo balance [R4])centered on the back. Also note the meter switch was moved from below the meter to above the power switch andthe power lamp was no longer used (probably because the Beede meter had a lamp were the Simpson did not.)


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PHOTO GALLERY 31

LA2

LA2aAn LA2 and a late model LA2a.One major difference between the LA2 and the LA2a is the use of terminal blocksto hold the components in the LA2a. The inside bottom of the LA2a is lined with 5 5x2 terminal blocks. You cansee a sixth terminal block attatched to the rear of the VU meter above ( it has 4 resistors soldered in it.) Buss wireconnected the terminal block pins.


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\I

CHAPTERSCIRCUIT HARDWAREThe most difficult part of reproducing a vintage piece of gear islocating the parts. That work is done for you here. Simply chose theappropriate parts and contact the listed supplier. I have included boththe original parts used in the LA2a with part numbers, when available,and improved components when applicable. The choice of vintage orimproved, in each case, is up to you.

ChassisThe original chassis was a 3 space x 2.5" deep x 19" standard rackmount unit. The front panel was hinged at the bottom allowing theentire face plate (except the rack ears) to fold downward allowingaccess to the inside of the unit. (All the inside view illustrations hereare shown with the front panel down.) Two locks were located towardthe top on either side of the front face plate.A reproduction of the original chassis would be expensi ve to manufac-ture but there are many suitable chassis floating around at hamfestsand surplus electronics outlets. With a little imagination you couldsave a lot of money here. I have built several into old CCA amplifierchassis and one in an old test instrument chassis (not rack mount). Inthe later, everything is safely enclosed and very portable. Ifyou insiston a new chassis, check out the boxes in the Newark catalogue or havea local machine shop manufacture one. Avoid using a plastic enclo-sure or you'll have to design a ground layout for the compressor.

)- - -


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3 4 HOW TO BUILD ATUBE COMPRESSOR / LIMITERTransformers

A UTC (United Transformer Company) HA-I00X input transformerwas used until the late 60's. There are some of these transformersfloating around on the used market but "they're gonna cost ya." TheHA-I00X was superseded by the UTC A-I0 input transformer andTeletronix used these in the later versions of their compressor. Manyengineers believe the sound of an HA-I00X equipped LA2 to besuperior to the A-I0 equipped units, but I looked at twelve LA2a's,and 6 were equipped with OEMA-I0's. Though theHA-l00Xequippedunits may sound slightly better to some, the A-I0 equipped unitssound just fine.The output transformer was always a UTC A-24. This transformer isstill produced to the original specifications by OPT/UTC. PeerlessElectronics Incorporated is the sole authorized distributor of the UTCtransformer lines in the United States but they don't always have themin stock and may require up to 10 weeks for delivery. Order yourtransformers early to avoid disappointment.Ifyou want a compressor with a li ttl e less distorti on, you could replacethe A-I0 and A-24 with a Jensen JT-llP-l and JT-I0K61-1Mrespectively. Although Jensen transformers were never used byTeletronix in the LA2's, they represent the state of the art in trans-former design today and are usually readily available. If you go withthe Jensen Transformers, tell the Jensen rep what they will be used for.Jensen has a reputation for being fanatical about transformer circuitperformance and they have some modifications you can do to thecircuit.Of course, you can use any compatible transformer so here are thenumbers.

Input Transformer Output Transformerprimary imp.sec. imp.freq. resp.Max.dBMMax.mW

primary imp.sec. imp.freq. resp.Max.dBMMax.mW

15KQ split*600Q

20-40Khz+1530

600Q50KQ split*

20-20Khz+1530 * the split winding is not required

V1.2


35/88

Resistors

Potentiometers

CIRCUIT HARDWARE 35The power transformer was a Triad R-4A on the earlier models and anunmarked unit in later production runs with a secondary winding of500vct (250-0-250v) @ 40mA and a filament winding of 6.3vct at 2A.In addition to the Allied units listed below, Hammond still producesa comparable unit available from several sources including AntiqueElectronics in Arizona.

The original circuit used all carbon composition resistors. Resistortechnology has come a long way since the tube days and high qualitymetal film resistors are abundant, readily available and very reason-ably priced. They are more stable, better sounding, and drift less withtemperature and age, however, there is a small contingency of audio-philes who believe that carbon resistors sound "warmer" than the"brittle" sounding metal film types. The parts list that follows listsmetal film types but Mouser, as well as most other suppliers, alsocarries carbon composition and carbon film in addition to metal filmresistors. Once again, the choice is yours.

RI and R2 are Allen Bradley J series lOOK log taper pots. The are stillavailable by special order through Mouser or Digi-Key as UnitedRecording part # J99030 lOOK 20% clockwise log taper. Early La2sused part # JAIN048PI04RA but Allen Bradley recommended theformer.R3, R4 and R37 are also Allen Bradley J series pots. These are 1Megwith a linear taper. They are still available by special order throughMouser and others as United Recording part # JAIL040S105MC 1M20% linear taper.Your choice of pots will be determined by your chassis, knobs,placement and budget. Make note of the value, taper, shaft length,diameter and component size when making your selections.


36/88

36Capacitors

HOW TO BUILD A TUBE COMPRESSOR / LIMITER

C4 and C14 are Arco 465 50-350pF variable capacitors. None of themajor suppliers carry these although they are available in smallquantities through Ampex and MCI tech shops as they were used inAmpex and MCI multitrack tape machines. Digi-Key carries a 10-180pFplastic dielectric cap., two of which can be paralleled to achievethe proper values and Antique Electronic Supply has a 40-500pF unit(part # C-VT500). Or you can use a fixed value.C4 adjusts the frequency response of the unit. A higher capacitance =more high-end (brighter) The variable cap is the best choice here butfixed values work just fine. Ifyou use a fixed value, 100 - 200pF is agood starting point.C14 was added to the latest production runs. Prior to that it didn't existin the circuit. A fixed value of 320pF works just fine here.C12 and C13 are mica capacitors; C6, C8 and Cll are general use(lowest quality) ceramic disks; and C1 and C9 are film capacitors. C2and C3 are Cornell- Dubilier WMF4P1 Kfilm( 10% tolerance). Polypro-pylene caps such as Sprague "Orange Drops" are a great replacement .here. They are more expensive than the originals but the capacitorparts count is so low, the additional cost is only a few dollars.If you like the look of that big Mallory capacitor can (C7a-d) hangingoff of the back of the compressor, then by all means, get a 2 x 40pF +2 x 30pF @ 450V can. Antique Electronics usually has these, orsomething comparable, in stock. They look good and work well but areusually more expensive than modern individual units and take up moreroom. Most of the working vintage gear in service today utilizesmodern individual replacement caps in place of the can capacitors andthe original cans are retained only for show so as not to have a largehole on the chassis's back . Ifthe vintage look of those caps on the backisn't important, use four 47pF @ 450V electrolytic capacitors forC7a-d. Sprague "atoms" are great for this purpose and take up lessroom than the capacitor cans.C5 is a 10pF electrolytic coupling capacitor (Mallory TC72A FF).That beautiful audio signal you've created with the circuit has to passthrough this capacitor so use the highest quality capacitor you can get


37/88

T4b

Switches

Misc. Parts

CIRCUIT HARDWARE 37a hold of. Again, Sprague "atoms" are good here or if you want tospend the big bucks, use a Multicap, Axon or MIT cap. CI0 wasoriginally a SOjtF @ 3SV electrolytic but it was replaced by a 100jtFcap in the LA2's I looked at.

This is a proprietary device available through JBL pro dealers. An-thony DeMaria Labs also sells a comparable unit. The UREI(JBL) T4bcomes with a matching resistor (R2S) to insure the VU meter tracksaccurately. If you use the DeMaria T4b or you don't have a matchingresistor see the appendix, pg. 88.

SWI and SW3 is a SPST onloff switch of 12SV. SWI is the powerswitch so it should be able to handle 3 amps or more. SW2 is a 3position I dual throw that allows the user to select between thecompressor output level referenced to +4 or + 10 or the amount of gainreduction. In actuality, most users only use the +4 output and the gainreduction settings. For this reason, I usually use a DPDT onion switchwired to select between output level at +4 and the gain reduction.DPDT switches are cheaper, easier to find and come in a greatervariety of packages. But the choice is yours.

The parts listed in the misc. ordering section in the next chapterbecause the choice of components depends on the chassis you buildthis into.The terminal blocks may be ceramic, pressed paper, or plastic and thesize and quantity will depend on the chassis. You may also elect to skipsome of the terminal connections and connect the wire or componentstraight between two components.I never use the 3 position switch for the meter switching because Idon't need the +10 output and there is much greater selection withDPDT switches. And while were talking about meters, the 6.3V lamp

V 1.3


38/88

38 HOW TO BU ILD A TUBE CO MPRESSO R / LIM ITERmayor may not be needed depending on the VU meter you use.Twisted pair console wire comes in very large rolls (1000' typical). It'sbest just to go to the local music store (the one the pro's hang out at)and buy about 6' unless you plan on wiring a studio or building acouple hundred of these compressors. Hookup wire comes in a varietyof colors. The truly anal retentive will want to get a number of colorsand follow the hookup wire colors listen in theappendix. The rest of uswill use one to three colors.Lastly, if the unit will reside primarily in a permanent installation, likeyour studio, then use barrier strips for the input and output connec-tions. They are cheaper than XLR and very reliable if the unit isn'tplugged and unplugged often. If you are going to travel from studio tostudio then XLR's may be more appropriate. Of course, you couldalways make some pigtails with XLR connectors on one end andterminal connectors on the other. One thing to consider is that depend-ing on the grounding scheme of the facility you're working in, youmay need to connect the shield to either ground or the centertap of theinput and output transformers. This is much easier with the barrierstrips.

What Is S urplus E quipm ent?Much of the hardware, that is, non electrical parts, of the compressorcan be salvaged from surplus or decommissioned equipment. Surplusor used parts can greatly reduce the cost of building gear. For example,I have never bought a new chassis but I have plenty of professionallooking chassis is my racks and shop. But, you may ask, what issurplus and where can I get it?Prior to the mid seventies, most television, radio and test equipmentconsisted of tube based circuits. With the widespread use of transis-tors, the interest in general use tube equipment has waned as siliconbased components are cheaper, more reliable, and consume lessenergy than their tube counterparts in most circuits. All of us haverealized the benefits of this progress as reflected in the price andquality of our TV's, computers, Nintendo's, etc., but there is anadditional benefit to us who build gear.


39/88

CIRCUIT HARDWARE 39There is a plethora of old tube equipment carcasses awaiting our"recycling". A single non working oscilloscope can supply an indi-vidual with more terminal blocks than he'll need all year, a number ofgood switches and pots, knobs, a power transformer, and even someusable tubes. And it is amazing what some sandpaper and paint can doto an old abused chassis. I have several scopes I got for next to nothingthat are full of ceramic terminal blocks complete with mountinghardware (try finding those new), Allen Bradley J series pots inseveral usable values, lots of multi position switches (some with silvercontacts) 7, 8 and 9 pin tube sockets, and the list goes on.It's a gold mine, but a few cautions are in order. Carbon compositionresistors have a tendency to change values with age and because theyare so cheap now ,just store them away for a rainy day (or emergencySunday night repair) or discard them. And don't use the old capacitors,especially the larger value ones. Unless the gear was never used, theyare always bad; and even if they test good, they will be bad soon.There is an additional problem concerning old can capacitors. PCB'swere used extensively in the dielectric of these caps and PCB's havebeen found to cause cancer when exposed to tissue. Additionally,PCB's do not break down so if you dump them in your back yard theywill still be there when dinosaurs return to rule the earth. It's best justto dispose of them with the proper agency and not mess with themfurther. Some of these capacitors also used various acids as thedielectric. Don't puncture the cans and if it is already leaking, bag itand dispose of it properly - and don't touch the liquid acid!PCB's were also used in some transformers so the same rules apply totheir handling and disposal as well, although they may be safely useduntil failure. Please, do not just throw them into the dumpster or thelocal landfill.To find surplus and used gear look for Hamfests in your area, checkthe pawnshops, flea markets, the classifieds, privately owned elec-tronics stores and get the word out that you are looking for old andbroken gear. It doesn't hurt to talk with members of the local radiocollectors or ham operators clubs either as they usually have a goodhandle on where the retired and surplus gear is.

) Happy hunting.


40/88



41/88

CHAPTER 6ORDERING INFORMATION

OK, get out your checkbook and order off the list ...

800- 285-2121516593-2121516594-3500 fax

Pearless E lectronics Inc.

http://peerlesselectronics.com

1 OPr/UTC A-I0 input transformer1 OPr/UTC A-24 output transformer

Pearless quotes delievery times of 8-10 weeksso order these early.
http://peerlesselectronics.com/http://peerlesselectronics.com/


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42 HOW TO BUILD A TUBE COMPRESSOR / LIMITERMouser

958 North Main StreetMansfield, TX 76063-4827

800 346-6873817483-0931 fax

http://www.mouser.come-mail: [email protected]

Resistors (metal film)cnt stock # value cnt stock # value cnt stock # value4 271-1k 1.0k 1 271-6.8k 6.8k 4 271-68k 68k1 271-1.2k l.2k 2 271-10k 10k 1 271-100k lOOk1 271-1.5k l.5k 1 271-22k 22k 4 271-220k 220k1 271-2.7k 2.7k 1 271-33k 33k 1 271-330k 330k1 271-3.9k 3.9k 1 271-47k 47k 5 271-470k 470k1 262-4.7k 4.7k12W 1 262-10K 10kl2W

Capscnt stock # value cnt stock # value1 140-XAlA50V1O 10 jlf/450v 4 140-XAlA50V 47 47 jlf/450v1 140-ESRL50V1 00 100jlF/50v

Pots31VF601 * 1M linear31VJ501 * lOOKaudio 3

* These are very inexpensive pots. Better quality pots may yeild a better sounding unit.Diodes2 583-1N4007 1N4007Neon Bulbs

36NEOO2 NE-2V1.2
http://www.mouser.com/mailto:[email protected]:[email protected]://www.mouser.com/


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ORDERING INFORMATION 43

2 12AX7a 1 6AQ5A 1 12BH7a

New Sensor Corpo ration20 Cooper Square

New York, NY 10003212 529-0466800 633-5477

212 529-0486 faxhttp://www.sovtek.come-mail: [email protected]

CapacitorsSprague "Orange Drops"

22

value (uF/V).022/600.11600

cnt. value (uF/V)1 .00116003 .01/600

cnt

Sprague "Atoms"(You may purchase the the electrolytics through New Sensorinstead of Mouser if you want to use the "Atoms")

Tubesent. tube number ent tube number ent tube number

Tube sockets:ent. part ent part ent part3 9 pin sockets 1 7 pin socket 1 octal socket
http://www.sovtek.com/mailto:[email protected]:[email protected]://www.sovtek.com/


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44 HOW TO BUILD A TUBE COMPRESSOR I LIMITERAllied Electronics7410PebbieDrive FortWorth,Texas 76118 817595-3500 817595-6444faxhttp://www.allied.avnet.coml

1 power transformer (stock #705-0113)(best value)or

1 Hammond 269X power transformer

IMS1756 Victory Boulevard Glendale, CA 91201 818243-0416MS knows meters. The Se1co A139WF is close to the same size meter as the Beede used irhe LA2a. (Beede is making electronic parts for Ford now and is no longer making meters.or a great vintage look, check out the Selco "Vintage" Meter types 20A SQ and 25A SQ.

Empire West Audio (authorized lBLdealer)4255 Main Street Riverside, CA 92501 909 788-4898

ororder through your local JBL dealer

orAnthony DeMaria Labs818340-0228 sales 818340-4331 fax

914256-0034 service

1 T4b with matching resistor (R25 on the schematic)

V 1.2
http://www.allied.avnet.coml/http://www.allied.avnet.coml/


45/88

ORDERING INFORMATION 45

Misc.All these parts choices are dependent on your choice of chassis. Useof surplus or used parts can create a vintage look and greatly loweryour building cost.9 3 pin terminal blocks4 2 pin terminal blocks or posts3 terminal lugs (ground only)1 3/8 fuse with holder2 SPST switches1 DPDT. switch1 6.3 vac lamp with holder6' console wire (24-26 gao twisted pair wI shield)1 roll hookup wire (24 ga stranded)3' filament wire (12-18 gao stranded)1 power cord with desired connectors5 knobs for the pots1 Chassis (enclosure)2 5 screw barrier strips

or1 XLRmaie1 XLRfemale1 114in TS female


46/88

46 HOW TO BU ILD A TUBE COMPRESSOR / LIM ITER:::.ADDITIONAL SUPPLIERS

Digi-key701 Brooks Avenue South P.O. Box 677Thief River Falls, NM 56701-0677800 digi-key 218681-3380 fax

http://www.digi-key.comBig catalog. Everything ordered from Mouser could have just aseasily been ordered from Digi-key, and they have a variable capacitorthat could work in parallel for C4 and C 14. Every tech has this catalog.

Jensen Transformers7135 Hayvenhurst AvenueVan Nuys, CA 91406

213 876-0059Jensen's name is synonymous with great transformers. They also havea circuit modification for changing the LA2 output circuit from acathode follower to a casco de circuit. If you want a "cleaner" LA2sound give Jensen a call.

JBL Professional! UREI Parts Dept.818 894-8850

If you can't find a local dealer to get a T4b module, call here.

Newark E lectronics800 4-NEWARK

http://www.newark.comHuge Catalog. A must have.
http://www.digi-key.com/http://www.newark.com/http://www.newark.com/http://www.digi-key.com/


47/88

ORDERING INFORMATION 47Magic Parts1537-AFourth St. #198SanRaphael, CA 94901415453-8606

A great guitar amp parts supplier.

Tech America800 877-0072800 813-0087 fax

Another mail-order catalog

Ford Electronics8341 CommonwealthBuena Park, CA 90620714521-8080

In addition to the parts every electronics store has, these guys alwayshave a supply of old chassis of various sizes and origins. Ifyou're inSouthern California, definitely stop by.

Apex E lectronics8909 San Fernando RoadSun Valley, CA

213 875-1308This is the promised land of surplus gear. Imagine a warehouse full ofold test gear, navy surplus electronics, wires, old mainframes, vendingmachines, land mines, .. , and a back lot for the overflow. You reallyhave to go there and look around but if you want "vintage" this is theplace.


48/88

48 HOW TO BUILD A TUBE COMPRESSOR / LIMITERPacific Radio E lectronics

969N. La BreaAvenueLos Angeles, Ca 90038

213 969-2035213 969-2053 fax

800 303-0969 (Ca only)800 634-9476 (outside Ca)

Great Pro Audio / Video oriented electronics supplier

Yale Electronics6616 Sunset BoulevardHollywood, CA 90028

213 465-3186Another great electronics store geared toward pro audio / video.

Antique Electronic Supply6221 S Maple AvenueTempe, AZ 85283480 820-5411

800-706-6789 fax (U.S. and Canada)or 480 820-4643

http://www.tubesandmore.comlmain.htmlSpecializes in tubes and radio supplies. Very large inventory.

V 1.1
http://www.tubesandmore.comlmain.html/http://www.tubesandmore.comlmain.html/


49/88

SECTION 3Mechanical DrawingsandAssembly Instructions


50/88



51/88

COMPONENT CHART

Component Chartschematic symbol illustration

resistor

potentiometers (pots)123

1 11 11 11 -I~ 1--------------1 11 1 ceramic disk1 1 I1 . 1-1------1--------------1 11 11 electrolytic

! T ~ - - - - - - - - - - - - - -1 11 11 1 electrolytic can~------L--------------1111

filmcapacitors (caps

variable

51


52/88

52 HOW TO BUILD A TUBE COMPRESSOR / LIMITERschematic symbol illustration

2 3 4

transformers

IIIIIIII,---------------------IIIIIIII

1 5

e.

67

input and output

power

tubes

IIIIIIII duel triodeI l2AX7aI 12BH7a~---------------------IIIIIIIIII

1 62 r;E-~ 73~9~8

4 5 68 7

5 2 3\~.\4- 7/7653 4pentode6AQ5a


53/88

" \I COMPONENT CHART

schematic symbol illustration

7 t , ( ) ~: : 83 + - u - + - 25 ~ (t) ~: : 6

T4b

diodes _I--

switches

111 -O...,._111 S~T1L _

3b 2b 1br-0- - - -C ;- .,.Jo.. . , ._ -0...... 1I IL _J 3a 2a 1a

2 positionViewed upside down as it is

placed on the open/rant cover

53


54/88

54 HOW TO BUILD A TUBE COMPRESSOR / LIMITERschematic symbol illustration

vu meter

incandescent lamp

h.-

neon lamp

barrier strip a a a a a

3 pin tenninalblock 123terminal block

and posts(no schematicsymbol) ost -centerisolated with

ground lugpost -ground lug

only


55/88

NN .-t0::: N0:::

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58/88

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-o C 1 JE-.... .QU roC 1 J 1 -CCo 0......1/1C '-o C 1 J...... c~.0'-uLL.

C"ItnooOC"ltnOO- 01 C"I tn \0 r- 000000000II II II II II II II

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59/88

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60/88

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61/88

(r-

CHAPTERSBUILDING THE COMPRESSOR

By now, you are familiar with the basic components, you've success-fully completed at least 200 solder connections and you have severalboxes in front of you containing resistors, caps, other electronic parts,an enclosure and your tools. It's time to build a compressor.First, a suggestion. Resist the temptation to dump all the parts out onthe table in front of you. Leave them in the packaging they wereshipped in. It's far easier to locate a 68K resistor by removing it fromit's own marked plastic bag than to sort through a pile of mixedresistors.

Chassis Layout and cuttingWe'll start with the chassis. We need to determine the componentlayout and cut the appropriate holes. If you bought a brand new 3 rackspace box use the mechanical drawings in this book for the layout asthis will yield the fewest surprises. Pencil mark the cutout of eachcomponent. Once the layout is complete, punch, drill or cut the holes.After the holes are cut, remove the larger burrs with needle nose pliersand deburr the edges with a small grinder or fine sand paper. ALL thedrilling, cutting and punching should be completed before going on tothe next step - even if you don't have all your hardware yet (yourinput and/or output transformers may be back ordered, for example.)When cutting or drilling, small pieces of metal drop around and in theenclosure. Ifcomponents are already mounted in the enclosure, theseshavings will find their way into the electron path.

) Ifyour box is substantially different than the one in the mechanicaldrawings or already has some punched holes (ex: a used box) , you'll


62/88

70

Painting

HOW TO BUILD A TUBE COMPRESSOR / LIMITER

have to determine your own layout. A few thing to keep in mind whendesigning a layout: Keep the power supply section, particularly the power transformer asfar from the audio section as possible. It is customary to mount thepower transformer on one side of the unit and the audio transformersand tubes are on the other. Keep the 12AX7#1 and 12BH7a tubes close to each other for propergrounding but far enough from each other to allow for air circulation.About 2 inches is good. The A-I0 and A-24 shouldn't be placed rightnext to each other either. Insure there is enough clearance between all components. Note thatcomponents mounted on the front panel take up room inside the boxonce it's closed up.Itmay be helpful to mark a piece of plain or graph paper with a full sizeoutline of the enclosure and mark the location of various components.When you're satisfied with your layout, transfer this layout to yourenclosure.

Once you've drilled all the holes, try each mounted component for fit.Ifeverything was measured and cut perfectly, they should fit just fine,but you may need to trim one or more of the holes for a proper fit. Onceall the components are fitted it's time for painting.

Remove the hardware (including electronic components) from anyenclosure parts that will be in the paint booth. At this point it's easierto remove everything than to mask it. Ifyou are only painting the frontpanel, which is all you really have to paint, and it is detachable fromthe rest of the enclosure, remove only the front panel and the compo-nents on it. Paint the panel. After the painting is finished and the paintis completely dry, wipe all the enclosure surfaces with a damp clothreassemble the enclosure and fit all the mounted components to theenclosure.


63/88

BUILDING THE COMPRESSOR 71Star Ground

We'll start the soldering with the star ground connections. Thepurpose of the star ground is to insure a good ground path for the audiosection of the compressor. So fire up your soldering iron, tin the tip,and lets get going. The star ground is made of a single strand of busswire (uninsulated 18-22 gauge) running from the ground lug of eachcomponent listed on page 60 to a single ground lug on the T4b socket.Itmay help to cut 5 6" buss wires and solder them to the ground lugof the T4b first. Then trim each one before soldering the other end toeach of the 5 components listed. Mark off each connection as youfinish it.

Buss Wire ConnectionsNow connect the buss wire connections. It's helpful to loop the endsof these wires through the pin before soldering as most of theseconnections will have additional wires or components soldered to thesame pin. Mark off each connection on page 60 as you finish it.

Twisted pair ConnectionsDo the same with the 5 console wires also listed on page 60. Consolewire is a pair of twisted wires wrapped in a foil or braided shield. Oftenthe shield is only connected on one end. The wires should be kept asshort as practically possible. It's OK to have some slack in the wire runbut avoid leaving rolls of wire in the bottom of the box. Mark off eachconnection as you finish it.

Power ConnectionsConnect the power connections. After wiring the AC plug, tape up theend of the plug so it can't be plugged into an AC socket. Or don'tconnect the AC plug until the rest of the compressor is completed. Thisis to prevent anyone, including yourself, from plugging in the unfin-ished compressor. Mark off each connection as you finish it.)


64/88

72 HOW TO BUILD ATUBE COMPRESSOR / LIMITERFilament Wire Connections

Connect the filament wire connections listed on page 62. Avoidrunning filament wires in parallel with other wires particularly thosethat carry audio. Also, most of the pins carrying filament voltage havea wire coming to the pin and another going from the pin. You may wishto solder both these wires at the same time by gripping both wires closeto the end with needle nose pliers or hemostats.Ifthe VU meter has a 6.3V AC lamp, that voltage can be obtained bytapping the power lamp pins or by omitting the power lamp altogetherand using those wires for the meter lamp. And, as you've previouslydone, mark off each connection as you finish it.

Shielded Wire ConnectionsConnect the shielded wires next. Avoid running these parallel to anypower or filament wires and try to leave at least a 1 inch clearance inall directions between these wires and the power and filament wires.Don'tfeel you have to use the wire colors listed. Youcouldjustas welluse gray (or any other color for that matter) wires throughout. The wirecolors were listed only to reflect what was used in the original LA2.Wires marked magenta were not used in the LA2 and were added toimplement the stereo adjust pot (R3) and compressllimit switch(SW3). And mark off each connection as you finish it.

Check your workNow is a good time to go back and check your work. Double checkeach connection you made. Insure that it is connected to the correctpins of the correct components. Also, make sure that it isn't touchingany adjacent pins or the enclosure. Also insure that all the solder jointsare somewhat smooth and shiny. If one or more are dull or look likea solder ball resting on top of the pin and wire instead of over them,then resolder those joints. Reread the section on soldering if you needa refresher.


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BUILDING THE COMPRESSOR 73Res is to rs and O th er Components

We'll start with the diodes. Place the two diodes as indicated on pages64-65. Diodes are one way devices so make sure the line on the diodeis closest to T8. Solder them into place. Trim the leads (the wires oneither side of the diode) so that the leads extend about 1/8 inch past thepin on either side.Now place the neon bulb on T1 pins 2 & 3, with the bulb facing the leftas indicated in the drawing. Trim the leads if necessary and solder intoplace. Mark off the diodes and NE-2 as you finish them.Some of the resistors are placed over others so we won't do theresistors in numeric order. We'll start with the easy ones. Place R5,R7, R11, R16, R22, R23, R24, R25 (if supplied), R29, as indicated. Itmay be helpful to put a half turn or hook in the end of the lead withneedle nose pliers before soldering the resistor into place. The hookhelps the resistor stay in place when the solder connection is meltedagain to add another component to the pin. So, for all the resistorslisted above: measure, cut, hook, and solder into place. Be sure theleads don't touch any other conductive material (buss wires, otherresistors, etc.) This goes for all the uninsulated leads. Mark off theresistors as you finish them.R12, R14, R19 and R20 are a little different than the rest. They aresoldered on one end to the tube socket ground. This can be either aground lug on the socket or the center of the socket depending on thesocket design. Since the leads on these resistors span a distance lessthan the length of the resistor body, it is easiest to stand them on endas illustrated. Mark off the resistors as you finish them.Solder all the resistors connected to the 12AX7#1 base. This includesR9, RlO, and R13. Mark off the resistors as you finish them.Solder all the resistors connected to the base of the 12BH7a. Thisincludes R15, R17, R18 and R21. The leads needn't be bent at rightangles as illustrated. Usually the shortest distance will suffice pro-vided you allow for clearance of 118 inch or more between wires,leads, etc. Mark off the resistors as you finish them.Do the same for the resistors connected to the 12AX7#2 and 6AQ5a

V 1.1


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74 HOW TO BUILD A TUBE COMPRESSOR / LIMITERsockets. This includes R31, R32, R33, R34, R35, R36, and R3S. Markthem off as you finish them.And finally, connect R6, R26, R27, R2S, and R30. Mark them off asyou finish them. The resistors are now finished.

Check your workDouble check each connection you made. Insure that each resistor isconnected to the correct pins of the correct components. Also, makesure that it isn't touching any adjacent pins or the enclosure and thatall the solder joints are good.

Capacitor PlacementPlace all the capacitors as indicated on Pages 66-67. If you orderedC7a-d as 4 discrete capacitors instead of the cap can as recommendedearlier, you may need to support C7d. You can mount it on anadditional terminal strip on the top or back right side of the enclosure;or strap them in with cable ties and a self adhesive nylon cradlemounts; or glue it to the enclosure with silicon adhesive (leastpreferred).CSa and CSb are 2 capacitors combined in parallel to get the correctvalue. C4 may be replaced with a 150pF to 200pF fixed value capacitorthe larger the value, the brighter.) C14 is for adjusting the .(\fequencyresponse of the side chain and may be replaced with a 330pF fixedvalue capacitor or eliminated altogether (as it is on earlier LA2's andat least one LA2 copy currently manufactured).Mark off the capacitors as you finish them. Then double check eachconnection you made. Insuring that each capacitor is connected to thecorrect pins of the correct components. Also, make sure that it isn'ttouching any adjacent pins or the enclosure and that all the solderjoints are good.

Fire it up!After you've double (and triple) checked your connections, it's timeto add some power. I usually use a power strip to test gear. Ifsomething is catastrophically wrong, like AC connected to ground

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BUILDING THE COMPRESSOR 75before the fuse, then the power strip circuit breaker will trip before thehouse or shop breaker. This will save you having to grope around inthe dark looking for the failed breaker.Install or unwrap your AC connector. Very Important: Check theground connection on the AC cord! Connect one multi meter probe tothe third prong of the AC connector and the other probe to theenclosure and reading the resistance. Ifit reads more than 1 ohm or so,then check, and if necessary, resolder the connections until it readsless than 1 ohm. Although it is common practice in some circles to liftthe ground on audio equipment, itis unsafe to do so while testing. Alsoinsure that your AC wall socket is AC grounded. There is an inexpen-sive Radio Shack tester built for this purpose.Now, with the tubes and T4b NOT installed in the sockets yet, and theAC switch (SWl) in the off position, plug the compressor in. Wait aminutes to see if anything is going to happen - Itshouldn't. Stand tothe side of the enclosure so you're not directly in front of anycomponents. Ifthere is a dead short or one of the power capacitors iswired backwards you don't want to be in the line of fire, should itexplode. I'm both cynical and cautious so I expect everything toexplode. Now, turn on the power switch. Watch for flames and snifffor smoke. There shouldn't be any (obviously) but if there is turn theunit off immediately. Unplug the unit and discharge the electrolyticcapacitors (C7a-d, and C5). Do this by touching one side of a lKIlOWresistor to the + side of the cap and the other end of the resistor to the- side. (Double check that the unit is unplugged first) You may see asmall spark, which is not a problem. Then try to locate the offendingcomponent(s). If something smoked, it will probably have scorchmarks on and around the failed part. If there are no scorch marks,carefully feel each component for excessive heat (after you've dis-charged the caps). Any component that is above room temperature atthis point is suspect but note that failed new components are usuallya symptom of another problem such as a misconnected wire or wrongcomponent value. Check all components and connections. Afterrepairing or replacing the failed part(s) and making any wiringchanges, go back to the top of this paragraph and retest the circuit.If everything was done correctly, the power indicator lamp and/or theVU lamp has lit. Wait a couple of minutes for the power caps to form,then look in the box - the NE-2 should be glowing. Ifit isn't, tum the


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76 HOW TO BUILD A TUBE COMPRESSOR / LIMITERshaft on the meter adjust pot (R4) until it does. If it still doesn't lightup, power down and recheck the connections. There is a very smallpossibility that the NE-2 is bad so you can replace it but chances arethe problem is a misplaced wire or a bad solder joint. Once the NE-2 is lit, let the unit run for 15 minutes or longer to better form the powercaps. Then turn the unit off.Place the tubes and T4b in the appropriate sockets. Turn the limitresponse pot (R37) and Stereo adjust pot (R4) all the way to the right(clockwise). Turn the gain and gain reduction pots all the way to theleft (counter clockwise). Switch the meter select switch (SW2) to theleft (output). Turn C4 (if you used a variable cap) all the way to theright (clockwise) then back about 170 degrees. This controls thefrequency response of the audio portion of the circuit and 170 degreesless than max will get you in the ball park. Turn C14 (if you used avariable cap) all the way to the right (clockwise) then back about 20degrees. Connect an audio source to the input and connect the outputto your mixer. Stand to the side of the enclosure so you're not directlyin front of any components. Turn the power switch on. Watch forflames and sniff for smoke - if there is any, turn the unit offimmediately and follow the debugging procedure outlined above. Ifyou used a pot for R25 go to the appendix, pg. 88, Switch the meterselect switch (SW2 - gain reduction) to the right and adjust the meteradjust (R4) to 0 on the VU meter. Let it warm up for a couple ofminutes, then turn on your audio source. Bring up the gain knob slow lyuntil the desired level is achieved (usually 9-11 o'clock). Now turn thegain reduction pot to the right and watch the VU meter and listen forgain reduction.

If the compressor is workingCongratulations!!! You now have a world class compressor. If youwish, you may adjust the frequency response. You may attempt this byear (not easily done but possible, I suppose) or by using a test tonegenerator and an oscilloscope. Adjust for a flat frequency response to15kHz. Further extending the response to 20kHz results in a bump ataround 16kHz which mayor may not be to your liking.

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BUILDING THE COMPRESSOR 77If the compressor is NOT working

If the compressor isn't operating correctly: Recheck your wiring (again). Check it against both the mechani-

cal drawings and the schematics. Verify all the component values. Check each individual compo-

nent against the schematic. Mark each component as youverify it.

Compare the circuit voltages against the voltage chart in theappendix. All references are between the specified test pointand circuit ground. If you are at all uncomfortable withworking with high voltages or you have limited experience inthis area, now is the time to consult an audio tech. If youchoose to continue on your own remember the warning in thefront of the book. This is a high voltage project and it can killyou. Be very very careful., or better yet, let someone whoreally knows their stuff finish it up.

Swap out the tubes in case one is bad.If all else fails, contact your local audio tech and have him take a lookat it.


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APPENDICES


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APPENDIX 81SPECIFICATIONS

Input Level: +16 dBM maximumInput Impedance: SO, 150, 250, and 600 ohms, balanced or unbalancedOutput Level: +10 dBM nominal; +16 dBM maximum peaksOutput Impedance: SO, 150, 250, and 600 ohms, balanced or unbalancedOutput SourceImpedance: approximately ISO ohms at 800 hertzGain: 40 +/- 1 dBGain Reduction: up to 40 dBAttack Time: essentially instantaneous (10 Jlsec.)Release Time: approx. 0.06 seconds for 50% release, 0.5 to 5 seconds for

complete release depending upon amount of previous reductionResponse: +/- 0.1 dB, 30 - 15K hertzDistortion: < 0.35% @ +10 dBM, and


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82 HOW TO BUILD A TUBE COMPRESSOR / LIMITERBlock Diagram

~tiCQI h Uoltage Cathode- At tenucltor ftIIpli fier . . Folla.erttodule ~ - ( 12RX7a) (1211H7a)


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APPENDIX 83Differences Between LA2 Revisions

Throughout the production life of the LA2, the design remained the same but some componentsand component values were changed either because components became unavailable or to makethe circuit more stable. This table lists the differences between an original LA2, an early (1965)LA2a and a UREI LA2a reissue.

LA2 LA2a (1965) LA2a (1979)TI ' r .HA~IOOX,+'t.

'j1'

R4R7RI6R30 47KR34 22K5%2W 22K5%2W IOK5%2WR36 IK IK 4705%R38 not used not used 220K5%C7a 20JlF/450V 40JlF/450V 40JlF/450VC7b 40JlF/450V 40JlF/450V 40JlF/450VC7c 20JlF/450V 30JlF/450V 30JlF/450VC7d 20JlF/450V 30JlF/450V 30JlF/450VClO 50JlF/5OV 50JlF/5OV 100JlF/50VC13 not used not used 510pF/450VCI4 not used not used Arco465CI5 not used not used 470pF/450VDiodes IN3I96 IN3I96 IN4007


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84 HOW TO BUILD ATUBE COMPRESSOR / LIMITER

RESISTOR COLOR CODE

Color digit digit multiplier tolerenceBlack 0 0 1Brown 1 1 10Red 2 2 100Orange 3 3 1,000Yellow 4 4 10,000Green 5 5 100,000Blue 6 6 1,000,000Violet 7 7 10,000,000Grey 8 8 100,000,000White 9 9 1,000,000,000Gold .1 5%Silver .01 10%

20%


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APPENDIX 85

CONSOLE WIRECOLOR CODE

colorblackbrownredorangeyellowgreenbluevioletgreywhite

circuitgrounds, grounded elements and returnsheaters(filaments), off groundpower, B+screen gridscathodescontrol gridsplatesnot usedA.C. powerabove and below ground returns, a.v.c., etc


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JUMPER PLUG & INPUT PADEarly LA2's were outfitted with an 8 pin socket and a jumper plug,pad, or pre-emphasis network. The jumper plug and pad may be usefulin some recording applications and are described below.

1 7 Pad Parts List2 RI-R4 200Q 1/4W

R5 250Q 1/4W3 8-94 OEM markingseWE5 RN70D10 2000F(Rl-4) or 2500F (R5)pad schematic -

1 287

34

6 5jumper plug

R1 2 1R28

7 RSR3 6

9R4 8 pin socket installationpad plug-in


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APPENDIX 87VOLTAGE CHARTV a l u e s i n v o l t s , l i s t e do n t h e T e l e t r o n i x L A 2 a s c h e m a t i c ( d r a w i n g # s-1 0 1 4 - 1 1 , r e v i s e d 7 - 1 3 - 6 6 ) a n d m e a s u r e d o n a n L A 2 a n d L A 2 a a sm a r k e d .

Teletronixtest point scherrrattc LA2 LA2aC 7 b (+) 2 7 5 2 4 1 2 4 51 2 A X 7 a # 1

p i n 1 1 4 6 . 1 1 1 1 9 6p i n 2p i n 3 1 . 2 0 . 9 1 . 0p i n 6 1 2 0 1 2 2 1 0 1p i n 7p i n 8 . n l a 1 . 4 1 . 0 8

1 2 B H 7 ap i n 1 1 0 0 9 5 9 2p i n 2p i n 3 n l a 3 . 4 3 . 8p i n 6 2 2 0 1 9 5 2 0 0p i n 7 n l a 6 5 6 5p i n 8 n l a 1 0 0 9 8

1 2 A X 7 a # 2p i n 1 n l a 8 9 9 0p i n 2p i n 3 0 . 7 5 0 . 6 9 0 . 7 5p i n 6 n l a 8 9 9 0p i n 7p i n 8 n l a 0 . 6 9 0 . 7 5

6 A Q 5 ap i n 1p i n 2 n l a 5 . 8 3 . 8p i n 5 8 5 1 2 5 1 0 0p i n 6 n l a 1 0 4 7 0p i n 7


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matching resistorADL doesn't supply a matching resistor with their T4b. IfR25 was notsupplied with your T4b, you should use a lOOK linear pot in place ofR25 and use the following set-up procedure.

1- Power up the compressor and let it warm up.2- Adjust the lOOK pot to about 25-50K (the value is not criticalat this point); turn the gain reduction pot fully counterclockwise(no limiting)3- In meter compress mode, set the meter to Ovu using R4.4- Switch to meter output.5- Feed a IK tone at +4 (Ovu) into the compressor and adjust thegain pot so that the meter reads Ovu. If you have an outboardmeter, such as the channel strip on you mixer, feed the output ofthe compressor to the input of console and adjust the console sothe meter reads Ovu. The outboard VU isn't necessary but makesthe job easier.6- Turn the gain reduction pot clockwise until the VU meter(s)read -5. You are now compressing the tone 5 dB.7- Switch the compressor meter switch to compress mode and turnthe lOOk pot until the meter reads -5dB.8- Kill the tone. Allow a minute or so for the CdS cell to dischargeand check the meter. If it isn't at 0 dBvu (and it probably won'tbe) readjust the meter adjust pot (R4, not the lOOK pot).9- Repeat steps 4 thru 8 until the meter doesn't move whenswitching between output and compress and also reads Ovu incompress mode with the tone off.


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Documents

Jahnsen LA-2A Tube Compressor Handbook