First Watt F3

8/10/2019 First Watt F3

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Now for something CompletelyDifferent:

First Watt model F3

Power JFET Amplifier


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PART 1: Introd!tion and Philosoph" #$o !an s%ip this part&

There are two basic elements to an amplifier design the parts you use and whatyou do with them. Of the parts, the active gain devices are the most importantcritical they contribute most of the distortion, and their characteristics heavilyinfluence the rest of the circuit design.

The better the parts, the simpler the circuit can be. The more subtle the circuitdesign, the more you get out of the parts, and the better the sound.

Simplicity not only reduces the number of parts that the sound has to go through,it requires design that maximizes the linearity of each gain stage and minimizesthe feedbac.

!or the last twelve years " have been exploring power amplifier designs with only

one gain stage #the $en series www.passdiy.com% in contrast to commercialdesigns using as many as nine gain stages. The simple amplifiers often don&tmeasure up as well in demanding applications, but when the speaer is easy todrive and the music isn&t too loud, a simple amplifier often sounds better.

To get these single stage amplifiers to wor, "&ve been using power 'OS!(Ts asthe gain devices. These were the only devices which had the characteristics tomae a practical single)stage power amplifier high input impedance, highvoltage and current capacity, and high gain. The $en amplifiers were all *lass +designs, both single)ended and push pull, and they all delivered about - watts orso. Some used feedbac, some didn&t, and they all measured between .-/and ./ harmonic distortion at watt.

0ower 'OS!(Ts were chosen by default. 1ou can&t get a practical single stagepower amplifier from a tube the gain is too low. + bipolar transistor has too lowan input impedance. +nd there were no power 2!(Ts on the maret.

2!(Ts, or 2unction !ield (ffect Transistors, are routinely used in the input stagesof the finest solid)state amplifiers and preamplifiers, where they give very high

input impedance, high linearity, and very low noise. 2!(Ts are often extolled inpromotional literature for their 3tube)lie4 qualities.

To my nowledge, power 2!(Ts as such have not previously made it to themaret. "n the 56-&s, Sony and 1amaha offered a series of 2!(T poweramplifiers using their own semiconductors, but discontinued them after a fewyears. " have been told authoritatively that product reliability was an issue.7evertheless, the amplifiers remain highly regarded for their sonic performance.
http://www.passdiy.com/http://www.passdiy.com/


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>hy power 2!(T&s you as@ Aon&t we have enough transistors already@ >e dohave lots of choices in devices, but a power 2!(T brings some particularadvantages to the table that we don&t get elsewhere.

!irst off, for comparable devices at a given bias current, we see that the power2!(T has much less distortion. The original !? circuit was designed with a

'OS!(T, and comparing 3apples to apples4, the power 2!(T operated with one)fifth the distortion of the 'OS!(T.

That&s only the beginning. The 2!(T has much less distortion operated as if itwere a 'OS!(T, but it&s not a 'OS!(T, and it has characteristics which allow foreven better performance.

>hen we loo at the anode #or tube plate% voltage curves for devices we seewhat " would call the 3triode character4 and the 3pentode character4.

+bove is an example of the triode character. On a triode, for a given grid voltage,as the plate voltage rises the current goes up in an exponential fashion.

+bove is the pentode character. "n contrast to the triode, the current tends to levelout more with increasing voltage in what loos to be a roughly logarithmic fashion."t has a 3convex4 shape as compared to the triode&s 3concave4 shape. + distortionfree linear device such as a resistor would have a straight line.


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Since no gain device does a perfect :ob of approximating a resistor, varioustechniques have been developed over the years to mae them more linear.

+nother curve common to field)effect devices is the transfer curve of the deviceoperated at a constant voltage across the device, as seen below. >e notethat lie the triode curve, its shape is exponential in character.

Triode designers linearize the tube circuit by operating the voltageBcurrent loadline so as to get some cancellation between competing voltage #plate curve% andcurrent #transfer curve% distortions. This can be done with either single)ended orpush)pull circuits.

The way it wors is lie thisC !or a fixed Drid)*athode voltage, the current risesexponentially with 0late)*athode voltage. !or a given 0late)*athode voltage, thecurrent rises exponentially with Drid voltage. 9oth these curves loo similar onpaper. >hen you use the tube as an amplifier, these two curves oppose eachother while the gain is rising at higher voltage, it is also declining because of thereduced current, and these two effects can be made to cancel, resulting in morelinear performance.

Aesigners will choose the voltage and current operating conditions to exploit a3load line4 which is the region the tube traverses over its operating cycle. 9ycarefully positioning the load line, tube distortion can be dramatically reduced.#Enfortunately, this wors best for resistive loads, and not as well if the load is

reactive or has the wrong impedance value%

That&s all very nice for triode circuits, but gain devices with pentode)type curves,which include 'OS!(Ts as well as pentodes, cannot tae as much advantage ofthis technique. >hile they have the exponential transfer character, their anodevoltage curve does not cancel in the same manner. They must use differentapproaches, most commonly feedbac.


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9ut as you loo at the curves for these power 2!(Ts you can see a narrow regionbelow volts and - amps where the anode characteristic loos lie a triode, sothat a load line can be constructed which further linearizes the device.

"f we have a gain device that behaves a bit lie a triode, then it is natural to try itout in a popular triode amplifier topology. !or many aficionados that topology

would be single)ended *lass + operation. ?--9&s, F&s and their cousinsoperated single)ended and coupled to an output transformer have been held bymany as the low power musical standard. Single)(nded *lass + tube amplifiersare not very powerful, and their measurements are nothing to write home about,but there is no denying that they have strong musical appeal to a sizable segmentof the audiophile population.

Detting the most linear performance from these power 2!(Ts relies on a topologynown as cascode operation. The 2!(T is operated with a *ommon Date partner

device which contributes so little of its own character to the output that it doesn&trate as a gain stage per se, but it shields the actual gain device from the outputvoltage. *ascoding maes the voltage across the 2!(T constant, and so gainvariation due to fluctuations of Arain)Source voltage #the music% vanishes.*ascoding also dramatically reduces the need to charge and discharge the inputcapacitance of the 2!(T.

*ascoding also allows a relatively low power, low voltage power 2!(T to operatein a high voltage, high wattage environment with little dissipation. The cascodingdevice acts as an umbrella, taing the heat while leaving the 2!(T completely

controlling the current through the circuit.

!reezing the voltage across the 2!(T lowers the distortion by itself, but does notgive us the most optimal load line for the device. !or absolutely lowest possibledistortion, the 2!(T must see some voltage fluctuation across its Arain andSource pins. !or this purpose " developed a 3modulated cascode4 topology #youheard it here first%, which imposes a slight voltage variation across the 2!(T as afunction of the current through the device.

The advantage of this circuit is that the load line truly becomes a function of theoutput current, not the output voltage. This wors better than the classic triodeapproach because it accounts for reactive loads or loads which are not the correctresistance. The !? is set up so that the load line of the 2!(T can be ad:usted withtwo degrees of freedom, tweaing the circuit to the individual character of the2!(T. >ith this ad:ustment, the !? gives about -.-/ distortion at watt. This isa remarable achievement for a single)stage power amplifier ) the original $enamplifier had sixty times that figure at -.G/


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8ere is the simplified schematic of the !?. "t bears a superficial resemblance tothe $en ;ariations


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>hile much of the explanation here has used tubes as examples, " am notsuggesting that " wish to emulate tube amplifiers as such. " want to give a basisfor understanding the advantages of these new devices and the types of circuitsthat elicit the best from them. +t the same time, you will hear some similarities,partly because there are similarities to the measured performance, and partlybecause both types of design have a similar philosophy and minimalist discipline.

" am mindful of what happened when 'OS!(Ts first arrived, when mediocredesigners simply threw them into the same old bipolar circuits that they had copiedfrom the H*+ manual and the mareting departments trumpeted how tube)liethey were. They weren&t, and it has taen years for the reputation of 'OS!(Ts torecover and for designers to put them into circuits where they are mostappropriate. 0ower 2!(Ts are remarable new devices that offer some specificadvantages, and " fully expect them to tae their place in circuits which uniquelyaddress their qualities.

" note that it was :ust about this date ?- years ago when Hene 9esne and " satdown to listen to the final prototype of the Threshold =--+ #shown above%, whichwas my first commercial offering.

Times flies when you&re having fun.

"f you have questions, comments or problems, you can email me at

nelsonIpasslabs.com or www.firstwatt.com

Those of you eager to build your own instead of buying an !? can get informationon the $en ;ariations


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PART ': (etp #Pro)a)l" $o (hold Read This Part&

The initial setup of the amplifier is very straight)forward. 0lace the amplifier in awell)ventilated location, as it draws nearly F-- watts during operation and requiresas much opportunity to cool itself as possible. 1ou should be able to put yourhands on the heat sin during operation. "f you canKt do this for seconds or so,

they need more ventilation.

On the front panel there are two blue J(A lights, one for each channel, indicatingregulated power to the channel. "f the light is on, the power supply regulator forthat channel is delivering voltage. On the rear panel you will find pairs of H*+inputs, speaer outputs, a fuse holder, an +* power receptacle, and onBoff switchand a label.

The label will indicate a serial number and also what +* line voltage the amplifier

is set for. "f the voltage is F- ;+*, then the fuse value will be a ?+D slow blowfuse rated at F. amps. "f the voltage is FL- ;+*, then the fuse will be rated at.F amps. Ao not substitute a larger value fuse. *ontact !irst >att if you haveany questions.

"Km assuming that you now how to attach the speaer cables to the way outputconnectors provided. " recommend that you mae all the connections with theamplifier power switch in the O!! position, but you will not damage the amplifiercircuit by accidentally shorting the output or overdriving the input ) my concern ismore for the safety of any fragile loudspeaer driver you might be using.

7ote that the #M% red terminals of the outputs are a ground potential, and that the#% blac terminals are live. This is important to remember if you decide to bridgethe channels or if you are using the output of the amplifier to drive an activesubwoofer&s input.

>ith everything connected up and the source equipment powered up first, you canproceed to turn on the power switch to the amplifier. Turn)on and turn)off thumpsand noise are small in this amplifier, and should not present any hazard to delicate

drivers. "t taes about minute for the regulators to come up to full voltage, sodon&t expect full output power for that time.

+t this point you should be able to listen to music. This amplifier has less gainthan most, but at watts rating, it should not need it. "f you need to turn the gainup on your preamp, then do so. "f you canKt get enough gain, then you areprobably using either the wrong speaer or the wrong amplifier. Tal to yourdealer if this is the case.


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The power supply of the amplifier is isolated from the chassis and +* earthground by a thermistor which connects the circuit ground to the chassis and earthground. This helps to prevent ground loops, but the thermistor stands by toconduct +* line voltage to ground until the fuse blows in case of transformer orother such failure.

The input impedance is the "8! standard of - Nohms, and the input capacitanceis very low, so you should find it easy to drive with tube equipment if you lie. "twould be a pitiful source that would not be able to deliver FL- micro)amps requiredto clip this amplifier. The amplifier is largely indifferent to the source impedance ofyour preamp, so a high source impedance is not a problem.

The !? has enough damping factor #=% to wor well with loudspeaers that matewell with tube amplifiers in general, and it delivers good performance into L and Gohms also see the distortion curves at the bac of this manual. "t is designed

around relatively high efficiency speaers and it particularly shines with those thathave 5- d9Bwatt sensitivity or greater, but you can hoo it up to anything you lie,as long as you ad:ust your expectations as to how loud it will play.

The amplifier requires about hour of operation to reach normal operatingtemperature, and this warm)up time is appropriate for the most critical listening,but is not otherwise an issue. The amplifier&s final ad:ustments were been madeafter an hour, but the performance difference between minutes and G- minutesis marginal.

" do not personally see a reason to run the amplifier all the time, but you can dothat if you want to. The power supply capacitors are liely to last about yearsor so, and while they will slowly dry out :ust sitting there, they will have a shorterlife span with the amplifier running constantly. +lso, at F-- watts it maeseconomic sense to shut the amplifier off if you aren&t planning on using it for therest of the day.

+gain, the heat sins on this amplifier run fairly hot, and you want to mae surethat they get adequate ventilation. They will run at around F degrees *. above

the ambient temperature, which puts them around - degrees in the averagelistening room. +t this temperature you should be able to put your hand on themfor about to - seconds or so.

"f you have any questions, contact !irst >att. " answer all questions, even if theanswer is no.

www.!irstwatt.com nelsonIpasslabs.com
http://www.firstwatt.com/mailto:[email protected]://www.firstwatt.com/mailto:[email protected]


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PART 3: #A)soltel" *st Read This Part&

Now the following is for "or prote!tion +

Ao not defeat the +* line (arth ground connection on theamplifier power cord. "t provides an extra barrier to preventpotential shoc hazard.

Ao not replace the fuse with a type other than specified.

Ao not operate the amplifier outside in the weather, or in and

around water or anything resembling water. "f you spill a drinin the amplifier or if your dogBcatBchild urinates on it, turn it offimmediately, unplug it, and do not operate it until cleaned by aqualified technician.

"f something gets loose or rattles around inside or smells funny,or if you can&t touch the heat sins for seconds or so, then turnit off, unplug it from the wall, and contact !irst >att.

There are no user serviceable parts inside. Ao not open theamplifier, and if you do anyway, don&t operate it with the coveroff. There are hazardous voltages inside. "f you need tochange the operating +* voltage, contact !irst >att.

"f you have a problem, contact !irst >att. >e are much happier

helping you solve problems so that we can be certain that it&sdone properly. "f you are far away and don&t want to ship theproduct for repair, we will assist your technician with informationand parts.

*ontactC www.!irstwatt.com nelsonIpasslabs.com
http://www.firstwatt.com/mailto:[email protected]://www.firstwatt.com/mailto:[email protected]


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0+HT LC Technical 0oop #Optional 9athroom Heading%

(mmar" of the nominal spe!ifi!ations:

'easured at F- ; +* with a F ohm source and an = ohm loadC

Aistortion I watt .-/ I N8z, typical .--/

"nput "mpedance - Nohm

"nput Sensitivity G- m; for watt, = ohms

Aamping !actor G

Output power = ohms watts I -.F/ T8A, N8z

Output 0ower G ohms - watts I -./ T8A, N8z

Output 0ower L ohms - watts I / T8A, N8z

Dain F.6 d9 #MB) -.F d9%

'aximum unclipped output MB)? volts, MB) F. amps

!requency response ) ? d9 I . 8z, 6 N8z

7oise -- u; unweighted, F-)F- N8z

0ower consumption F-- watts

!use ?+D slow blow type, F. +mp for F-;+*.F +mp for FL- ;+*

Warrant"C 0arts and labor for ? years, not covering shipping costs or

consequential damages.

*opyright F-- Deneral +mplifier

Deneral +mplifier "nc.0O 9O 6G-6H(7O 7; =5-)6G-6


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A"STOHT"O7 ;S >+TTS I = O8'S, N8$

A"STOHT"O7 ;S >+TTS I L O8'S, N8$


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A"STOHT"O7 ;S >+TTS I G O8'S, N8$

A"STOHT"O7 ;S !H(PE(7*1 I = O8'S, >+TT

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First Watt F3