Project 5 MiniAmp p78-82

8/12/2019 Project 5 MiniAmp p78-82

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(a)

MINlAMP

Deflnltlon: A portable, low-power audio amp capable

ot driving a loudspeaker o moderate volume levels.

Figure 5-29

(a) I already have a couple of smalI practice amps,

so I built this one in a plastic enclosure for mini-

mum weight. It sounds pretty good, although

you 'II Ket better hass response with a wood cabinet.

(b) A closeup of the loaded circuit board.

(b)

Background:

Large amplifiers are not needed or ali occasions:mini-

amps will suffice or practicing and many recordingap-

plications. They are usually physically smaliand battery

powered or portability.

Level of Difficulty: Beginner o intermediate

ConstructionTips

Becausehis circuit hand1es ore power han other book

projects,certainprecautions houldbe folIowed.

Shield the wires going from the input jacks to both

controls, and from the controls to pad A and B. Keep

output leads physically separated rom the input wires

and the area of the board where he input wires termi-

nate.

For best results,build the miniamp nto its own case

and give it its own power supply. I recommendusinga

battery holder capableof holding eight C cells; for

smallestsize, useeight AA cells. Either of thesecom-

binationsproduces12 V.

The lA706 usesanonstandardpackage. he it tle dot

on the top of most ICs that normally indicatespin l is

Features

. Single IC design,no output transformer required

. Switch-seIectabIe peaker or headphoneoutputs

. Operates lam + 2V DC battery pack or car bat.

tery

. ReasonabIerequencyresponse nd distortion

characteristics

. Suitable or recordingwark:

-SmalI size alIows for unusual miking schemes

(seeUsing he Miniamp in the Studio)

-Simplicity of repair and maintenance

-Low output power minimizes Ieakagebetween

instruments,simplifiesbaffling requirements

-May be overdriven o simuIate arge amp sound

without Iargeamp power evels

-Dual, individualIy mixable inputs for jarnming

with anotherplayer


2/5

Figure 5-30

Top view o the ,tL4706. A dot on the bottom o

the C indicatespin 14.

A smalI speakerbaffIe makesan excellent enclosure

for the miniamp and batteries.Stuff with acoustical ad-

ding for besthass esponse, ut keep he areaaround he

IC heat sink elearofpadding.

If you expect to play at the loudest possiblevolume

level of which this amp s capable or extendedpeods

of time, glue a beat sink to the top of ICI with a ther-

mally conductive silicone glue (available at hardware

stores).Figure 5-34 shows he heat sink attached o the

IC.

1

2

3

4

5

6

7

Figure 5-34

Heat sink mounted on top o[ the IJA 706.

on the bottom of the jlA706 and ndicatespin 14. The

top of the IC bas a metal strip that aids n dissipating

beat rom the IC. (See igure 5-30).

The volume of the miniamp is highly dependenton

the speakert drives.Paradoxically, igh-cost/high-quali-

ty car radio speakers ill usually produce ar lessvolume

than the inexpensivespeakers ou find in table radios

and the like. Efficient musical nstrument speakers8"

to 15 ) typically provide the loudest sound pressure

levels.

Figure 5-31

Artwork for the foil side of the circuit board,

shown1 to 1.

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Using he Miniamp, Genera nstructions

. Che k a11 onnections horoughly before applying

power. Plug one or twa instruments nto the inputs, se-

lect speakeror headphoneoutput, then adjust volume

controls o suit.

. If you beaTexcessive oise or squealing,or if ICl

getsextremelyhot immediatelyartel tum-on, shut down

power and determine he sourceof the difficulty. Other-

wise,play away.

. You may power twa setsof headphones ithout

problems; however, hese should be of the samemake

and model.

Using he Miniamp n the Studio

. There should be no hum (except for stray hum

picked up through the instrument being amplified),

since he amp s battery powered.

. The smalI size allows maRYunusual mic place-

ments o alter the ton al quality of the miniamp'ssound.

Hereale some . .

- Boost bossby miking close o speaker.Cut boss y

movingmic further away.

-Create phase changesby placing amp on floor;

hang mic about 20 cm (8 ) above he floor, pointing

at the floor, about 8-40 cm (4-16 ) from the amp.

Moving the height of the mic in relation to the floor

changeshe soundby creatingphase hifts.

-Point amp at wall; also point mic at wall to pick up

the retlectedamp sound.

-Reduce treble by plac ng a piece of thin cardboard

between he speaker nd mic.

-Boost midrange, and create resonances, y placing

amp and mic inside a large cardboardbox willi the

mic pointing away from a direct ine willi the speaker.

Change ize of box, cut holes n box, or changemic/

amp positioning o vary the effect.

Figure 5-32

Component layout lor the miniamp.

~

C11

A4.

.R1

@~

e

@)

~

C6

C2( )

C3 c==)

-' -RS-

C 1 c:::>

.A5

'~n'

R3-

.P1.

R2

s

GBADCH+

83


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In Case f DifficuIty

. Distortion: Back orf on input controls. Check

rower supply voltage while playing through amp; if it

drops much below 9V or gO, he batteriesneed o be re-

placed.

. Low volume: Usemore efficient speaker.

. Oscillations, excessivenoise: Check lead layout

and shielding, eferring o ConstructionTips section.

. ICl overheats r bums out: Increase eat sinking.

. Resonances,uzzes:Checkenclosure or tightness.

Make SUTell joints are glued together well and are pre-

ferably caulked.

Modifications

. ReducingClI to 250jlF trades orf hass esponse

for longer battery life. This hasa negigible ffect on the

sound, since most smalI speakerswill not respondeffi-

ciently to low hass requencies nyway.

. For more power, you may use 10 C celIs con-

nected n series o create a + 15V supply. In this case,

a heat sink for ICI will be mandatory.

. Lowering the values of R2 and R3 (typically to

lOn) trades off more headphonevolume for slightly

more noise. RaisingR2 and R3 (typically to 56 or B2n)

tradesoff lessvolume or lessnoise.

Specifications taken with 12V battery pack)

Quiescent urrent consumption: ypically 15mA

Typical maximum current consumptionwhile playing:

175mA@800mWout

Frequency esponse:2dB, 100Hz-10kHz

Maximum nput before clipping @ 1kHz: 250 mV pk-pk

Maximum power nto SS 2@kHz: approximately W

Figure 5-33

Miniamp schematic

R4


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How it Works

The biggest radeoff in a miniamp is battery life vs. output power; mOle

power means decreased attery life. The chosenoutput power, approxi-

mately l W, seems ike the best compromise between output level and

power consumption.

ICI is an audio power op amp, and is similar to a regular op amp ex-

cept that it has been optimized to provide arge amountsof output cur-

rent. Signals ouple n flam the iwo inputs via R6/R7 and C4/CS.CI adds

stability and discourages adio frequency interference. RS, located be-

tweeDpin 8 of ICI and ground, sets he overalloperating evel of the amp.

The variousother capacitorsassociated ith the op amp provide requency

shapingand/oTbypassing o improve stabilityand power supply rejection.

The output couples nto the speakerhrough ClI; C6 s, again,designed

to promote stability (power ampsdo not like feeding nductive oadssuch

as speakers).Switch SI allows you to chooseheadphones r speaker, ut

not both. R2 and R3 cut down on the signalgoing o the phones n order

to attenuate he noise coming out of the amp a ittle bit, and o bring the

output level down to something easonableor OUT ars.Use 8- to 16-Q

headphonesor best results, although most any type will wark satisfacto-

rily.

Project No. S PARTSLIST

Resistors alI are 1/4W, 10% olerance,exceptasnoted)

Rl-R3 33n

R4 4?n

R5 270n

R6, R7 100k

R8 470k

R9 1M audio or linear taper pot-controls channelllevel

RIO 1M audio or linear taper pot-controls channe121evel

Capacitors rated at ~2V or moce or + 12V supply exceptasnoted)

CI

C2

C3

C4

C7

C8

CI

CI

20pF, ceramicdisc

390pF (polystyrenepreferred,disc acceptable)

O.OlJjF mylar preferred,disc acceptable)

O.22JjF,mylar or ceramic

l OOJjFelectrolytic or tantalum

1OOJjF, lectrolytic or tantalum, 6V or more

220JjF,electrolytic

1OOOJjF, lectroytic

Semiconductors

FairchildJJ.A706

r TBA641B audio amplifier

MechanicalParts

J1,J2

J3,J4

SI

Misc.

Opencircuit 1/4 mono phone ack

Opencircuit 1/4 stereophone ack

SPDT oggleswitch-selectsspeaker/headphones

Case,knobs, 14-pin IC socket, circuit board, 12V bat

tery pack, solder,wiTe, oudspeaker, tc.

85

-C6

,C9

o

l

Documents

Project 5 MiniAmp p78-82