gain control ic for audio signal processing hr 0777

8/18/2019 Gain Control IC for Audio Signal Processing HR 0777

1/8

gain control 1C

for audio signal processing

Asamulti-purposed 1C,

theNE570analogcompandor

fulfillsmany

audioprocessingneeds

Tw o ICs recentl y i n troduced by S igneti cs , the

NE570and NE571, permitthedesignof efficient and

prac ti ca l aud io -s igna l con tro l func tions w i th a

minimum overall parts count. These devices are

primarily designed to act ascompandors; the com

plementary processes of compression and expan

s i o n . 1^ They are both dual-channel ICs and either

portion can be used individually as a compandor.

However, asw ill beseeninthisarticletheyarealso

well suited to a variety of other tasks useful to the

amateur.

basic device operat ion

Each channel of the 570and 571 consists of the

functional componentsshown in f ig. 1A. Packaged

ina 16-pin DIP, the only items common to the two

signal channels arethe power supply, ground con

nections,andaninternal1.8voltbiasregulator.

The three principal components of each section

are a AG cell, fu ll-wave rect if ier, and an output

amplifier.TheAGcellisusedtocontrolthegainover

a rangegreaterthan 80 dB. The controlvoltagefor

this cell is generated by rectifying an input signal

(RECTIN). Thefinal output is thendeveloped bythe

bufferedo utpu tamplifierfrom thescaledsignalcu r

rent supplied by the AG cell. The 570 and 571 are

identicalelectrically, butthe570isselectedfo rlower

inherent d istor tion and a h igher supply voltagerange.

TheAGcell,asshowninf ig.1B, consistsofanop

amp, A1, andtransistorpairsQ1-Q2andQ3-Q4. The

input signal is first converted by R2 into a current

thatdrives A1. Thefeedback forthisop amp is via

thetransistorpair Q1-Q2. Therefore, theam ount of

currentinthispairisthesameasthecurrentthrough

R2. In addition todriving Q1-Q2, theopamp isalso

connected to Q3-Q4. Unlike Q1-Q2, this transistor

pair does not have a constant-current source. By

scaling the Q3-Q4emittercurrent, their outpu t is a

linear product of the input signal from A1 and thescaled current. This circuit is a linearized transcon

ductance multiplier4-7 which cancels the inherent

non-linearity and temperature sensitivity of the dif

ferential pairs, greatly enhancing the usefulness of

thisgain-controltechnique.

The rec tif ie r port ion consis ts o f op amp, A2,

class-B transistors Q5-Q6, a pnp current mirror Q7,

and an npn cur rent mir ror Q9. When rectifying a

signal at the RECT IN terminal, Q5and Q6 produce

pulses of current proportional to the positive and

negative input signal swings. The output currentof

Q6 is used directly, while theQ5cu rrent is mirrored

byQ7.Thus, thedrivetoQ9 isa positivegoing,full-

wave rectified pulsating dc. These pulsesarefiltered

byanexternalsmoothing capacitor attached to the

C r e c t terminal.

The output stage is a s imple inverting op amp

similarin performance toa741. Various optionsare

possible byuse ofeitherR3, externalinput, orfeed

backresistors. Theoverallcircu itgainisun ity (AG IN

to OUT) , with R3 connected as a feedback resistor

and70/iA rectifiercurrentintoQ9.

In addition, the T HDT RIM terminalallowsasmall

offset to be introduced into the AG cell to null its

distortion. The two inputop amps (A1 and A2) are

connected tothe internal1.8vo ltregulator. Eachop-

amp input should be capacitively coupled while the

inputimpedance isdetermined by R2or R1, respec

tively. Circuitoperation isverystableand immune to

power-supply var iations. A single supply voltage

from +6to +1 8volts(571)or +6to +24volts(570)

can be used, though the following applicationswil l

usea +15vo ltsupply.

basic com pando r c i rcui ts

The570and571 canbequitesimp lyconnectedfo r

theirbasicfunctions ofexpansion and compression,

asillustratedin fig.2. Thesecircuitswillnotbedealt

wi th in great detail because most amateurs wi ll

probably be more interested in some of the other

uses. Also, compandoroperationiscovered indetail

inotherliterature.1-3

Thegainthroughtheexpandorshownin f ig. 2A is

1.43 VIN, where VIN is the average input voltage.

By Walter G. Jung

ju ly 1977 E3 47


2/8

8 THD TRIM 13V+

570/571

(A SECTION)

J 5 (-) IN

j 6 R3

570/571

(BSECTION)

> 12(- ) IN

i II R3

f ig . 1 . Funct iona l d iagramo f the S ign et icsNE570/571.A s im pl i f ied schem at ic d iagramo f the device is shown in B. C ons tant cu r rent sources l -j

and I2 feedtransistorpairQ1-Q2.

The 570/571 circuit constants are set up such that

unitygainoccursatanrmsinputlevelof0.775volts,

or0 dBm in 600-ohm systems. The Cin and Coare

coupling capacitors, chosen for the desired low-

frequency rolloff. Cr ec t is selected for the desired

timeconstant(10ms)inconjunctionwiththeinternal

10-kilohmresistor(R5).

Resistors Ra , Rb , Rc and Cb are not essential to

basic operation, but are desirable. Rr furnishes

short-circuitprotectionfortheoutputandcapacitive

loadbuffering,whileRa andRcpolarizeCin andCa -

Cr isa powersupply bypass, typicallyanaluminum

electrolytic.

The compressorconfiguration in f ig . 2B also has

unity gain at 0.775 vol t (rms) input , but , a com

p lementary in /ou t characte ris tic . The main d i f

ferenceinthiscircuitisthattheAGcellisconnected

asafeedbackimpedanceviaCf ,andtheinputisap

pliedtoR3throughCin - Biasfortheo utputstageis

set up by the RC-decoupling ne twork , w i th the

valuesshownappropriatefor15-voltpowersupply.

In general, the OUTterminal should be biased to

one-halfthesupply voltage. Useo fa570or571 asa

compandor is not limited to the gains shown, but

maybeextendedtootherrangesbyuseofadditional

components.

t r imm ing techniques

Deviceperformancecanbeenhancedbyjudicious

trimm ing, as shown in f ig.3. Each technique is op

48 0 ju l y1977


3/8

tional, and can be applied in anycombinationwhen

the highestperformance isdesired. Themost useful

ofthethreemethodsisprobablytheT HD trim,which

minimizes the gain cell harmonic distortion. In this

case, a smallvoltage (0to 3volts) is usedto injecta

currentintotheT HDT RIM terminalthroughthe100k

resistor. By biasing the rectifier terminal as shown,

the inherent currentflow in the rectifieris compen

satedforandpermitsbetterlow-levelsignaltracking.Typically, thegain-controlsignalto A1 shouldnotbe

reflected in the output. The contro l feedthrough

trimmer will minimize that signal during periods of

lowinputvoltage.

appl icat ions

An interest ing and versat ile group of circuits, the

gated orswitched-mode amplifier, can be builtfrom

the 570/571. With thedevice controlled by external

logicappliedtothe RECT in input,theongainisnor

mallysettoanyvalueandtheo ff attenuationcanbe

in excess of 80 dB. Use of the 570 or 571 is ad

vantageousinthatallportionsofthefunctioncanbeperformedentirelywithinthe IC. Further, theo n/o ff

transition timescanbesetto a valuedetermined by

thetimeconstantfrom C r e c t -

Fig. 4A is a logic controlled amplifier configured

for a HIGH input to be on, and LOWoff. When the

control input is HIGH, CR1 is off and the current

V+

lOpF

rh

f ig . 2 . Schemat ic d iagramof the devicesconnectedasanexpander ,

A , an d a co m pre ss o r, B. Th e vo ltage ga in th ro ug h th e ex pan de r is

1 . 43 * V j j y , w h i le f o r t h e c o m pr es s or i t i s \ / 0 .7 /V j js j . - V j n is t h e

average input vo l tage.

developed by Rgain f lows into the rect if ier input,

which turnson theAG cell allowing thesignal tobe

amplified. Rgain can be selected for the desired on

stategain,whichisunitywitharectifiercurrentof70

nA. R1 and R3 also effect device gain, but R3 is

selected basicallyfor an optimum ou tput bias of7.5

Vdc. R1 canalsobeadjustedfo rgain, but asshown

thevalueallowsupto3voltsrmsinput/outputsignal

levels.

f ig . 3. B y a p p ly in g t h e d i ff e re n t t r im m i n g m e t h o d s , d i s t o rt io n

t h ro u g h t h e 5 70 /5 71 c an b e r e d u ce d . T h o u g h e a c h m e t h o d is o p

t iona l , theycanbeappl ied in anycom binat ion.

As can be seen in the contro l ch aracte ris tic s

plotted in f ig. 4C, the gain is unity (or its nominal

value, ifchosenotherwise)forco ntrol inputsgreater

than 3volts. Switching is quiteab rupt, with full at

tenuationbeingachievedatlevelslessthan 1.5volts.

This narrow t ransit ion width and the nominal dc

center of 1.8voltsallowsdirectcon trolfrom CMOS,

T T L, D T L, or o ther pos it ive log ic . The u lt imate

voltage ofthe HIGH stateis non-critical, due to the

100-voltratingofCR1. Unfortunate ly, thisc ircuit hasone inherentweak point. Gain issensitive to supply

voltage due to the connection of R g a i n - Thus, the

supply voltage should be stable whi le choosing

R g a i n for70/xAintotheRECT IN terminal.

A companion circui t w ith complem en tary contro l

characteristics is shown inf ig . 4B. In thiscase, the

gainisdeterminedbythecurrentdevelopedthrough

R g a i n in con junction w i th the in te rna l vo ltage

reference(1.8V). With a lowc ontrol input, thenor

mal currentwil l f low outthrough R g a i n - When the

control signal is high, CR1 is forward biased, inter

rupting the currentflow. Therefore, the output will

beattenuatedsinceQ3-Q4havebeenturnedo ff.

Both circuits can be tai lored for specific on-off

transitiontimesbyselectionof C r e c t - Thetimecon

stantissimply 10k-CREc r HOkilohmsistheinternal

resistor). Thus, the audible switching effect can be

smoothed,eliminatingthetransientsproducedbyan

asynchronousfast switch. The C r e c t value shown

yieldsnominaltimesof5milliseconds.

UseofC r e c t inasw itchedamplifierofthistypeis

ju ly 1977 [(3 49


4/8

+15V

INPUT

f ig . 4 .The log i c con t ro l led amp l i f ie r s can be con f igu red to p rov ide

an o u t p u t w i t h e it h er a H IG H o r L OW i n p u t. T h is a ll ow s th e

a m p l if ie r s t o b e i n te r fa c e d w i t h m a n y d i f f e r e n t l o g ic t y pe s , T T L,

CMOS, DTL , e tc . The res i s to r Rq a IN shou ld be se lec ted to p rov ide

70 fiA a t un i ty ga in .Eachamp l i f ie r hasanon /o f f t ime determined by

the t ime constan t o f C r^C Tw i th in terna l 10k resistor .

optional and not absolutely necessary. However, .to

minimize noise pickup some capacitance wi ll be

founduseful. Also, theultimateo ff stateattenuation

willbelimitedtoabout60dBduetotheinternalbiascurrent. Thisfeedthrough errorcan beeliminatedby

connect ing a 1 megohm resistor f rom C r e c t to

ground to bleed awaythe error current. This allows

attenuationof80dBormore.

Fig.5illustratestwosectionsofa571 combinedas

a two-input multiplexer, for FSK orother uses. This

circuitoperation issimilartothe others, but is biased

and switched in a simpler manner. Gain of eachon

channelisunity,asdeterminedbyRgain-Theoutput

ofthe B channelAG cell issum med with channel A

by connecting the ( - ) IN terminals of the A and B

sections.Therespective channelsaregated o ff bya

low control logic input, which clamps the rectif iercurrent,switchingtheAGcelloff. Forfskoralternate

channel use, the CONTROLAand CONTROLBsignals

shou ld be comp lemen ta ry . T hus, the inpu t is

"instantaneously” switched between the A and B

inputs.

Control signal suppression can be optimized with

the CHOPPER NULL contro l, wh ich trims the control

signalc omponen tin theo utput. Suppression is bet

terthan60dBa ftertrimm ing. Responsetimeisquite

fast, and is actually l imited by the slew rate of the

output op amp rather than the AG cell itself . This

makes the sw itch ing in te rval a function o f the

signal's peak amplitude. For instance with a 4-voltpeak amplitude signal the 0.5 V//xs slew rate wil l

f ig . 5. Byprovid ingco mp leme ntarycon tro l s igna ls, the FSKgenera

t o r w i l l s w i t c h b e t w e en t h e t w o s ig n a l i n pu t s . T h e o u t p u t s , w h e n

ON,are summ ed throu gh the f i rs t ope rat iona l am pl i f ie r .

50 E3 j u l y 1977


5/8

al low switching in 8 ^s; lower amplitudes wi ll be

proportionally faster. Although the circuit is touted

as a multiplexer, it can also be used as a summing

switch,withbothsignalsonatanygiveninstant.

automat ic level control

Automatic level control isa relative ly common re

qu ir emen t in aud io s ignal p rocess ing . A 571

automatic levelcontrolc ircuitcan provideconstant,

h igh percentage modu la tion w i th vary ing inpu t

levels, yet without danger of overload i f properly

handled.8 This circuit ( f ig. 6) is adapted from the

570/571 data sheet. There is one additional feature

which may be useful, however: an optional resistor

allows thethreshold of level regulation to be varied.

IfRxisleftopen,thecircuitwillhaveitswidestrange

of gain control. As this resistor value is lowered, a

larger input signal is required for full output. The

general effect, for var ious Rx values, is shown in

f ig.6B.

Since the 570/571 device operates on the prin

ciplesofaveragelevelde tection , itwilleasilysaturate

on large crestfactor inputlevelssuch asspeech, or

large transient envelope changes. This condition is

highly undesirable, but, fortunatelyitse ffectcan be

negatedquitesimply. Sincethe resultof overshoots

are peak-to-peak amplitudes in excess of the regu

lated output level, it follows that appropriate peak-

levelclipping can effectivelycontrol theovershoots.

Inthiscase, thermsoutpu tamplitude is0.775voltor

2.2 volts p-p. This part icular level is conveniently

clippedw ith a pairo f reverseparalleled LEDs, which

willlimitto3.2vo ltsp-p. TheLEDscanbeconnected

asshown in f ig. 6with a series resistorRy, wh ich is

usedtoregulatetheclippedamplitude.

This clipping technique, while not a requisite part

of the automatic level control, greatly enhances its

regulationw ith nonsinusoidal signals. Useofthis cir

cuit with speech inputs wil l necessitatethe diodes,

which willtypicallybeclipping a good portion ofthe

time. This, of course, adds audibledistortion to the

output.Therefore,ausefulitemwiththeALC/clipper

isaspeechfilterto removethe superfluous highand

low frequencies. Thefilterw illalsogreatly attenuate

harmoniccomponentsgeneratedbyclipping.

The circuit in f ig. 6C is a bandpass speech filter

which usesa single IC.Thecircuitissimply a pair of

cascaded Sallen and Key10 highpass filters (3-pole

Bessel type) . The Bessel response is one of the

poorest intermso fcu toffsharpness, but goodfrom

500 IOOO 5000

FREQUENCY IN CYCLES PER SECOND

-4 0 -3 0 -2 0

INPUT,RELATIVE dB

f i g . 6 . Schem a t i c d iag ram o f an NE570 connec ted as an au tom a t i c l eve l con t ro l (A ). The speech f i l t e r (C) i s added to r emove the undes i r ed com pon

createdby thepeakc l ipp ing. Thea t tenuat ion curve (D) , thoug hno t sharp, providesexce l lent re ject ion o f the sharp pu lsesp ikesgeneratedby thec l ipp i

ju ly 1977 C2 51


6/8

a pulse response standpoint. This feature is impor

tantto minimize amplitude overshoots which could

occurwithseverelyclippedinputs.

A comm on 1458 (dual 741) op amp is used with

nearest5 per cent component values for the fil ter

elements. Iflow-powe roperation isdesired, the1458

canbe replaced directlyw ith a 358. Ifa 358 is used,

10 kilohm resistors should be added from each outputterminaltocom mon. W ith unitygain, thecircuit

candriveloadimpedancesgreaterthan10kilohms.

Oneveryeffective usefor the570and571 deviceis

an amplitude-regulated RC sine wave oscil lator.

Typically, such circuits use a Wien bridge or other

frequency-selective RC network, with some form of

amplitudestabilization to maintain cons tantand cor

re ct lo op g ain , and also t o g ua ra nte e o u tp u t

waveformpurity. A 570or571 is nearlyoptimum for

this type of circuit because it contains the required

functions of ampli fier, recti fier, and gain-control

circuits.

Two types of sine wave oscil lators are shown inf ig. 7. The oscillator circuit ( f ig. 7A) based on the

WiennetworkisformedbythecombinationofR1-C1

andR2-C2.Thisnetworkisplacedaroundtheoutput

amplifier ofsection A, which effectively makes it a

bandpassamplifierresonantat

/ = 72-rrRC

With equal values o f R and C, the inpu t/ou tpu t

voltageratiois2to1.

Tooriginateandsustainoscillations,the571Bsec

tion is used as an inverting amplifierw ith a nominal

gain of2. A slightlygreater initial gain is establishedbythecombinationofR6,R7,and R8,wh ichensures

startup.TheBsectionAGcellisconnectedasacom

pressor, which regulates this stage's gain at the

precisevaluerequiredtomaintainundistorted,stable

amplitudeoscillations.

Thereare two main steps taken to enhanceflex

ibilityofthecircuit.Aseparatedcfeedbackpath(R3,

R4, C4)isusedaround theA stage, to removevalue

restrictions on R2 due to bias considerations. This

a llows R2 (R1) to range f rom 10k to 1 megohm

without a major performance compromise. C1 and

C2 have an even greater range, from 1 /*F down to

100 pF. To minimize errordue to strays, the lowestvalue should be used. W ith the values shown, the

circuitis capable of reasonably low harmonic distor

t ion. For example, 0.03 per cent d istort ion was

measured at 1.6 kHz and THD (Total Harmonic

Distortion)can generally be heldbelow0.1 percent.

This will vary according to the specific frequency,

and the selected impedance of the Wien network.

The lowvalueof distortion isdueto thelightdegree

ofAGcellregulation.

+I5V

+I5V

f i g . 7 . The NE570/571 can be connec ted as a s ine -wave osc i ll a to r .

TheWien br idge type osci l la tor is sho wn in A. For R= R1=R2and

C= C1=C2 , the ope ra t ing f r equency i s 1 /27 rRC. Res is to r R shou ld

be l im i tedbetw een10kand1 meg ohm w i th Cbetwe en 1000pFand

1/xF. The no rma l f r equency range can be va r ied f r om 10 Hz to 10

kHz. Thepha se-sh i f t osci l la tor should beused to generate d iscre te

f requencieson ly. Dependingupon these lect ion o f par ts, the output

frequ enc yw il l be1/27rRCV3.

The circuit wil l operate as shown over the range

from 10 Hzto 10 kHz. Below10 Hz comp onentsize

becomesimpractical,andabove 10kHz slewlimiting

in theoutpu tamplifiercauses distortion to rise. The

circuit is useful as a fixed frequency oscil lator, butcan also be tuned if a matched dual pot is available

for R1-R2. Outputamplitudeissetby R6, and is op

timum at1.5 volts rms output,from section A. Ifa

higher output level is needed, section B outp ut can

alsobeused,at3volts.

The circuit of f ig . 7A maybe undulycom plexfor

some uses, so an a lte rnate and much s imp le r

sinusoidal oscillator is shown in f ig. 7B. Thiscircuit

is a form of phase-shif t osci llator, similar to that

52 Q3 july 1977


7/8

describedbyTobey,Graeme, andHuelsman.9A571

is well suited for a phase-shift oscillator because it

contains the necessaryinverting amplifierto sustain

oscilla tion. In thecircuitshow n, C1, C2, andC3are

the t iming capacitors, while R1 and R2 are the

resistorsfor the phase-shift network. R3 mustbeat

least 12 times the R1-R2 value for adequate loop

gain. AGC is provided by using the AG cell as acompressor.

Thiscircu itisnotsuitablefo rtunable use. Itshould

only be used as a spot frequency oscil la tor, by

varyingC1,C2andC3. ThisisbecauseR1 andR2are

related, bythedesign, to R3; in thisspecific caseR3

cannot bevariable becauseit is used tosettheo ut

putdcbiaspoint.

Although it uses a simple design, this cir cu it pro

ducesexcellentresults.A tthefrequency indicated,a

laboratorytes tindicatedaTHDof 0.01 per centat 3

voltso utput, which is remarkable in viewo f the cir

cuit's simplicity. Totake full advantage ofthis per

formance, an output buffer may be useful; for thisyou could simply use the remaining channel as a

simpleunitygaininverter.

conclusions

Thisdiscusssion hascoveredafe w usesfora new

and interesting chip. In the course of this article's

preparation several other potential uses suggested

themselves, such as phase comparators, phase-

locked loops, voltage-tuned oscillators, and others.

Unfortunately, space and time restrictions did not

permittheircompleteexamination.

references1.C.Todd, "A MonolithicAnalogCom pandor,"IEEE Journal o f

Solid State Circuits, VolumeSC-11, number6, December, 1976,

page754.

2. C. Todd, "The MonolithicCompandor— A HighPerformance

Gain Control Integrated Circuit," Aud io Eng inee ring So ciety

Preprint, number1100, May,1976.

3. W. G. Jung, C. Todd, "Operationand Usesfor the570/571 IC

CompandorChip," The Audio Am ateur, 4/1976,page3.

4. B. Gilbert , "A Precise Four-Quadrant Mult ip lier with Sub

nanosecond Response,” IEEE Journ al o f So lid State Circuits,

December, 1968,page365.

5.W . G.Jung, "GetGain Control of80 to 100 dB,"Electronics

Design,June21, 1974,page94.

6. W. G.Jung, "ICOpAmpC ookbook," HowardW . SamsCo.,

1974,page251 and451.

7. W. G. Jung, "Application of the Two-Quadrant Amplif ier

Multiplier in Audio Signal Processing," Journal of the AES, Volume23,num ber3,April, 1975,page207.

8.J. Fisk, W1HR, "Novel Audio Speech ProcessingTechn ique,"

ham radio,June, 1976,page30.

9. Tobey, Graeme, Huelsman, "Operational Amplifiers; Design

andApp lications," McGraw-Hill, 1971,page391.

10. R. P. Sallen, E. L. Key, "Practical Method of Designing RC

Act ive F ilte rs," IRETransactions, VolumeCT-2, 1955,page74.

ham radio


8/8

describedbyTobey,Graeme, andHuelsman.9A571

is well suited for a phase-shift oscillator because it

contains the necessaryinverting amplifierto sustain

oscilla tion. In thecircuitshow n, C1, C2, andC3are

the t iming capacitors, while R1 and R2 are the

resistorsfor the phase-shift network. R3 mustbeat

least 12 times the R1-R2 value for adequate loop

gain. AGC is provided by using the AG cell as acompressor.

Thiscircu itisnotsuitablefo rtunable use. Itshould

only be used as a spot frequency oscil la tor, by

varyingC1,C2andC3. ThisisbecauseR1 andR2are

related, bythedesign, to R3; in thisspecific caseR3

cannot bevariable becauseit is used tosettheo ut

putdcbiaspoint.

Although it uses a simple design, this cir cu it pro

ducesexcellentresults.A tthefrequency indicated,a

laboratorytes tindicatedaTHDof 0.01 per centat 3

voltso utput, which is remarkable in viewo f the cir

cuit's simplicity. Totake full advantage ofthis per

formance, an output buffer may be useful; for thisyou could simply use the remaining channel as a

simpleunitygaininverter.

conclusions

Thisdiscusssion hascoveredafe w usesfora new

and interesting chip. In the course of this article's

preparation several other potential uses suggested

themselves, such as phase comparators, phase-

locked loops, voltage-tuned oscillators, and others.

Unfortunately, space and time restrictions did not

permittheircompleteexamination.

references1.C.Todd, "A MonolithicAnalogCom pandor,"IEEE Journal o f

Solid State Circuits, VolumeSC-11, number6, December, 1976,

page754.

2. C. Todd, "The MonolithicCompandor— A HighPerformance

Gain Control Integrated Circuit," Aud io Eng inee ring So ciety

Preprint, number1100, May,1976.

3. W. G. Jung, C. Todd, "Operationand Usesfor the570/571 IC

CompandorChip," The Audio Am ateur, December, 1976,page1.

4. B. Gilbert , "A Precise Four-Quadrant Mult ip lier with Sub

nanosecond Response,” IEEE Journ al o f So lid State Circuits,

December, 1968,page365.

5.W . G.Jung, "GetGain Control of80 to 100 dB ,"Electronics

Design,June21, 1974,page94.

6. W. G.Jung, "ICOpAmpC ookbook," HowardW . SamsCo.,

1974,page251 and451.

7. W. G. Jung, "Application of the Two-Quadrant Amplif ier

Multiplier in Audio Signal Processing," Journal of the AES, Volume23,num ber3,April, 1975,page207.

8.J. Fisk, W1HR, "Novel Audio Speech ProcessingTechn ique,"

ham radio,June, 1976,page30.

9. Tobey, Graeme, Huelsman, "Operational Amplifiers; Design

andApp lications," McGraw-Hill, 1971,page391.

10. R. P. Sallen, E. L. Key, "Practical Method of Designing RC

Act ive F ilte rs," IRETransactions, VolumeCT-2, 1955,page74.

ham radio

gain control ic for audio signal processing hr 0777

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