Ne570 Applications

Operationandusesfor the chiP lC compandor 5701571 PARTI b y t , l a l t J u n g , C o n t r i b u t ' i n gE d i t o r a n d C r a i g T o d d ,S i g n e t i c s C o r P . IFHIS

A R T I C L E D I V E R G E Ss o m e w h a t I fror past op Anps fot Audio colunlrrs, and is also a great deal more Notewotxhg than one of the tYPical and New itens. The reason for these will be evident as You differences begin to grasp the details of oPeration and'working of a new and fasIC chip. The Signetics cinating NE570 ahd NE571 are monolithic IC compandots, devices which can be used for the audio gain control functions of compression and expansion, as well as other gain-controlled functions. for Although designed PrimarilY telephone trunk line conrnunications the 570 and service applications, 571 devices possess Perfonnance flexiand built-in soecifications for a host of sufficient Uifity audio signal processhigh quality surarticle ing uses. This first veys what the 570 and 571 are, how and what theY do, and how to aPPIY them to audio uses. We think Yourll agree this sonewhat unusual anount is warranted when You of interest see what these devices, with relative ease, can do with audio signals, and we think You will quicklY grasp their imPortance to audio in is general. Before this article conplete, you will be able to build been projects which have heretofore or impractical. sinply inpossible is bY intent both This article and feaand Practical, tutorial tures nany readY-to-use 570/571 Inforrnation on 570/571 circuits.. is included at the end availability of the article.

B a s i cO p e r a t i o n Fig.l is a block diagrarn of the 570 and 57I. Both the 570 and 571 units comPonconsist of the functional ents shown, and are in fact derived fron the same basic chiP. The difbetween the two devices fetence the lies in their sPecifications, 570 being the prirne of the two units, with lower inherent distortion perforrnance. The 571 has nore but relaxed basic sPecifications, nay be trirnned with outboard conponents to a Perfonnance level eoual or near that of the 570.

Thus, unless otherwise stated hereall of the ci-rcuits shown after, may use either device, with the choice being uP to the user. final The 570 and 571 are dual devices, of two sets of the funcconsisting tional signal blocks indicated. These are a AG cell (for variable a full wave gain, if you will), the gain which controls rectifier and an output oP of the AG cell, amp stage connected as shown. An on-chip 1.8V bias reference voltage is included, which suPPlies bias voltage to both halves of the dual This reference voltage, circuit. along with the supply voltage and ground pins, are the onlY things c o m m o nt o t h e t w o r r h a l v e s i l o f t h e with the renainder of the circuit, shown being duplicatpin functions ed, one set per side of the 16 Pin laybut. The 570 and 571 oPerate fron single power supply voltages in the range of +6 to +24Y, with quite 1ow power, typical current drain being only 3mA (both sections). The devices are therefore well suited to battery operation, for not onlY is the current drain low but internal makes operation highly regulation immuneto supply voltage, in general. The lG Cell In operation the audio signal to be controlled is applied to the AG IN input through a coupling capacitor. AC coupling is necessarY due to a +1.8V bias level present at the AG cell. The AG celi also constitutes

Diagtam

570, 571 FunctionalFig.7: (f of 2 channefs).

a virtual ground node, therefore the input impedance is sirnPlY the 20K value of R2, and the resulting innut current will be Vin/2OK. The AG cell is designed to accePt a naxinun of tlOouA of inPut current (200yA p-p), or a 1.4V nax RMSsine wave level. Wherenecessary, higher input levels can be acconnodated by including an external series resi s t o r . In its gain control nechanisn, the net action of the AG cell in effect proportions the applied input current, and Passes it on to the output op anp, where it is ultimately converted into the circuitts output voltage. The proportionality (or scaling factor) of the AG cell is in turn controlled which Produces an by the rectifier, conoutDut current which directly trois the AG cell. The AG cell rnay be viewed as a current controlled, current scaling circuit. The concurrent is the DC outPut trolling and the con' of the rectifier, trolled current is the audio signal. We havenrt roon hete for the actual details of the AG cell circuit irnplenentation, but they nake interesting reading. Consult the references for further insight into the technique. The AG cell, in short, conpensates for natural transistor nonand temperature sensilinearities tivity, providing a stable and Predictable prograrnrnablegain element with a dynarnic control range of 100dB, with low signal distortion. The ultinate linitation on distortion is deterrnined by the matching of the gain cellsr internal transistor pairs, and this Practical lirnit accounts for the aforenen-

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J

| .,lu l?iilriiriiiur

I

*F----i Issue4/1976

tioned differences between 570 and 571 devices. Solace arrives, however, in the news that either device can be trirnned using the THD TRIM terninal. Whentrimned, tyaical devices can show THD figures of 0.1% or less (while untrimrnedTHD can be up to 0.5%). Distortion is this high, however, only at full scale audio signal inputs; at lower levels distortion drops rapidly, and noise becones the limit to dynarnic range. Noise in a bipolar transistor transconductance multiplier such as is used here has been a topic of controversy in the audio fraternity, with nany designers disnissing the technique as being practically useless because of its ilinherently limited dynamic range,t' or because it is rrinherently noisy.rrThese objections are simply not justified when a linearized transconductance multiplier is irnplernented,as it is here. Signal to noise ratio in a 570 or 571 can be as high as 90dB in a 20kHz noise bandwidth, which of course easily exceeds alnost aIl available progran sources. Further, signal to noise irnproves (even beyond this figure) at lower working gains, where the circuit is nost apt to be used. Recti fi e r

'

The rectifier circuit of the 570/ 571 accepts a signal at the RECTIN terminal, and full wave rectifies this signal, converting it to a proportional DC signal which is used to control the AGcell. Like the AG IN input, the RECTIN input is biased at +1.8V and is AC coupled in normal operation. Input imp e d a n c ei s t h e v a l u e o f R l , o r l 0 K . Full scale rectifier input current i s 2 0 0 u Ap e a k , e q u i v a l e n t t o l . 4 V RMSinto 10K. After fu11 wave rectification within the circuit, an external capacitor attached to the CRECT Fig.3a:

Circuit

hookuP,

Basic

!---'TfrnTr"---il----l I

! L

....t ---

t---T 'L?R E C T

rxorntu

I r 4!? a G r N

szorstt i (B) rslnEcrrru I I c"ect L---+---------J t6 Fig.2:570,571 bvice gram and pinout.

I t-)rrrr { rz ! n. {rr i o u r{ r o i

sgnboT dia-

0 u t o u t C ir c u i t The output op amp used in the 570/ 571 is similar to a 741, and like the rest of the circuit is biased for nininun quiescent power drain. This amplifier can be used only in the inverting mode, and is set up for operation by connection of R3, either as an input or feedback resistor. You may also use external resistors, as access is provided to the (-) input. The op amp is biased at its inputs to a DC level of +1.8V; connection of R3 as a feedback resistor biases the output to 3V, optinun for a 6V supply level. You can achieve output biasing to other DC levels for greater swing by using external resistors in se-

L

I { I

nr #lFourPur*

GND

OUTI-J = |

I

t_ G a i n= 1 . 4 3 v t N ( A v g . )f o r c o n n e c t l o n s s h o w n - - i n g e n e r a l: t.. = I2

Ga i n

llf6l

fne nuOioO*r"r,

v1 x(A vs') n,

Basic Expander Connection and operation of a 570 or 571 as an expander is shown in Fig.3. The hookup is shown in general form in Fig.3a. The signal is applied in cornmonto the AG IN and through a corunon RECTIN terninals, coupling capacitor, Cttt. h averaging capacitor, CRECT,-is connected

filters the DC sigterminal nal, which is now equal to the average AC input level. The current fed to the AG ceII as a gain control signal is then the average of the input AC signal.

57o/57t cnscr

€-----€r---J

9 l----rJ--

Ex-

ctN"

nr 06 I our+z cND !

ezorszt i (a) - rH -zle necr

pandet.

VIN

(-)rN{5 I

; 39aGrN I

ries or shunt conbinations. We gain further insight into the nanner of. 570/57Lrs working by examining the two basic connections, as an expander and as a compressor. The subsequent schenatic representations follow the device s"yrnbol d i a g r a r no f F i g . 2 , w h i c h a l s o s h o w s the pinout. Although it is not obvious fron this drawing, the 570/ 571 is laid out in a synnetrical pattern (with the exception of supply pins), which facilitates PC layout.

lu = 140uA R , = l O K , R , = 2 0 K ,R , = 2 0 K ** Select clN, co for-desired inPut/output corne r r req .

frorn the CRECTterninal

to ground.

The output is jurnpered for use of R3 as a feedback resistor. Cr* and CO are input and output coupling capacitors, chosen for the desired circuit low end frequency response; they will typically be on the order of several microfarads for wide range audio work. (Note: observe correct polarity when using electrolytics.) The gain of this circuit is described nathenatically as 1.43 VIN, where V'U is the average input

values voltage. The 570/571 circuit are set up in such a way that the is gain of this expander circuit u n i t y a t a n i n p u t l e v' \e l o f 0 . 7 7 5 rr /^+ n 7tf , a leVel ob-

"(Rl.rs)'"''''(Avc;''

viously convenient for conmunications work. In the input/output graph of Fig.3tr, you will note that indeed the output is 0.775V when the input is 0.775V. The expansion ratio of this circuit is l/2, thus an inPut change yield an output change of 6dB will of l2dB, which can be noted from the curve. Although this is a somecurve intended to what sinplistic it is true the effect, illustrate in practice that a 570/571 circuit over wilI follow this relationship an input dynamic range of 40dB or more (or 80dB or nore of outPut range) . Errots which do occur at the low end of the dynamic range are largeand can be ly due to the rectifier, minimized. You may use separate into the AG and coupling capacitors eroffset RECT inputs to eliminate rors here, and a trin technique can be applied to the RECT IN terminal to optimize low level tracking (to be discussed below). designers out For the innovative there, we include the circuitrs pertinent complete desi.gn equations. You can shift the unity of crossover point by alteration R3. 0r you may also adjust the effective value of Rl (or R2) bY usat these tering series resistors minals. I. is an internal circuit constant,oand is not alterable.

B a s ci C o m p r e s s o r Fig.4 is a basic 570/571 compressor. Fig.4a details the circuit hookup, and 4b shows the input-outYou will note that put relation. this is the exact conplement to the expander curve of 3b, as the addition of the two curves will yield a curve. linear input-output is soneThe compressor circuit what more complex than the expander, because it requires sone additional bias components. In this

TO AG CELL THDTRIM ( P r NI O R9 )

TO RECT, INPUT (PrN 2 0R 15) b. LOW LEVEL ATTENUATION C A L I B R A T I O NT R I M

+l5v

TO AG CELL INPUT ( P r N3 0 R t 4 )

roK o

LEEDTHRU C .C O N T R O F Fig.5 : Optionaf

trinnning ^netwotks g

the AG cell is the AC feedcircult back path, and a separate DC output bias path is provided bY the ROa bias the These resistors resistors. output up to a stable DC point, and COa renoves any AC feedback. ROa be in the range of 20 can typically to 30K, while c." is on the order l0uF. CO is a coppling capacitor to the AG and RECTIN inputs,

while

C^

and C^ are input and output cou(typically chosen pling"capacitors as described previously). You nay have already intuitively reduces noted that this circuit gain for increasing input levels, as more signal feedback to the rectifi-er increases the AG cell culrent, which reduces gain by being in the feedback path. Again, both the si.nple and general forns of design equations are shown. Unity gain crossover Point in this circui-t is also 0.775V, to complement the expander. The output Circuit

Fig .4a : ptessor.

hookup'

will change only SdB for a 6dB input change, which is a 2/1 conpression ratio. Modification of the unity gain crossover point (when desired) rnay be acconplished by using an external resistor in place of R3, connected to the (-) IN terninal. This circuit can also be optirnized for low level tracking, by to the AG using separate capacitors as in the conand RECT IN inputs, pressor. Also you may trin as described in the next section.

T r i m m i nT gechniques You may apply several trimming techniques to various 570/57l terminals, to optimize or enhance operation of either device. All of the techniques discussed here, and and shown in Fig.S, are optional, need be applied only when you want the highest perfonnance. Fig.5a shows the most useful of for a trin the three techniques, corrects nininun TllD. This circuit for the offset voltage in the AG cel1, in so doing mininizing its output second harmonic distortion and content. It is quite effective, in some cases can reduce distortion by as much as a factor of ten. The trimmer is adjusted for minimum output THD with the AG input driven to its maxinum level. of Fig.5b is useful The circuit in cornpensating for the bias curwhich is rent of the rectifier, 50-l00nA. This DC bias typically current places a lower linit on tracking range, as it appears as an equivalent AC input, if unconected. The trirnmer is adjusted for tracking, with an correct rectifier input 50-60dB below full scale. Note that this trim will not be effective unless a separate coupling capacitor is used at the rectifier input, as mentioned above. Thp Fio (e eirerrif can be USed tO minimize feedthrough of a gain change signal to the output. Ideally gain change signals should not be seen in the output, but if input signal levels are very low they nay The tTililner is adbe noticeable. in the iusted for minimun shift

Basic Com- u+

RDC

R0c O U T P U *T

ctN *

\t+

v tN o---rr Grin

=

, i n7 l---;:l-l

.t*

Gain =

lvtu\^vs.r; I s s u e4 / l 9 7 6

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output as gain is varied, with no AC signal applied to the AG cell. Applications We are now ready to discuss more circuit uses for the 570/ specific which follow 57I. Alf the circuits are for single supply, +lsv operaopertion, but nany can be battery ated if desired. A nunber of mixer thus are included, type functions the reader could readily use 570rs or 571rs to build a portable mixer features. with sorne interesting

rct-B

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570/571

ct* O.OlpF IL

S i n eW a v e0 s c i I I a t o r

20K
-A r.c_l

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F i g . 6 s h o w s h o w a 5 7 O / 5 7I c a n b e used to build a simple, self-AGCrd of the Wien sine wave oscillator is reasonably type. This circuit clean in terms of THD content and for a medium perfornis suitable ance, fixed frequency sine wave source. the Wien network In this circuit, is conposed of components R1-Cl and C2-R2, which in conjunction with the output op anp of the 570/571 section A form the frequency deterresoThis filter nining filter. nates at a frequency of 1

t=,:

ZTKL With equal element values as shown (RI = R2 and Cl = C2) the networksf signal loss is 2/7 at the resonant frequency, and the Phase from the input at Cl relationship to the output at R2-C8 is inverta secing. To sustain oscillation, ond gain stage nust provide signal for in-phase feedback, inversion and a gain of 2. The gain nust be held at 2 to sustain undistorted constant amplitude oscillations. Section B of the 570/57L provides the required gain and phase inversion, in its output stage. BY connecting the AG cell of this section as a cornpressor, the gain is autoregulated to the required matically value to sustain undistorted oscilResistors R3-R4i form the Iation. of the op amP gain input resistance network, and the chiprs internal is used as the feed20K resistor Note that the i-niback resistor. tial noninal gain of this stage is 2/7 {about higher than the required this factor is neces4/l in fact); Once sary to start oscillation. bulld uP unstarted, oscillations the AG celt connected as a comtil pressor reduces the gain to 2/1, self-sustaining. and is thereafter the Due to DC bias restrictions, resistance has a range of tining narrow spread, about 4 to I fairly as shown. However, the capacitance unrestricted. value is relatively The values shown are for 400H2 oPbut other frequencies over eration, a wide range, up to about 20kHz, are possible.

The Audio Amateur

2rRC (as shown f = 400H2)

Fig.6:

^a

toPF x o

rooo

=

Modulatot

AnPfitude

id;

-r

fiir-ox |

of

A m p li t u d e M o d u l a t o r With the wide dynamic range gain of the AG cell, control capability various modulation schemes are possible using the 570/57f. One of these is an arnplitude nodulator, shown in Fig.7. requires a circuit AM nodulation which can vary the anplitude of a signal (the carrier) between 0 and 100% in a linear fashion. In this the AG cell of the 570/571 circuit, nodulates the gain of the signal (carrier) channel, up to 100%. With no modulating signal apis a plied, the gain of the circuit nominal unity, as set up by the res i s t a n c e R 3 + R 4 f r o n R E C TI N t o ground. This resistance biases the LG cell to a mid-gain point. With rnodulation applied, negative nodulation peaks double the gain, acc o m p l i s h i n g 1 0 0 %r n o d u l a t i o n . T h e to resistance R3 is nade variable, signals beacconrnodate nodulating tween 0.5 and IV RMS in amplitude. can be used as a nodThe circuit effects or tremolo generaulation tor. For very low modulating fre-

I

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c3

s7o/57t *r iI _--'"" "li-ll-?r' [3n

IB 1 W F[' 1, lilo*'

(o.775vRMS)

resistor

be replaced by a single 5k.

Xt

rNPUr *+---+L-facli---ii sTGNAL s C 2 l O p F| MOoULATION f ,tt+ I rNPUr tnz I

OUT I

r

sine Wave osciTTator

is reasonably low, but Distortion If a can vary due to two factors. 570 unit is used it will yield disof about 0.25%, without tortion trimning. A 571 can also be used, trim network shown and the optional at both outcan reduce distortion Duts to about 0.1% or less. The u1(for eiti-mate level of distortion ther device) is dependent upon the howvalue of C4. Unfortunately, ever, C4 cannot be increased indefas to decrease distortion, initely tine and lengthen settling it will For frethus compromise stability. quencies greatly rernoved from the example shown, C4rs value should be optimized, going lower for higher higher for lower frefrequencies, quencies. is set by trim Output anplitude of the R3-R4 resistance, which acin the 570/ commodates variations resistance. Output 2 571 internal can be adjusted in the range of 0.5 follow to lV RMS, and output I will at half the output 2 1evel. Distortion is lowest at output 1, due to of the Wien network. the filtering Either or both outputs may be used, and they are well buffered against If exact output loading effects. levels are not desired, R3-R4 can

Fig.7:

R2T 39K

570/571

l---lnr -f t0K l@

|

R4 4.7K

IN

| |

|

* sEE r EEXxTr SEE T

lruuK uOouuareo I

|

ourpur

bI

RGATN(ext).

This

can be nodi-

f-ied for higher gains, ,ith RGAIN (ext) = O.

up to

SdB

M ix e r -F a d e r SENSITIVITY

R6

rooo R7

rooK O U T P U T

?.2pF

Fig.8: ouencies such as below 10H2, C2 ihoufd be increased to about 50uF. Cl is lOuF for wide band audio, but frequencan be reduced for carrier cies above l0OFlz. Both inputs are conpatible with the 570/571 sine outPuts, as shown wavl oscillator here.

S q u e l c hC i r c u i t Fig.8 operates as a gated amplifier controlled bY the signal level or the squelch. In this aPPlication, as a fixed gain 5io/571 functions stage when on. Signal level sensing are performed by control and on/off transistors Ql-Q2. circuit In the Ql-Q2 rectifier emitter PNP Ql is a conbination foLlower and half wave rectifier. as seQl buffers the inPut signal control Rl, iected by sensitivity and discharges C2 on the negative peaks. Ql can discharge C2 quicklY, but C2 nust recharge through R4. Fig.9:

Squelch Circuit

With no signal applied, resistor R4 provides base drive for Q2 which causes it to saturate, clamPing the " ' ^i ' n e I t o s r o u n d , T h i s r e "cR E C T t- 'e- m duces the 570/S7l gain to -80d8 (or less). When an input signal is aPplied which overcones the threshold bias set up bY R2-R3 at the base of Ql, C2 is discharged, turning off Q2 and gating the amon by releasing the clamP plifier on C3. Switching actj-on is snooth, due times. A to controlled transition long tirne constant is relatively provided for by R4-C2. This time constant provides a delaY Prior to which prevents Prenature turnoff, switchoff between sentellces, or other natural pauses. is a maxiThreshold sensitivity mum of 200mV RMSwith Rl fu1lY advanced, and can of course be reduced as Rl is lowered. As shown, the on state gain is unity, as set

Mixet-Fader

R7

rooo

IN P U T

CONTROL IN P U T

R3+ IM E G

c2 lpF

H I G H . C HA LOW.cH B

R4 | I M E G. I1

When the control input is low, the conduction path through R2 is b r o k e n a n d t h e A c h a n n e l A G .c e l l switches off. The A channel gain goes snoothly to 0, as controlled by a tine constant of (10K)C2. At the -same time as the A channel goes high, inverter logic iriverter IC2-b goes low, which prograns the with B channel to an 0N condition, the B channelts AG cel1 oPerating way to the A channel. in a sinilar (pin 12) The output of this AG cell is tied into the A sectionrs amPlifier summing input, at pin 5. With the B channel AG cell on, the B channel signal is passed to the output at unity gain (as deternined

by Rs). In the transition from off to on

+l5V

B INPUT

shows how a sinThe Fig.9 circuit gIe 570/571 device can be used as a logic level controlled mixer-fader uses the This circuit amplifier. two halves of the device as gated with a commonoutput anplifiers, which is nixed in the output op anP of section A. For either the A or B Portion of the 570/57I AG cells the circuit, gated anpliperform as controlled bY the which are controlled fiers, drive applied to the respective R E C Ti n p u t s , p i n s 2 o r 1 5 . F o r t h e A s e c t i o n , C M O Si n v e r t e r s t a g e I C 2 a gates the 570/57L AG cell on with a HIGH control input level. This provides a DC path to ground, the R2. l5K gain programming resistor, in the A chanwith an 0N condition nel, the A signal input is passed to the output with a nominal unitY gain (as deternined by R2).

s

'Jl ii-r

I ,^l

rcre 571

c6 lO;.tF

the B channel tine constant is set by (10K)C4. Thus while channel A is going to zero gain, the oPPosite side, channel B is going to maxinurn gain. There is an overlapping transition period where both signals are on, due to time constants of network. The effect the rectifier is a I'soft lap'r type switching, very snooth and pleasant to the tine ear. You nay vary transition if you like, by adjusting C2 and C4 values. can also be rnodified This circuit for gated rnixing by separating the EiA and B channel control lines. ther or both may be 0N as desired, in any combination. Adjust signal gains of each respective channel via R2 or R5.

V o l t a g eC o n t r olle d F a d e r and useful forn of An interesting is an ancircuit gain controlled

I s s u e4 / 1 9 7 6

plifier with a gain which is an expnentiaT function of an input conSuch a circuit nay be trol voltage. voltage used as an all-electronic, a controlled fader, by utilizing linear control pot excited by a DC control The exponential voltage. law gives the circuit a control funcwhich is a linear "dB/voltt' tion. Fig.10 is an exarnple of this type of circuit. In this hookup a section of a 570/57I is used as a DC currentThe current attenuator. controlled is which detennines the attenuation supplied by Q5. Q1-Q5 and IC2-a form an exponential voltage-towhich gives the current converter, an exponential whole conbination characterisvoltage gain control is suptic. This characteristic plied by matched transistors Ql and Q5, part of a 3046 IC array. is the The input of the circuit of diode connected transisenitter tor Ql. Directly at this point, the is one dB of control sensitivity gain change for 3mV of control voltage change. The input voltage to divider scales this sensitivity a more usable form, at the Vc j.nput. With the values as shown, the has OdB of gain with a +3V circuit and a control sencontrol voltage, of 46nV/dB. Gain is -60dB sitivi"ty I +.231V of input, the limit of useful working range. For further reductions of input voltage, the switches to an OFF stage circuit below about -62dB, where attenuation is more than 80dB. Within the 60dB working range tracking accuracy is within about ldB of an ideal exponential law.

features are Several circuit and can aid the user to noteworthy, realize naximum performance. Gain is calibrated by R4 to unity, with 3V applied to R9. At the other range extreme, gain is calibrated to -60dB via R7, wj-th +.23\Y applied to R9. If the IC2-b stage, R18, R19, LED and Dl are used l'these na rts are not essent-iaI to gain should drop basic operation), ^L-..-+1.. +^ o^rD ^- Iess and the au1 uPLr/ as the input voltage LED extinguish is taken down through +.128V. If these components are not used, gaj-n should be -o5dB wj-th 0V input at R9. The THD trim network is also opand may not even be deemed tional, necessary as the additional AG cel1 R2 reduces AG celL input resistor Input overload is nonlinearity. raised to more than 2V by this recicfnr

Af

l\/

innrrf

se lr l u

urr L L/

For nanually operated faders, use a lk pot for a low source inpedance. The 3V for pot excitation can be obtained fron a three-terrninal (which can drive a number regulator of pots in parallel). A suitable circuit for this use is shown in the Fig.lO inset; it can drive at least two dozen pots with good regulation. For further technical information on the 570 and 571, write: Si g n e t i c s 811 East Arques Ave. Sunnyvale, CA 94086 The 570 and 571 devices are available from: Janes Electronics, PO Box 822, Belnont CA 94002 Prices: NES7OB: 1 0 . 5 0 e a . ; N E 5 7 1 B :g t 0 . O O e a .

Ba rrr t

0.1% or less THD is typically (without trim) and S/N greater than 72dB wideband (equivalent to 80dB or better in a 20kHzbandwidth). inprove at gains Both these figures of -20dB to -15dB, where a fader is normally operated. Control voltape is scaled to 3V for unity gain, a voltage which undictates a non-standard fortunately le" R9 value. R9 and R10 are ideally components, but a quite close approximatj-on to R9 can be had with parallel 15K and 300K 5e,units. The cjrcuit can be built up in a dual format with only four ICrs: one 570 or 571 , one 324 quad op amp, and two 3046 IC arrays. It may be used as either a dual channel unit, or strapped for stereo by Lying the Vc inputs together.

Quest Electronics, P0 Box 44J0, Santa Clara CA 95054 prices: N E 5 7 0 B$: 5 . 0 0 e a . ; N E 5 7 t B :$ 5 . e a on page 1 3 concl-uded rid

1n

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source

+ NATIONAL SEMICONOUCTOR

L M 3 1 7 H* OR L M3 I 7 P

RI

rooo O.lyF ADJUST FOR 3.OOV OUT

3 VOLT S O U R C E

Fig.70:

+l5V

voTtage ConttofJ-ed Fadet 33K

tooK THD TRIM (9)

R3 r50K

THD TRIM AG IN

rctA(B) 570 / 571

INPUT 0.775 VRMS

ils) i

CT IN

L Rt4

c3 .i7ox

+l?v

fl3*

CONTROL VOLTA6E 46 mvldB 5V. OdB

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o . 2 5 t v .- 6 0 d 8 r c 2b ( d ) x Po 3046 I C T R A N S I S T OARR R A Y

The Audio Amateur

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