symmetry voltage (<

matches within the VCA IC. For this purpose, the 2150

series devices were designed for optimum performance

with an impedance of approximately 50

Ω at pin 4. A

trim pot is used to adjust the voltage between pin 4

and pin 2 as shown in Figure 3, Page 3. For supply

required adjustment range.

It is also possible to use pin 2 and pin 4 together as

an opposite-sense voltage control port. A typical circuit

using this approach is shown in Figure 14. Pin 3 may

be grounded, and pin 2 driven against the symmetry-

adjustment voltage. The change in voltage at pin 4

does have a small effect on the symmetry voltage, but

this is of little practical consequence in most applica-

tions. Using the opposite sense of control can some-

times save an inverter in the control path.

It is also possible (and advantageous) to combine

both control ports with differential drive (see Fig-

ure 15). While the driving circuitry is more complex,

this configuration offers better performance at high

attentuation levels (<-90 dB) where the single-control-

rates, the internal opamp will accomodate the change

in current demand by responding with a small change

in its input offset voltage. This leads to an accumula-

tion of charge on the input capacitor, which in turn

can cause thump when the high attenuation is sud-

denly removed (e.g., when a muted channel is opened).

Differential control drive avoids the large dc levels oth-

erwise

required

to

command

high

attenuation

(+610 mV for -100 dB gain at pin 3 alone, vs.

±305 mV

when using both pin 3 and pins 2 and 4).

Control Port Drive Impedance

It has already been noted that the control port

should be driven by a low source impedance for mini-

mum distortion. This often suggests driving the control

port directly with an opamp (see below under Noise

Considerations). However, the closed-loop output im-

pedance of an opamp typically rises at high frequencies

due to falling loop gain. The output impedance is

therefore inductive at high frequencies. Excessive in-

ductance in the control port source impedance can

cause the VCA to oscillate internally. In such cases, a

51

Ω resistor in series with a 1.5 nf capacitor from the

control port to ground will usually suffice to prevent

the instability.

Noise Considerations

It is second nature among good audio designers to

consider the effects of noisy devices on the signal path.

As is well known, this includes not only active devices

such as op amps and transistors, but extends to the

choice of impedance levels as well. High value resistors

have inherent thermal noise associated with them, and

the noise performance of an otherwise quiet circuit can

be easily spoiled by the wrong choice of impedance lev-

els.

Less well known, however, is the effect of noisy cir-

cuitry and high impedance levels in the control path of

Ec+

Ec+

Ec-

OUT

-IN

V-

GND

V+

-

+

240k

VCA

Series

2150

Rsym

Ec+

10u

LF351

47p

20k

50k

ADJ

SYM

51

5.1k

20k

+15V

-15V

-15V

+15V

OUTPUT

INPUT

1

5

6

2

4

8

3

7

2150

Series

VCA

300k (2155)

390k (2150A)

470k (2151)

Figure 14. Positive Control Port Using Pins 2 and 4

THAT Corporation; 734 Forest Street; Marlborough, Massachusetts 01752; USA

Tel: (508) 229-2500; Fax: (508) 229-2590; Web: http://www.thatcorp.com

Rev. 10/25/96

Page 9