■ FIGURE 10. External connections to the circuit board,
including one possible way to implement the power supply.
■ FIGURE 11. The controller — which has an
LED bar graph display — was built into the box of a
surplus oscilloscope plug-in module.
Referring to the circuit diagrams in Figure 6, the gain
control element (IC3) is an LM13600 dual operational
R5, R6 220K
R7, R8 4.7K
R9, R10 1K , 20 turn Trim Pot
R11, R12 15K (Note 1)
R18, R19 2.2K
R20, R21 10K
R22, R23 220K
R28, R29 20K (Note 2)
R31 1 M
R32, R33 10K
R35, R36 470K
R38 25K (Note 3)
C1, C2 0.47 F, 35V
C3, C4 47 F, 25V
C5, C6 0.47 F, 35V
C7 2.0 F, 35V (Note 4)
C8 0.47 F, 35V (Note 4)
IC1, IC2 LF351N
IC3 LM13600N or
IC4, IC5 LF351N
(Optional) 1 mA
Power ± 15 V 25 mA
1 - Dual 10K Audio Taper
4 - RCA Style Audio Connectors
IC Sockets: Five eight-pin, one
14-pin, one 16-pin
Breadboard, Chasis, misc.
transconductance amplifier. This chip functions as a voltage-controlled amplifier with a current source-sink output.
Fortunately, the chip has an internal current-voltage
converter that makes it suitable as a stand-alone VCA for
most applications. The simple current-voltage converter
that's used in this chip is not exactly suitable for
professional audio applications, so a revised chip with
better circuitry — the LM13700 — was introduced.
There are, however, still many sources for the older
LM13600 chip (including my parts drawers), so the
controller circuit adds its own current-voltage converters
that allow the circuit to operate with either chip. These
converters — which are just operational amplifiers (IC4-
5) — also double as convenient output buffer stages.
Other operational amplifiers (IC1-2) buffer the inputs.
The circuit is socket compatible with the LM13700.
This is a stereo gain controller which is not exactly
the same as two monaural gain controllers in the same
box. In stereo, it's important to maintain the stereo
image, which is the virtual location of sound sources in
the audio. In classical music, especially, you don't want
to have the violins or horns wandering all over the sonic
space. For this reason, the control voltage for the
processor is derived from the sum of both stereo
channels. IC6a is the summer. The gain is reduced or
enhanced in both channels simultaneously.
Rectification of the audio to obtain a DC control
signal is done by a full wave rectifier built around IC6b-
IC6c. This signal is fed to the platform circuit, and the
peak holding capacitor Cp is buffered by a high
impedance amplifier, IC7. A level-shifter derives a
control voltage that's suitable for the VCA and an
You'll notice that the platform circuitry is slightly