BC550C VCA 2

This is the same circuit as the BC550C VCA 1, except that some resistors have been changed. As the original circuit exceeded 1% distortion at 10 V p-p input signal, the input divider resistors were changed to reduce the signal further. Instead the control current was increased, as this improved performance slightly.
This circuit has the lowest signal bleedthrough at 10 kHz, but apart from this the performance isn't particularly good. As with the BC550C VCA 1, CV bleedthrough and temperature sensitivity are problem areas.

Noise & signal attenuation

Red = signal bleedthtrough at 0V CV. Blue = 10V CV, no signal. Green = 0V CV, no signal.

Distortion (THD+N) vs. input level

Frequency response

Test results

Dynamic range 10 V CV, no signal 80 dBr A
  0 V CV, no signal 97 dBr A
  0 V CV, 1kHz 10 V p-p in 95 dBr A
  0 V CV, 2 kHz 10 V p-pin  94 dBr A
  0 V CV, 10 kHz 10 V p-pin  87 dBr A
  Headroom (over 10V p-p)  2 dB
CV bleedthrough with careful trimming 220 mV



In contrast to what some people seems to think, a simple discrete circuit like this has no advantages over an OTA-based one.


Low signal bleedthrough

High CV bleedthrough

Temperature sensitive

Needs matched transistors