BC550C VCA 1

A VCA built with discrete transistors. The circuit is taken from René Schmitz' web site. The transistors were matched acording to the instructions in the Moog Modular service manual. This simple circuit does indeed work, but it has a couple of serious problems.
To start with, it is very temperature sensitive. The DC offset will go up if the transistors are not at the same temperature. They definitely need to be thermally coupled. But even with a constant temperature the CV bleedthrough is too high.
Noise and distortion figures are not better than for OTA-based VCAs. Signal bleedthrough is low, especially at high frequencies, but the LM13600 has even lower signal bleedthrough.
The reason for the fairly low signal-to-noise figures is that the op-amp contributes to the noise. In this circuit it has a gain of 17 times, which adds noise. Thanks to Jürgen Haible for pointing this out. René Schmitz had specified a 5534 op-amp, which should be a little less noisy than the TL071 that I used.

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  83 dBr A
  0 V CV, no signal 95 dBr A
  0 V CV, 1kHz 10 V p-p in 95 dBr A
  0 V CV, 2 kHz 10 V p-pin  93 dBr A
  0 V CV, 10 kHz 10 V p-pin  81 dBr A
  Headroom (over 10V p-p)  -4 dB
CV bleedthrough with careful trimming 200 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

Too much distortion at 10 V p-p

Temperature sensitive

Needs matched transistors