BC550C VCA 2

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

Summary

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