MC3340 VCA

MC3340 VCA

The Motorola MC3340 VCA chip is used here. It has logarithmic control response. Because it is designed for single supply operation, the output has a DC offset of about half the supply voltage. An op-amp is used to get the control voltage range to 1 – 10 volt. This circuit has very low distortion at moderate signal levels and low signal bleedthrough. But the CV bleedthrough is far too high for synth use. Another problem is that it won't produce 10 V p-p output level. You could add another op-amp at the output to fix this and the DC-offset. But then you would lose the main advantage with this chip, which is low parts count.

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	93 dBr A
	0 V CV, no signal	103 dBr A
	0 V CV, 1kHz 10 V p-p in	78 dBr A
	0 V CV, 2 kHz 10 V p-pin	79 dBr A
	0 V CV, 10 kHz 10 V p-pin	78 dBr A
	Headroom (over 10V p-p)	0 dB
CV bleedthrough	no trimming possible	500 mV

Summary

In some areas very good performance, but is unacceptable in others. A shame really.

• Low distortion at moderate signal levels

• Low signal bleedthrough

• Low noise

–

• Very high CV bleedthrough

• DC offset on output due to single supply operation

• Output level too low