CA3080 VCA 1
The trusty old CA3080 is used here in a
logarithmic VCA. The circuit is similar to the one in Elektor
Formant, except that the FET buffer is replaced by a FET input
op-amp. The load resistor is connected to ground and the op-amp
is a voltage follower. This chip is frowned upon by some people.
It has a reputation for being very noisy. In reality it is only
slightly noisier than the alternatives. The noise is only present
at full gain, which makes it a problem only if the VCA is fed
with very weak signals.
What can be more of a problem is that the signal bleedthrough at higher frequencies is rather high. But even at low frequencies the signal attenuation is not as good as with the LM13600 or the SSM2024. The CA3080 has very low CV bleedthrough if it is carefully trimmed.
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
|Dynamic range||10 V CV, no signal||77 dBr A|
|0 V CV, no signal||117 dBr A|
|0 V CV, 1kHz 10 V p-p in||83 dBr A|
|0 V CV, 2 kHz 10 V p-pin||76 dBr A|
|0 V CV, 10 kHz 10 V p-pin||65 dBr A|
|Headroom (over 10V p-p)||5 dB|
|CV bleedthrough||with careful trimming||6 mV|
This was expected to be a poor performer but turned out to be quite respectable. It is a little noisier but the difference is surprisingly small.
Very low CV bleedthrough
High signal bleedthrough at higher frequencies
Yes, it is the noisiest of the lot