The MC3340 VCA

[jorgen.bergfors at idg.se]

I recently did some experiments with the MC3340, to find out if it is of any 
use to us synth DIY types.
It is a complete logarithmic VCA in an 8 pin mini-DIP package. All external 
components are two or threee capacitors. Price is around $2 to $3.
The chip is designed for single power supply, but with an input capacitor, you 
can feed it any audio signal. It won't work for DC or very low frequency signal 
though. IE not for control voltages.
The control voltage range is from 5,65 to 2,8 volts. The response is -30 
dB/volt. This means it will get louder if you reduce the voltage. By adding an 
op-amp connected as an inverting amplifier with offset, you can get a contol 
range of 0 - 10 volts or whatever you like.
Maximum audio input signal is 1,6 volts peak to peak. This will give 8 volts 
p-p out at maximum gain. The output will have a DC offset of 8,7 volts. All 
these figures are for a supply voltage of 15 volts.
Noise and signal bleedthrough at maximum attenuation is around 0,3 mV p-p on my 
scope. There is a control voltage bleedthrough of 0,5 volts. That is, if you 
change the control voltage between max and min with no signal in, the DC level 
on the output will change 0,5 volts.
I suppose the control bleedthrough is what rules it out for synth use. That it 
can't be used for control voltages is also a bit daft. Anyway, I thoght you 
might like to know.

/Jorgen