[sdiy] Cheapest, simplest but still reliable VCO & VCA for polyphonic project

[Tom Wiltshire]

Karl,

On 8 Jan 2009, at 18:52, Karl Ekdahl wrote:
>
> Next question is in regards to the fact that since i'm doing a poly  
> synth where one set of knobs controls all the voices, i'll have to  
> make *every* parameter voltage-controlled, so i'll need a hell of a  
> lot of VCAs. Any suggestions on good, cheap and very low parts- 
> count VCAs? I'm guessing a LM13700-design.. ?

I'd have thought Paul is right that the SSM2164 would be a better  
choice than the 13700 for audio VCAs. More VCAs in the same size chip  
for less money, and probably better quality too.
Paul is also right that perhaps you don't need *so* many VCAs. If you  
have various modulation routes with selectable sources and selectable  
destinations, you only need one VCA per route, plus analogue switches  
or similar for the routing.

My own thinking along these lines started off with a very analogue  
approach with everything controlled by CVs, then developed to a semi- 
analogue approach with ADSRs and LFOs generated by CV-controlled  
PICs, and has now got to the point where I've got all the modulation  
sources and routing in a dsPIC, with only the outputs ever seeing the  
analogue light of day via a multichannel DAC. The modulation dsPIC is  
controlled via an SPI link. This whole learning process has been an  
incredible journey these last couple of years.

Even if you only went to the second of those stages, you can have a  
CV-controlled ADSR in a 14-pin chip with a CV-controlled output  
level, thereby saving you one of those VCAs you thought you needed.

Let us know what you finish up with. I'm always intrigued by the  
solutions other people come up with to these perennial problems of  
synth design.

Regards,
Tom