[sdiy] OTA to voltage-control resonance?

[Magnus Danielson]

cheater cheater wrote:
> Hi guys,
> I've been looking at some monosynths and one thing has me wondering:
> if I have a potentiometer somewhere in the filter's resonance
> feedback, can I safely replace it with an OTA based 'voltage
> controlled resistor' without changing the sound?
> 
> I understand the best answer could be 'depends' - so, what does it depend on?
> 
> Do OTAs work like buffers, similarly to op-amps?

OTAs is quite different animals than pots. They provide a current output 
which depends on a control current multiplied with the 
voltage-difference between the input terminals multiplied by some 
conversion factor. This is not at all a good description of a normal 
pot, it does not even come close and does not even attempt to describe 
the non-linear properties. So there is no drop-in replacement, it takes 
analysis to figure out the best way for each and every topology.

So yes, it depends...

OTAs work different from op-amps and buffers in that you have a voltage 
input and current output where as most op-amps and buffers have voltage 
in and voltage out. Also, the gain (or transconductance) is programmable 
through a control current.

Cheers,
Magnus