[sdiy] Unique sounding modules that can't have voltage control?

[brianw]

On Feb 9, 2024, at 8:08 AM, cheater cheater wrote:
> On Fri, Feb 9, 2024 at 4:33 PM Didrik Madheden wrote:
>> 
>> I think the boring answer is that there really are no examples where you absolutely can't make it voltage controlled. Which is why people in this thread, myself included, have had to go to increasingly obscure devices to find counterexamples. (And creating an interesting discussion in the process I might add.)
> 
> Honestly I don't think that's necessarily true! Especially I feel
> there might be examples when talking about controls that are in
> feedback loops.

It's true that feedback loops are the most difficult to replace, but they're not impossible to replace without changing the sound.

Transconductance and transresistance are terms to study. There are voltage-controlled voltage sources and voltage-controlled current sources. Each of these can be useful in challenging applications. They also happen to be powerful solutions in situations where an inductor is needed, but is too expensive, and thus using a capacitor but translating the results to behave like an inductor can both save money on parts and improve accuracy and consistency.


Reviewing the ways that variable resistors are used, each poses a different level of difficulty:

A) A good number of pots on synth control panels are merely connected between power and ground, and thus deliver a DC voltage to the circuit. These are obviously easy to replace with CV.

B) There are also many cases where a variable resistor is used to attenuate a variable signal (rather than a DC voltage source). These are cases where a VCA might be useful to allow CV to control the same result.

C) The circuits where a variable resistor affects a feedback circuit (or even a connection to virtual ground) can be voltage-controlled using transconductance or transresistance. There are challenges, as folks have pointed out, but not unsurmountable.

Brian