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

[Gordonjcp]

On Thu, Feb 08, 2024 at 03:14:57PM +0100, cheater cheater via Synth-diy wrote:
> Thanks Florian, not sure what you're trying to say - does an expo
> converter make things impossible to control via CV? (of course I know
> it doesn't but I don't understand your email)

Alright, think about it this way:

I want you to build me two sets of two matching filters. They're going to be Sallen-Key filters, so two resistors, two capacitors, and a buffer. Feedback through one cap to the junction of the resistors, other cap from buffer input to ground. You know, like an MS20 filter.

Build one pair of filters like a "late MS20" filter using OTAs as the "resistors". It's easy to make me two identical ones of those, any two OTAs you pick out of the box will be identical, behave identically and predictably, and it's all perfect.

For the second pair of filters, use vactrols as the resistors.

Holy hell.

You're never matching those. You're not even getting close to matching one pair, never mind all four, and they'll all have to match for the two filters to even vaguely be the same. The construction of the LDR, the position of the LED, exactly how the LED responds to the current through it - all massive tolerances right there, that you can never quite line up.

Now consider how easy it would be to just make me 100 identical *fixed frequency* Sallen-Key filters because you can just buy capacitors and resistors with sufficiently close tolerances.

Boy, did we get in a mess trying to make them voltage controlled!

-- 
Gordonjcp