[sdiy] Untangling the Oberheim OB-8 filter

[Tom Wiltshire]

On 15 Jun 2017, at 00:08, Declare Update <declareupdate at gmail.com> wrote:

> "But in this case I can't see where the "inverted phase" is coming from. What am I missing?"
> 
> The VCAs in the chip are all inverting, I think!

Are they? I wondered that, but I've never heard it before, and the datasheet doesn't mention it.

I suppose that would explain it - the Resonance VCA must be inverting too. I'll have to do some experiments and check.

> all that gain applied to the input mixer with frequency (already pointed out), anyone know why?

Is it somehow to compensate for the Vgm of the OTA being varied by the CV? They're using that stage as an op-amp, but it's actually an OTA, and it's connected to the exponential filter CV, so its gain varies with frequency CV unless you do something about it.

Tom