[sdiy] NPN pair VCF?

[np]

Dan Snazelle wrote:
> been noticing lately that just about every VCF out there uses a PNP pair.
> is it possible to use an npn pair in a vcf? 
>
>   
I think that has to do with the fact that all the current controlled 
gain elements (OTA's ect). are controlled from a current referenced from 
ground or the negative supply.

These inputs are current-suckers, and you need a current-source to feed 
them. That can only be done with a PNP.

You could use a NPN based expo-converter and turn it into a current 
source via a current-mirror, but this way you'll end up having a PNP 
pair in the current mirror, so nothing is gained.

For non OTA based filters such as a transistor ladder it should be 
possible to use NPN pairs by just turning the filter upside down. 
Haven't tried though.

Cheers,
  Nils