[sdiy] Steiner filter mods

[Michael Ruberto]

>
>   http://houshu.at.webry.info/200511/img_1.gif
>
>Constant current source Q2 reduces common-mode gain of the
>differential amplifier to almost zero, resulting in far less
>CV feedthrough.
>

hmmm this is very interesting as it sheds some light on another circuit 
that's currently giving me a headache.

>Another modification is the output point to the buffer U1.
>Response of the resonance pot is more like Korg MS20.
>
>I also used a diode limiter as Yves Usson.
>

I have to say this is slick. I have my version of the filter on the 
breadboard still and I'm gonna give this a try. On my VCF I have an FET 
boosting the output signal from the circuit but the side effect is it's also 
amplifying the gain changes from from resonance. It's interesting that you 
use 2 feedback paths here as this is what I found in the actual Synthacon I 
serviced a few years back.

M. A. Ruberto