[sdiy] Debugging Hammond filter circuit

[Aaron Lanterman]

On Apr 25, 2010, at 12:04 AM, Scott Gravenhorst wrote:

> Yeah, I see, not really a problem.  It's my birthday --- beer today! sorry.

Yay! Happy birthday!!! :)

> It _looks_ interesting, but weird too.  Sort of an uncommon/unconventional 4
> pole cascade without interstage buffering.

Yeah... I'm thinking an interesting modification would be to buffer the stages.

> The circuit allows easy voltage control because the JFETs are grounded on one end.  I'm guessing that is why the capacitors and JFET cascade was placed into the feedback loop so as to get
> easy voltage control of a lowpass filter.

Yup. 

Another possibility would be to use just one C-FET highpass 1-pole filter in the feedback loop of an op amp to generate a 1-pole LPF, and cascade those. The Buchla 191 does this, although it uses the dynamic resistance of a couple of diodes, tied to oppositely-signed CVs, instead of a FET.

> I will certainly be interested to see if this makes a decent lowpass VCF once all the bugs are worked out.

The sound on Bob's 102100 demo seems to indicate it's worth pursuing. I'm not going to push my student that hard on it, though - if a few tricks I'm thinking of don't work, I'm going to tell her to just go ahead and build a highpass filter. ;)

- Aaron