[sdiy] Debugging Hammond filter circuit

[Aaron Lanterman]

On Apr 24, 2010, at 9:20 AM, Paul Schreiber wrote:

> b) I would replace the TL072 (too high a BW) with a LM358 or other "bad" (low gain-bandwidth) dual op ap. The *last* thing you need here is a fast, wide-band op amp.

Kaylah, give this a try... I recall you were using TL072s, so you can (doublecheck pin out) probably swap a couple of 741s in their place.

> c) the "FETs" are a quad analog switch with a 600 ohm (holy crap!) on resistance. No prayer using discrete FETs, you need to maybe use a CD4016 but there is no chance the "CV input" will smoothly change the resistance like these ancient National Semi parts. Might have a better chance with unbuffered CMOS gates like in the Wasp filter (I forget, CD4007 or something?) and play with the voltage range on the Vgs to be in the 'linear' range (as I recall that is small like only a 1V range).

She's using J201s JFETs now, with a negative control voltage on the FETs. Is this a good idea or a bad idea?

I have another student working on a version of this with just the highpass part, and trying to put a negative feedback loop (Moog style) around the whole thing to make a resonant highpass filter; he's using J201s also right now.

Both seem to at least have a nice highpass filter over a decent control range.

Kaylah, if we can't get the lowpass part working on the breadboard (which Paul suggests will be tricky if not impossible but I want to try it anyway), we'll just drop back to doing a highpass filter. ;)

- Aaron