[sdiy] Debugging Hammond filter circuit

[Paul Schreiber]

>
> http://users.ece.gatech.edu/~lanterma/patents/hammond_fet_vcf.pdf
>

a) you have no chance of this thing working unless the layout is just right. 
No "white breadboard", no parts hanging up in space. It will oscillate at 
like 5 MHZ.

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.

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).

All in all, a total POS circuit.

Paul S.