[sdiy] Debugging Hammond filter circuit

[Scott Gravenhorst]

"Paul Schreiber" <synth1 at airmail.net> wrote:
>>
>> 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?) 

4069UB is 6 unbuffered inverters, 4007 is similar in that 3 inverters, also unbuffered can
be made from it.  4007 datasheets can have several example circuits for using the MOSFETs
in different ways.  I believe the Wasp used 4069UB.

>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.
>

And an education as well.

-- ScottG
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-- Scott Gravenhorst
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