[sdiy] Paia Fatman ic replacement

[Scott Gravenhorst]

I don't know what the advantage would be, really.  The reason for doing it
for the glide pot is input impedance.  With the 324, the glide pot affects
tuning (and does not with a JFET type).  The VCOs do not have tuning issues
so apparently, there isn't an impedance problem with the integrating cap
being watched by a 324.  I did the glide mod on one of my FatMan synths by
just replacing the 324 with a TL type and had to change it back later
because something didn't work right (it's been a while, I can't remember
the symptom).  I think the issue is that the LM324's output can go a lot
closer to the negative rail than can a TL opamp.  I had to remove the TL
and put the 324 back in, then I wired in just one opamp from a TL to take
care of the glide problem.  I believe that I piggybacked the power leads;
it makes it a bit of a hack, but it does work.

Hope that helps.

Antti Huovilainen <ajhuovil at cc.hut.fi> wrote:
>Hi
>
>I've ordered Paia Fatman a while ago and while waiting for it to cross the
>pond, I've been reading the mod pages on the net. One page (can't remember
>which) suggested replacing the LM324 following the glide pot with a jfet
>input opamp. Is there some reason why I couldn't replace all LM324s in
>oscillator sections with, say, TL084s? I can't think of any reason why
>exactly LM324 would be required in the oscillator sections.
>
>Antti
>
>Give a man a fire, and he'll be warm that day,
>Set him alight and he'll be warm for the rest of his life
>

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