[sdiy] Rene's ms20 filter clone

[Jay Schwichtenberg]

Karl,

Yes, your calc of the cap value seems right to me.

The feedback provides some of the signal being 'feedback' into the
'beginning' of the circuit. With filters its weird and they do not all
behave the same in regards to feedback. Depending on the filter circuit in
some cases more feedback lessens the resonance (and is actually required for
the filter to work) and sometimes more feedback increases resonance. I don't
keep this type of info in my head so I don't know how feedback is used in
this case.

Here's my logic:

1) You have a VCA so to me that says you are getting signal though the OTAs
(CA3080) and buffers (A1, A2). Also if you can control the amplitude the
exponental converter is working. But you don't have a filter.
2) The two things that make this a filter vs. a dual VCA are the caps and
feedback. Basically you have RC (Resistor Capacitor) circuits that forms the
filter. The OTAs being the resistors. So if you substitute the OTAs for
resistors you can look up a filter circuit that is simular and get the math.
The thing that I don't know about is that the first OTA is inverting and the
second isn't.

To check the feedback I'd look at the signal coming out of A3 and see if it
varies when changing the resonance pot. Might even disconnect it from the
cap and see what happens.

Always go over your work a number of times and if you know someone that
knows electronics you might have them give it a look over. Actually maybe
someone that doesn't know anything could be good. When you have to explain
and show someone what to do your looking at the problem with different
'eyes' and going over stuff in more detail so you may see the problem
yourself. A lot of times it takes me looking at something 3 or 4 times
before I see what is going on.

What test gear do you have?

How was the circuit built (solderless breadboards are very touchy)?

Jay S.

> -----Original Message-----
> From: owner-synth-diy at dropmix.xs4all.nl
> [mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Karl Ekdahl
> Sent: Wednesday, September 28, 2005 4:21 PM
> To: jays at aracnet.com; synth-diy at dropmix.xs4all.nl
> Subject: SV: Re: [sdiy] Rene's ms20 filter clone
>
>
> Okay, to make sure of this a final time (i've been
> wrong so many times before about this); a cap marked
> "102" means 10pF * 10^2 = 1000pF = 1nF right? The
> unfortunate thing is since i don't really understand
> the principals behind a resonant filter, i don't
> really know what to look for in the feedback circuit?
>
> many thanks
>
> Karl
>
>
> --- jays at aracnet.com skrev:
>
> > Karl,
> >
> > I'd check the resonance feedback circuit and 1nf
> > (0.001uf) filter caps.
> >
> > Jay S.
> >
> > Karl Ekdahl wrote:
> >
> > > ...behaves perfectly as a VCA right now but does
> > > nothing "filtery". I've measured everything and it
> > > seems ok, so i don't know...  ..any clues anyone?
> > >
> > > btw. here's the schematic:
> > > http://www.uni-bonn.de/~uzs159/ms20.html
> > >
> > > Karl
> >
> >
>