SV: RE: Re: [sdiy] Rene's ms20 filter clone

[Karl Ekdahl]

After looking at the schematic i can't possibly
understand how it would *not* work as a vca? Where is
the signal supposed to go when the cutoff is turned to
-15v? I found a shorted-out cap and a 470 ohm resistor
instead of 470k at the base of the expo-converter, so
now i can hear suttle resonance and frequency changes
while turning the pots. But still the volume goes to
zero while turning the CV pot all the way down? Where
am i supposed to find the signal when it's turned
down??

Karl

--- Jay Schwichtenberg <jays at aracnet.com> skrev:

> 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
> > >
> > >
> >
> 
> 
>