SV: RE: [sdiy] SVF resonance overdrive problem

[Scott Gravenhorst]

Karl Ekdahl <elektrodwarf at yahoo.se> wrote:
>Hi all,
>
>> I wonder if in the original question... if the
>> circuit is using an OTA
>> without the traditional attenuator on the input.
>> This would distort badly by
>> the time you reach 20mV ???
>
>This is the same thing that Paul said right, the 1K at
>the input of the OTA? I tried that and indeed that
>does raise the possible input volume a little bit.

I'm not sure what you did exactly with this resistor, but OTA based
SVFs usually have a voltage divider at the input, the FatMan's is a
series 100K to the OTA input with 1K from OTA input to ground giving
approximately 100:1 attenuation.  There is one such voltage divider for
each of the two OTA inputs.

>
>The problem is that whatever i do the output stays
>fairly low and that amounts up to noise in the
>background - this is the ultimate problem.
>
>I figured this had to do with the low input level, but
>perhaps not - any other way i should approach this?
>
>Thanks for all the info so far!
>
>Karl
>
>> 
>> H^) harry
>> 
>> 
>> --- Andrew Simper <andy-list at vellocet.com> wrote:
>> 
>> > A slightly different configuration that the
>> regular SVF is the ARP 
>> > structure, which drives the input signal more than
>> a regular SVF, but 
>> > you only get a low pass output, and if my analysis
>> is right a 
>> > high/band pass output depending on resonance
>> (output of first OTA).
>> > Here is a schematic:
>> > http://www.till.com/arptech/40235/40235.pdf
>> > 
>> > Andrew
>> > 
>> > -----Original Message-----
>> > From: Harry Bissell Jr
>> > [mailto:harrybissell at prodigy.net]
>> > Sent: 11 December 2006 20:10
>> > To: Karl Ekdahl; synth-diy at dropmix.xs4all.nl
>> > Subject: Re: [sdiy] SVF resonance overdrive
>> problem
>> > 
>> > The most common SVF configuration has a resonant
>> peak that exceeds the 
>> > passband amplitude (the peak has MUCH higher
>> gain).
>> > This is normal.
>> > 
>> > There is a way of injecting the input through the
>> resonance path, 
>> > which limits the peak amplitude to the same value.
>> The passband 
>> > amplitude goes down. This is how the Moog ladder
>> filter behaves.
>> > 
>> > Another way is to introduce a "Q limiter" into the
>> resonance feedback 
>> > path.
>> > I use back to back zeners to increase the
>> resonance feedback (lower Q) 
>> > when the signal amplitude gets too high. (I use
>> 5.1V zeners).
>> > 
>> > Typically, low frequency harmonics are stronger
>> and tend to overload 
>> > the filter. As the filter sweeps down, the zeners
>> conduct and reduce 
>> > the filter resonance.
>> > 
>> > This does NOT sound transparent... but it does
>> stop the overload. I 
>> > used the technique in my Hex Guitar processor
>> (Muffy III) because I 
>> > cannot fidget with signal levels while playing
>> live. The "Q limit"
>> > saves the day at high
>> > resonance levels.
>> > 
>> > H^) harry
>> > 
>> > 
>> > 
>> > --- Karl Ekdahl <elektrodwarf at yahoo.se> wrote:
>> > 
>> > > Hi list,
>> > > 
>> > > i've built a OTA-based SVF whose resonance
>> > distorts heavily if i have
>> > > to high input-volume. The problem is that "to
>> > high" is something like
>> > > >20mVpp so that won't do. Can anyone give me a
>> > hint on what's wrong,
>> > > is this a common problem with SVFs?
>> > > 
>> > > Karl
>> > > 
>> 
>> 
>> 
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>> 
>
>
>
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-- ScottG

---------------------------------------------------------
-- Scott Gravenhorst
-- Xilinx Spartan-3E Based Music Synthesizers
-- FatMan: home1.gte.net/res0658s/fatman/
-- NonFatMan: home1.gte.net/res0658s/electronics/