[sdiy] AS3320 VCF chip - some queries

David G Dixon dixon at mail.ubc.ca
Mon Nov 12 10:14:25 CET 2018


Yes.  In filter configurations like the internals of the 3320, the filter
stages are simply buffered RC tanks.  However, when 2164 is used as the
variable resistor, the capacitor must be part of a Miller integrator,
because 2164 can only terminate to virtual ground.  The point I was making
was that these integrators are not sensitive to what follows unless an
unsuitably low impedance is used to connect the output to the next stage.
This would have to be a very low impedance indeed to overwhelm the
current-sourcing ability of the opamp, and since 2164 filters are typically
connected together with 30k resistors, this is never a problem in such
filters.
 
 
  _____  

From: Andrew Simper [mailto:andy at cytomic.com] 
Sent: Sunday, November 11, 2018 10:55 PM
To: David G Dixon
Cc: pete.hartman at gmail.com; Tom Wiltshire; SDIY List
Subject: Re: [sdiy] AS3320 VCF chip - some queries



The op-amp is the buffer. 

On Mon, 12 Nov 2018 at 13:16, David G Dixon <dixon at mail.ubc.ca> wrote:



I don't think I have ever put a buffer between two filter stages.  Since
ever filter I've ever built uses 2164 as the variable resistor, the
capacitor is always part of an integrator.  I don't believe that what
follows such a filter stage is at all relevant to its performance, provided
that the following input impedance is reasonably high (and mine are always
30k, which is more than enough).


  _____  

From: Andrew Simper [mailto:andy at cytomic.com] 
Sent: Sunday, November 11, 2018 6:14 PM
To: David G Dixon
Cc: pete.hartman at gmail.com; Tom Wiltshire; SDIY List
Subject: Re: [sdiy] AS3320 VCF chip - some queries


Good point about the main filter caps David. Two other things to watch out
for to make low frequency self oscillation happen with filters: 

(*) make sure any DC blocking caps in the resonance signal path are well out
of range of where you want the filter to self oscillate down to, otherwise
the phase and gain changes can throw off normal resonance
(*) make sure you use good buffers between stages to not drain out the
current from your caps. 

With the 3320 the buffers are on chip so this makes things more difficult if
they are causing the issue. Do you know what they are? If they are a part of
the problem then one way to mitigate it somewhat would to use larger valued
caps, and then likewise increase the cutoff current so the relative drain of
the buffers is lower, but this may impact the highest cutoff you can get.

Cheers,

Andy



On Mon, 12 Nov 2018 at 02:17, David G Dixon <dixon at mail.ubc.ca> wrote:



You might find that the filter will oscillate easier if you use good film
caps.  I had problems getting a four-pole filter to oscillate at low
frequencies with monolithic ceramic caps, but it oscillated strongly down to
the minimum frequencies with WIMA polyester caps.
 
I just built an SVF with silvered mica caps (which I usually use in
oscillators) and it is being finicky with self-oscillation below about 50Hz.
Maybe I'll swap those out for WIMA polyester and see.


  _____  

From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Pete
Hartman
Sent: Sunday, November 11, 2018 9:20 AM
To: Tom Wiltshire
Cc: synth-diy at synth-diy.org DIY
Subject: Re: [sdiy] AS3320 VCF chip - some queries


On Sun, Nov 11, 2018 at 10:32 AM Tom Wiltshire <tom at electricdruid.net>
wrote:

2) The resonance response is tilted to the high end.
It seems to require a lot more resonance CV (ok, current) to make the chip
oscillate at the bass end than at the treble end. Similarly, the amplitude
of oscillation increases as the frequency rises. Why is this? Is there
anything I can do about it? Should I even care?



If I compare this with the behavior I've seen in other lowpass filters that
self oscillate, even without CV over the resonance, it seems consistent.
Typically I need the frequency at 50% or higher to get it to start
oscillating -- I can then commonly turn it down and get lower frequencies,
but my experience is that if the filter is going to oscillate, it's always
easier to start the oscillation at a higher frequency.  And typically if I
turn it "too low" then it stops oscillating again.

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