[sdiy] 4-pole filter a success!

[David G. Dixon]

Fellow DIY'ers,

After many weeks of fiddling around, thinking, reading, designing,
simulating, consulting with Ian Fritz, etc, I've finally tested the
breadboard version of my 4-pole filter.  It works exactly as designed, and
sounds fantastic with my modified Thomas Henry oscillators!  I'll try to get
a web link up this week and get some schemos, pix, sounds and scope vids up.

I set out to design the ultimate 4-pole COTA filter based on cheap, easily
obtainable parts.  Here are some of the features:

* Cascaded OTA 1-pole filter stages based on LM13700 and TL074
* Inverters based on TL074 at each stage
* Each stage switchable for HP or LP, giving 16 different configurations
* Resonance amps between each stage based on LM13700 and TL072
* VC cutoff and VC resonance
* 8 quadrature sine waves available (a la Mankato)
* Parallel pnp-based expo converter for cutoff frequency control
* Automatic gain control on the resonance amps to limit sine amplitude

The individual resonance amps ensure uniform amplitude of both the filtered
signals and the sine waves at high resonance.  This is distinct from other
COTA filters which have only one resonance amp in the loop.

The automatic gain control is based on a MAV (mean absolute value) circuit
consisting of a precision rectifier or absolute value circuit feeding an
integrator.  The output is fed through resistors into the linearizing diodes
of the LM13700s in the resonance amps. Without this circuit, the sine waves
suddenly appear at a certain resonance pot setting, and then hit the rails
and distort with just a slight increase in the pot setting, giving a fairly
harsh sound.  With this circuit, very nice sine waves form gradually over a
significant range of the resonance pot, and remain undistorted even at the
highest pot setting.

The only thing which is ever-so-slightly off spec is that the sine waves
from each stage do not all have exactly the same magnitude, in spite of
using 1% resistors everywhere.  However, they are close (within about 1/2 a
volt).  I suspect this is due to variations in voltage offset in the
LM13700s, which can be easily fixed by replacing the biasing resistors with
trimmers.

If anyone would like to see the schematics, please send me a message
offline.

David G. Dixon
Professor
Department of Materials Engineering
University of British Columbia
309-6350 Stores Road
Vancouver, B.C. V6T 1Z4
Canada
 
Tel 1-604-822-3679
Fax 1-604-822-3619
 
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