[sdiy] Looking for OB-8/CEM3320 filter gurus

[Tony Allgood]

CH > The sick filter has different DC values, compared to a good filter.

This appears to be the case in most dead 3320s I have seen, although I
had a dead one in a P5 that had no expo response but otherwise was
working OK.

JH > It's remarkable that this is stable...

Well, the OB-Sx two pole SVF filter is not. It honks at certain cut-off
frequencies and resonances. You can't hear it honking because it is
oscillating at several hundred KHz. At first I thought I had a bad 3320,
but I have tried several ICs, from different batches, and all honk. This
has the audible effect of making the filter appear to suffer from bad CV
breakthrough as the HF oscillation comes in and out during a filter
sweep. Putting a small cap on the input pin to ground controlled the
situation well enough. (albeit a complete and utter bodge)

The circuit of the OB-Sx is similar to the one used in the later OB with
the two 3320's per voice. I can't remember which one this is... but I
think the later ones with the CMOS switching all over the place used a
slightly different circuit.

Regards,

Tony Allgood                       Penrith, Cumbria, England

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