[sdiy] Hot Tips for Making a SV Filter Oscillate?

[Ken MacBeth]

Hi All! I have been playing around with the Oberheim SEM filter for some time now. It sounds great- remarkably pure and clean, and most of all, very useful. It is the design that I want to include in my larger M5 synth that I'm doing. Everything about that filter is great, but I know that users who buy the M5 will expect oscillation. I have played about with resistor values in the feedback loop, around the resonance pot and can actually get it to oscillate, but the sound quality is hellish. Also, the cutoff freq is greatly affected by any attempt to gain oscillation, ie, it goes from hardly any cutoff and then into osc when the res pot is turned on full. I have dropped the value of R157 (SEM Diagram) from 33K to 1K to do this. I have also removed as well as shorting out out R158 and all give a pretty similar poor result. So if anyone one out ther has experience or know-how of how to get what I am after, it would be great to get that help.
I appreciate that I am now doing this stuff on a commercial level, and that some of you diyers may not want to give away your own trade secrets. But at least I am open and up front with you all. I remember seeing various arguements/debacles over Wasp filters and filter ladder ripoffs some time ago and it all seemed very sneaky! I do not want to go that way!
Anyway, any help appreciated!
Best- Ken


http://www.macbethstudiosystems.com
Home of the M3X Analogue synthesizer...

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