Thanks Oliver. I'll try the 8580 and see how it performs. As I said I can post-process it once it's recorded if I want to make it sound less clean.
I have found this information on replacing the 8580 with a 6581:
1) The 6581 uses a 5v and a 12v supply, while the 8580 uses 5v and 9v.
2) Different filter capacitors are specified for each chip - 470 pF for the 6581, and 22000 pF for the 8580. This gives each chip a different range for the filters.
Aside from the voltage supply found on pin 28 (6581 = 12v, 8580 = 9v), the
two chips are pin-compatible.
To make a complete swap, you must provide the proper voltage on pin 28 and swap out the two filter capacitors on pins 1-4.
So if I just replace the capacitors then bend out pin 29, then find 12v from somewhere, could I use a 6581?
Anyway, as I said I'll see how I get on with the 8580. I do want to use the 303 bassline synth so maybe the 8580 will be better anyway.
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moto