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marc.hull
December 11th, 2009, 07:37 AM
I have been playing with a SID chip device recently and have a question. The datasheet calls for 1000pf caps for the filters but I have also seen other values bandied about. Does anyone know what size is in the C64 for a 6581?
Thanks in advance...

carlsson
December 11th, 2009, 08:10 AM
If it helps you, the schematics and service manual is here:
http://zimmers.net/anonftp/pub/cbm/schematics/computers/c64/index.html

From what I can understand, the capacitors you mention are the ones connected to CAP1A, CAP1B, CAP2A and CAP2B. In the datasheet I found, the recommended value is 2200 pF. However the schematics makes a mention that you should use 470 pF (!) for the 6581 and 22000 pF (!!) for the 8580. There also are C80, C81 which call for slightly different values between SID revisions.

Remember the 6581 is infamous for having highly individual filter settings, so barely two chips will sound the same. Possibly that is why people have exchanged caps for other values to try to make the chips sound anything alike. Listen for example to the music of Martin Galway, who was one of the pioneers in using the filter to its max. All his songs were optimized for his own C64 and some games even had a special filter adjustment screen where the user could tune in software parameters to make the music sound as good as possible.