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From: TLC (all posts)
Date: 2000-11-30
Subject: SID card

Hi all!

I think I found the answer for a question that I had since I had a SID card (by Solder / Synergy). ...Anyone having such a gadget?...

Well, the device acts a bit strange. In overall, it works O.K. - except for the subjective side if you listen to musics on it :-(. In short:
distortion, clicks, pops etc... something that one doesn't really expect from such 'simple' sound thing. I once contacted Solder, but neither he had good ideas on the reason, as I remember. I suspected overdriving the small amplifier part on the SID audio output.

Since few weeks, I'm experimenting with the SID filters and I think I found the answer ... somehow by accident. The answer is actually on the opposite side. ...I'm very angry. The SID external input line is pulled low by a resistor on the card. The EXTIN is at about 2-3 volts constantly, somewhere in the border where the NMOS 'op-amps' are still functional. This DC offset goes through the SID, overdriving either the mixer, the resonant filter stage or both, depending on the actual SID settings. I did a quick test simply disconnecting EXTIN (pin 26 of the SID) and voila! - it worked well!!! Although it picked up plenty of noise (I left the pin floating), it sounded 'correctly' ;-).

So finally: if you have such card and want to get rid of the distortion then you can fix it by removing the pulldown resistor: it is found next to the SID, between the SID and two similar small ceramic capacitors.
You can identify it on the board: it is the first resistor connecting to the route going to pin 26 of the SID. ...Be careful, do not ruin this otherwise cool device ;-).

(A note: after disconnecting the resistor, the SID will probably be unable to play digis, similarly to the C64-II with the same 8580 SIDs.
...I haven't seen any SID-card supporting stuff playing digis on the SID itselves, so it shouldn't be much of a problem.)

Best regards,

L.