[sdiy] SID output protection?

[Antti Pitkämäki]

Hi,

I have an ongoing project of adding 3 additional SID sound chips to an old
Commodore 128DCR computer for a total of 4 SIDs (I'm planning a C128DCR for
this project instead of a C64 due to the 128DCR's additional space for
modding). I've got the basic idea of how to implement it sorted out, but
there is one crucial aspect I'm not yet sure how to implement properly: SID
output protection.

SIDs are known to break/fry easily. A situation where it often occurs is
when the mixer connected to the computer is turned off while the computer
is powered (the computer should be switched off before switching off the
mixer). Another potential cause of SID death is (accidental) hot plugging
equipment. So I think my SIDs would need some protective circuits at their
output.

As it is, there is a already a single transistor buffer (2SC1815) at the
output of the SID in a stock C64/C128. However, apparently the transistor
doesn't do a very good job at protecting the SID.

I've found several suggestions for SID output protection: an optocoupler, a
ground loop isolator, diodes that short overvoltage or an additional op-amp
buffer.

I'm quite sure the best advice I can find resides in SynthDIY, so... If you
had a delicate audio chip such as the SID, how would you protect it's
output to prevent damage to the chip caused by voltage spikes, overvoltage
or other anomalies from auxiliary equipment? Would a more sophisticated
buffer than just the original transistor be sufficient or do I need
something else?

Any help would be greatly appreciated!

BTW in case anybody is interested, I can share information on how to add 3
additional SID chips to a Commodore 64/128. Schematic-wise it's really
quite simple.

Finally, some background on my project: I've made C64 music for years, and
some time ago I found out that my favorite native C64 music "tracker" has a
version that supports up to 4 SIDs for 12 channels of sound. So my dream is
to mod a C64/128 for 4 SIDs so that each SID has a separate output. It
would be like a mini synth studio. 12 channels with 4 outputs (3 channels
per output) would have great possibilities and it would be enough to make
synth music outside the chipmusic context. And the learning curve would be
close to zero since I know the tracker well. Also I like "closed musical
systems", I mean doing music with a single device (although I'd of course
add external mixing and effects).

Regards,
Antti
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20250728/957e4545/attachment.htm>