[sdiy] SID output protection?

[grenert]

For what it's worth, this is the output design for the SID version of
MIDIBox.  Looks like it's probably not too different from what you have
now.
[image: SID output.png]

I've used essentially the same output on SID Blaster type boxes.
[image: exSID.png]


On Mon, Jul 28, 2025 at 8:39 AM Antti Pitkämäki via Synth-diy <
synth-diy at synth-diy.org> wrote:

> 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
>
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