[sdiy] SID output protection?

[Ben Stuyts]

What is the mechanism that destroys the SID? Is it some transient voltage surge, or ESD? If so perhaps a few well placed TVS’s and/or Shottky diodes would help. (At output and power pins.)

And take precautions for people connecting it to a phantom powered input...

Ben


> On 28 Jul 2025, at 17:36, 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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