[sdiy] SID output protection?

[Oren Leavitt]

For protecting the SID from the "outside world", an op-amp buffer with 
the typical 1k series output resistor should be much more effective than 
that transistor buffer.

Don't know the details of your project, but in general.. a solid power 
supply design, a few carefully placed Schottky diodes and bypass caps 
may help safeguard the SIDs from other nastiness.

- Oren

On 7/28/25 10:36 AM, Antti Pitkämäki via Synth-diy 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
>