[sdiy] SID output protection?

[Antti Pitkämäki]

Thanks for all the information! I'll find a good basic op-amp buffer
circuit and implement that at the SID outputs.

My design will actually have 4 separate outputs, one for each SID. The
outputs will then be connected to a mixer, allowing EQ'ing, effects,
panning and volume control separately for each SID. This is a very
important aspect for me in the project. It allows me even to do live dub
mixes :)

J.P. mentioned that the original power supply is especially dangerous to
the SID. Many Commodore enthusiast warn that the original power supply
could damage the whole computer if something goes wrong. There is a
replacement power supply available for the Commodore 128DCR with twice the
power rating of the original. I'll definitely get one of those (available
here: https://www.c64psu.com/c64psu/91-commodore-c128dcr-internal-psu.html).

Regards,
Antti

On Mon, Jul 28, 2025 at 11:02 PM brianw <brianw at audiobanshee.com> wrote:

> I second the suggestion to use an op-amp buffer.
>
> Also, since you're adding 3 SID chips, you might as well mix their outputs
> together with an op-amp mixer, which would further disconnect the SID chips
> from the external output. If you want all SID chips to have the same
> maximum level, then a simple mixer with fixed gains would be easy. If you
> want each SID chip to have its own level, controlled separately than their
> internal level controls, then you'll need knobs or faders, but that might
> be a bit much for a small project.
>
> - Brian
>
>
> On Jul 28, 2025, at 9:27 AM, Oren Leavitt wrote:
> > 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 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.
> >>
> >> 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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