[sdiy] SID output protection?

Tue Jul 29 20:25:51 CEST 2025

Hi,

Unfortunately it seems my understanding on opamp circuits is quite limited
- it's been about 20 years since I had an electronics course. I understand
the very basics of what an opamp buffer is and the basic idea of how it is
connected, but I'm a little bit unsure on what kind of additional
components and connections my SID protecting opamp buffer circuit would
need. I didn't find a solution after several hours of online searching and
reading my old analog electronics course book, so I'm back with a question
again...

I'd like to connect the opamp buffer after the C64/128's original
transistor buffer, a picture of which I've attached below. Some people
might raise criticism on including the original transistor buffer in
addition to the opamp buffer - however, based on others' experiences I've
read about, the original transistor circuit has an impact on the sound, and
I really don't want to change it, even if the original transistor circuit
has a theoretically poor sound quality. It's just that that's how I've
always listened to the SID, through that transistor. I read about a fancy
opamp buffer circuit somebody used to replace the transistor buffer, and it
resulted in an "improved" sound - with so much additional treble that one
might need to turn the treble down on their amplifier. I doubt the SID
needs more treble as we are talking about mostly raw digital waveforms (if
not using the SID's filter)...

Anyways, I feel a bit lame to ask such a basic question here, but I was
wondering if anybody could point me to an optimal opamp buffer circuit that
I could use with my SID as mentioned earlier in this thread?

BTW this is the abovementioned alternative "fancy" buffer circuit in case
anybody is interested:
file:///C:/Users/User/Downloads/MOS6581_Alternative_output_V2_0.pdf

Thanks for all the help so far,
Antti

Here's the original transistor buffer of a C64/128:

[image: image.png]

On Tue, Jul 29, 2025 at 1:55 PM Antti Pitkämäki <antti.s.pitkamaki at gmail.com>
wrote:

> 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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