[sdiy] SID output protection?

Tue Jul 29 20:40:40 CEST 2025

Try searching for a Voltage Follower -
https://www.eeeguide.com/voltage-follower-circuit-diagram/

I think that you should keep the 10uF capacitor on the output like in the
previous examples, but swap out the rest with a basic opamp voltage
follower.

*Jacob Watters*
JacobWatters.com <http://jacobwatters.com/>

On Tue, Jul 29, 2025 at 2:28 PM Antti Pitkämäki via Synth-diy <
synth-diy at synth-diy.org> wrote:

> 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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