[sdiy] SID output protection?

[Ben Stuyts]

Hi Antti,

> On 30 Jul 2025, at 21:20, Antti Pitkämäki via Synth-diy <synth-diy at synth-diy.org> wrote:
> 
> So it seems I can just leave the original 10k/1000pF lowpass filter there for not-too-trebly-sound and forget about the original transistor buffer, going instead for the opamp buffer with protective diodes and the output resistor.

Yes, please do. The op-amp buffer costs next to nothing, compared to the rare and expensive SID chips.

> Thanks for all the help and links, I'll go through the information contained in them and let's see if I manage to come up with decent solution. I know it's not difficult, this is just not my field of expertise :P
> 
> BTW, here is another SID output protection circuit:
> 
> https://csdb.dk/release/?id=150177
> 
> It is based on diodes without any active parts.

The series-diode D1 doesn’t make much sense either. It could influence the sound (not sure about the output voltage levels involved), but it is definitely a non-linear device. In my opinion, it would make more sense to connect the D1 between the top of D2 (anode) and +12V (cathode). That way, anything over +12 V is shunted to the 12V rail.

And then loose the zener. Do not use zeners for this. Use a decent TVS (and check its datasheet where it actually starts clipping). The 12 V zener isn’t much of a protection. It is:
a) slow
b) not really 12 V (look up the data sheet)

In all, I would ignore that circuit.

If you settle on a circuit, feel free to post the schematic here for a go-over. There are lots of experienced designers here with solid opinions.

Ben

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