[sdiy] SID output protection?

[Ingo Debus]

> Am 29.07.2025 um 20:25 schrieb Antti Pitkämäki via Synth-diy <synth-diy at synth-diy.org>:
> 
> 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

That link doesn’t work for me. Is that probably a link to a file on your own computer?
However, I googled the file name of that pdf and found this one:
https://www.c64.cz/data2/download/x12/122790/MOS6581_Alternative_output_V2_0.pdf
Is it this one? This circuit looks a bit strange. Why didn’t they just connect the negative power supply terminal of the opamp to ground? What are the two 18k resistors good for?
Also, the 500R/220pF lowpass has a cutoff frequency in the MHz range. The 10k/1000pF lowpass of the original circuit has a cutoff frequency of about 16 kHz. Maybe this is the reason for the „additional treble“?

I still don’t see why an opamp buffer should protect the SID’s output better than the original emitter follower circuit. Can anyone explain?

 Ingo
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