<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body style="overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><br id="lineBreakAtBeginningOfMessage"><div><br><blockquote type="cite"><div>Am 29.07.2025 um 20:25 schrieb Antti Pitkämäki via Synth-diy <synth-diy@synth-diy.org>:</div><br class="Apple-interchange-newline"><div><meta charset="UTF-8"><div style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 14px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;">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)... </div><div style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 14px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;"><br></div><div style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 14px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;">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?</div><div style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 14px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;"><br></div><div style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 14px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;">BTW this is the abovementioned alternative "fancy" buffer circuit in case anybody is interested:<span class="Apple-converted-space"> </span><a href="file:///C:/Users/User/Downloads/MOS6581_Alternative_output_V2_0.pdf">file:///C:/Users/User/Downloads/MOS6581_Alternative_output_V2_0.pdf</a></div></div></blockquote><br></div><div>That link doesn’t work for me. Is that probably a link to a file on your own computer?</div><div>However, I googled the file name of that pdf and found this one:</div><div><a href="https://www.c64.cz/data2/download/x12/122790/MOS6581_Alternative_output_V2_0.pdf">https://www.c64.cz/data2/download/x12/122790/MOS6581_Alternative_output_V2_0.pdf</a></div><div>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?</div><div>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“?</div><div><br></div><div>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?</div><div><br></div><div> Ingo</div></body></html>