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</o:shapelayout></xml><![endif]--></head><body lang=EN-GB link=blue vlink=purple style='word-wrap:break-word'><div class=WordSection1><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial",sans-serif;mso-fareast-language:EN-US'>Hi Naoki,<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial",sans-serif;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial",sans-serif;mso-fareast-language:EN-US'>This seems an interesting problem, so I whacked the whole lot into my simulation program, and after checking that it performed approximately as my calculations suggested (at -5V CV, +/-5V input gives o/p swing of +/-3.27V, so gain of x0.65; unity gain at approx CV = -7.7V), I then stuck a largish cap, 12pF, from the diff amp o/p back to the positive input: immediate ~1MHz oscillation (and reducing the max time step in SPICE brought this down to about 570kHz, along with much 'smoother' curves). No sign of any large offset though, but this could well be down to the 'lack of real world' nature of SPICE. That you have discovered by physically separating Q3 from the board that the offset disappears, I'm thus going to go with the 'something is oscillating' hypothesis as suggested already, though perhaps it may not be the op amp.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial",sans-serif;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial",sans-serif;mso-fareast-language:EN-US'>I'd say that if it <i>were</i> the op amp there would be a fair chance of actually being able to <i>see</i> the oscillations on an oscilloscope (do you have access to one?); if it is one of the <i>transistors</i> oscillating, then (in my experience at least) it can become <i>much</i> harder to work out what is going on, as the frequencies involved can be so much higher, and probing around the circuit can alter things to the extent that the problem changes as you do so, and you can't observe it. (An example I have seen, and which I have yet to analyse fully, is a suspected transistor oscillating in an exponential converter of a VCO, and initially the only way to 'see' it was the effect it had on the '1V/oct' response; later I used a spectrum analyser with a 'sniffer' coil of wire, which suggests it is oscillating at something like 37MHz.) To this end, running a finger lightly around the PCB to identify which part is causing the problem by making things better or worse (rather like your bamboo stick), may help, especially if you can rig things up so that you can <i>hear</i> the difference of 'problem' vs 'no problem'.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial",sans-serif;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial",sans-serif;mso-fareast-language:EN-US'>Obviously if you cannot identify which part of the circuit is to blame it becomes a bit hit-or-miss as to whether re-laying out the PCB will make it go away or not. But if you <i>do</i> suspect a certain region of the PCB is causing problems, it is quite possible to actually <i>measure</i> the capacitance between tracks, down to just a handful of picofarads, using a signal generator that will output a triangle wave (at units to tens of MHz) and a scope – I've done so on a few occasions, and it is <i>very</i> useful to have some hard data to back any theoretical basis for whatever problem you have!<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial",sans-serif;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial",sans-serif;mso-fareast-language:EN-US'>Tim</span><span style='font-size:10.0pt;font-family:"Arial",sans-serif'><o:p></o:p></span></p><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial",sans-serif;mso-ligatures:standardcontextual'>__________________________________________________________</span><span style='font-size:10.0pt;font-family:"Arial",sans-serif'><o:p></o:p></span></p><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial",sans-serif;mso-ligatures:standardcontextual'>Tim Stinchcombe <o:p></o:p></span></p><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial",sans-serif;mso-ligatures:standardcontextual'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial",sans-serif;mso-ligatures:standardcontextual'>Cheltenham, Glos, UK<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial",sans-serif;mso-ligatures:standardcontextual'>email: tim102@timstinchcombe.co.uk<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial",sans-serif;mso-ligatures:standardcontextual'>www.timstinchcombe.co.uk<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial",sans-serif;mso-fareast-language:EN-US'><o:p> </o:p></span></p><div style='border:none;border-left:solid blue 1.5pt;padding:0cm 0cm 0cm 4.0pt'><div><div style='border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0cm 0cm 0cm'><p class=MsoNormal><b><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri",sans-serif'>From:</span></b><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri",sans-serif'> Synth-diy <synth-diy-bounces@synth-diy.org> <b>On Behalf Of </b>Naoki Iwakami via Synth-diy<br><b>Sent:</b> 27 August 2025 08:29<br><b>To:</b> synth-diy@synth-diy.org<br><b>Subject:</b> [sdiy] Bad offset on VCA output<o:p></o:p></span></p></div></div><p class=MsoNormal><o:p> </o:p></p><div><p class=MsoNormal><span style='color:black'>I'm developing a small trimless VCA of size 7/8" x 3/8", but suffering from bad offset on output such as 0.4V for zero input with 5V CV. I tried the same circuit on a breadboard and 3" x 2" universal PCB. Both worked fine (no bad offsets). I suspected crowded PCB layout did some harm so let the gain unit transistor pair hang in the air to take some distance from the PCB — the problem disappeared then. I will redesign the PCB to eliminate this issue but cannot figure out what to move to take distance from the transistor pair.<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='color:black'><o:p> </o:p></span></p></div><div><p class=MsoNormal><span style='color:black'>Does anyone have similar experience, such as cupper beneath a semiconductor changes the behavior, transistors and opamps work correctly only when they keep certain distances, and so on? I'm using relatively small SMD components for this project (SOT-363, TSOT23-8, 0603, etc.).<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='color:black'><o:p> </o:p></span></p></div><div><p class=MsoNormal><span style='color:black'>I posted a blog article about this issue <a href="https://gaje.jp/2025/08/26/7810/">https://gaje.jp/2025/08/26/7810/</a><o:p></o:p></span></p></div><div><p class=MsoNormal><span style='color:black'><o:p> </o:p></span></p></div><div><p class=MsoNormal><span style='color:black'>Thank you,<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='color:black'>-- Naoki<o:p></o:p></span></p></div><div><p class=MsoNormal><span style='color:black'><o:p> </o:p></span></p></div><div><p class=MsoNormal><span style='color:black'><o:p> </o:p></span></p></div></div></div></body></html>