JoaMat Posted July 25 Report Share Posted July 25 On 7/17/2024 at 12:13 AM, justin said: Last year I ordered a lot of different boards of KG's amps/PSUs and had someone here put them together as a project. One of them was this 6DJ8 hybrid that uses the triple output buffer of the CFA3. Single-ended output only. There was 1 missing trace on the board that prevented it from working, found and fixed that, so possibly nobody had assembled one of these before. Problem is there's a substantial hiss (very easily audible with even sensitive planars, something like 0.3mV rms of 'white noise'. The +100V is coming from a TL783 regulator, tried AC, DC, batteries for heater, different grounding, gate stopper resistors, feedback caps, w/out servo..exhausted a lot of stuff but couldn't get more than a few % improvement on the noise. Anyone have ideas? th.PDF 51.92 kB · 23 downloads What dual op amp are you using? Quote Link to comment Share on other sites More sharing options...
justin Posted July 25 Report Share Posted July 25 have to look, probably either a OPA2134 or 2132 But I also tried it without the servo and using a pot instead Quote Link to comment Share on other sites More sharing options...
JoaMat Posted July 27 Report Share Posted July 27 (edited) Have had a new bias servo working with my CFA3smd something for a few days. Schematically it looks like this, and the new thing is in red square. Now I get a good reference point to the servos. Don’t understand why I didn’t figure this out earlier – slow brain perhaps. All four servos are fed from the same point. So, one trimmer and full control over the BIAS of all channels. Diagram for new servo - 6 minutes from cold. and diagram for old servo (in mA for the first 40 minutes from power on) Please have in mind that the graph for no servo is for my CFAsmd something. I guess the original CFA3 have a different and better behavior – where all tripple Darlington and vbe transistors are mounted on heat sink. Edited July 27 by JoaMat 6 Quote Link to comment Share on other sites More sharing options...
kevin gilmore Posted July 28 Author Report Share Posted July 28 would like to see what effect this has on thd. especially large voltage swings. still think an optoisolator drive to the vbe multiplier. Quote Link to comment Share on other sites More sharing options...
JoaMat Posted July 28 Report Share Posted July 28 (edited) Like this? Think I got inputs to op amp right. Edited July 28 by JoaMat 1 Quote Link to comment Share on other sites More sharing options...
kevin gilmore Posted July 28 Author Report Share Posted July 28 if it works right then yeah. Quote Link to comment Share on other sites More sharing options...
kevin gilmore Posted August 3 Author Report Share Posted August 3 (edited) so this is the way to do it correctly. its the only way not to seriously effect the thd. unfortunately both opamps have to take the voltages of the output devices. which means they have to be opa445 you set rv2 to the maximum amount of current. Then B1 is a 10k pot to V+ and ground. r11 is actually 100k. Edited August 4 by kevin gilmore 4 Quote Link to comment Share on other sites More sharing options...
JoaMat Posted Wednesday at 01:12 PM Report Share Posted Wednesday at 01:12 PM So, I got myself QuantAsylum QA-403 Audio Analyzer. Now I can make some simple analyses of amplifiers I build. Screenshot – my CFA3smd something with HD800 headphone, 2Vrms into headphone. Probably room for improvements. But I’m satisfied and I’m not competent enough to make it better. 4 Quote Link to comment Share on other sites More sharing options...
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