29-09-2018, 05:46 AM
Thanks Simon and Trevor, nice tips. The stage is de-coupled from HT with a dropper and 10u cap.
Had a busy week on this amp, everything went wrong, it worked one day, the next oscillated like a demon on amphetamines., oh and the input stage started acting up. After much testing and investigation i found the LM317 regulated PSU that power the frontend and LTP was crashing to 1.2v ish, and found this was down to said reg going into current limiting foldback. bunged a TIP3055 series pass Q on, job's a good un.
Re oscillation, i ended up using a few RC step network's, I believe they are called to kick the oscillation in the head. So that 's a 2k2 and 2200p from each "anode" to ground, a 4k7 and 1000p across the front end anode R, and finally a 470r and 100n across the load. This is from memory, I was aiming for anything over 30khz attenuation , but think this is a tad low. I managed to pop a F counter on one oscillation briefly, it started about 50khz and ended at 100khz as the caps drained after SW off. Clocking HF like these is tricky as some are over 100v P-P in amplitude, one has to be quick.
Amp now steady as a rock, but I still have to test it on other speakers and alter the FB cap. briefly squarewave tested it yesterday = very good at 400hz and 1khz but iffy at 10khz, it's very rounded. Once I have a circuit that stay's the same for longer than a day, I'll draw it up if anyone's interested.
Andy.
Had a busy week on this amp, everything went wrong, it worked one day, the next oscillated like a demon on amphetamines., oh and the input stage started acting up. After much testing and investigation i found the LM317 regulated PSU that power the frontend and LTP was crashing to 1.2v ish, and found this was down to said reg going into current limiting foldback. bunged a TIP3055 series pass Q on, job's a good un.
Re oscillation, i ended up using a few RC step network's, I believe they are called to kick the oscillation in the head. So that 's a 2k2 and 2200p from each "anode" to ground, a 4k7 and 1000p across the front end anode R, and finally a 470r and 100n across the load. This is from memory, I was aiming for anything over 30khz attenuation , but think this is a tad low. I managed to pop a F counter on one oscillation briefly, it started about 50khz and ended at 100khz as the caps drained after SW off. Clocking HF like these is tricky as some are over 100v P-P in amplitude, one has to be quick.
Amp now steady as a rock, but I still have to test it on other speakers and alter the FB cap. briefly squarewave tested it yesterday = very good at 400hz and 1khz but iffy at 10khz, it's very rounded. Once I have a circuit that stay's the same for longer than a day, I'll draw it up if anyone's interested.
Andy.
