20-08-2020, 10:59 AM
Been mucking about these last few days trying to get my head round testing for stability which involves using a cap as load. I tried 0.1u, 0.470u and 2u. 2u2 was too much, sent the amp into instant oscillation. So, with various loads as described you do frequency sweeps looking for where THD jumps up/exceeds 2% THD or where the amplitude peaks. Results... I found an increase at 15hz to 18hz, an increase at 45khz and a peak at 111khz, I swopped 220p/2k2 for a 470p/3k which deals with the 111khz peak, swopped 12r6 (two 6r8's) and 270n zobel which deals with the 45khz.
Regarding the 15 - 18hz this is essentially the OPT core saturating I think, but I have had an issue where there's a thump and the ghost of the test tone on SW off, which is odd, as the hifi preamp vol cont was right down, therefore somehow the test tone is jumping over the vol control. The 47n and 1m step network should deal with any LF instabilty, but won't deal with the latter so I popped a jump lead from ground to g1 V1a, switched off, fault gone.
If you clock the preamp schematic, I can't see how a signal is jumping the VR4A therefore I'll try a small signal relay on the OP of the hifi amp so that when powered off, sigout is cut and V1A g1 connects to ground. With all this done the amp is as far as I can test, stable. An increase in NFB results in no instability.
One last issue found, on SW on it sounds like a bag of ferrets are inside the amp or restless Aunty Maud with piles and a bag of humbugs at the pictures is at the controls. I went though the amp with a stick and gave everything a good poke, looking for dry/iffy joints. Non found but did find V1a/b was microphonic, changed it, better, but still microphonic. There is something else going off, the noise settles down after 5 minutes. I checked all OP valves for jumps in Ik, checked the HT, 300v and a few other rails for voltage spikes, non found, reckon this could be a resistor, but all are new, what do you reckon?
Andy.
Regarding the 15 - 18hz this is essentially the OPT core saturating I think, but I have had an issue where there's a thump and the ghost of the test tone on SW off, which is odd, as the hifi preamp vol cont was right down, therefore somehow the test tone is jumping over the vol control. The 47n and 1m step network should deal with any LF instabilty, but won't deal with the latter so I popped a jump lead from ground to g1 V1a, switched off, fault gone.
If you clock the preamp schematic, I can't see how a signal is jumping the VR4A therefore I'll try a small signal relay on the OP of the hifi amp so that when powered off, sigout is cut and V1A g1 connects to ground. With all this done the amp is as far as I can test, stable. An increase in NFB results in no instability.
One last issue found, on SW on it sounds like a bag of ferrets are inside the amp or restless Aunty Maud with piles and a bag of humbugs at the pictures is at the controls. I went though the amp with a stick and gave everything a good poke, looking for dry/iffy joints. Non found but did find V1a/b was microphonic, changed it, better, but still microphonic. There is something else going off, the noise settles down after 5 minutes. I checked all OP valves for jumps in Ik, checked the HT, 300v and a few other rails for voltage spikes, non found, reckon this could be a resistor, but all are new, what do you reckon?
Andy.
