I have build 2 of these and 1 has developed a fault where after switch on everything is OK and I can leave it with no signal for as long as I like BUT as soon as a signal is applied the standing current goes crazy 10A+. It blows the +ve fuse but not the -Ve fuse and even stranger is that the speaker does not deflect. Also before the fuse blows you would not know anything was wrong judging by the sound from the speaker. Any help much appreciated
Are you using TO-3 outputs? You could have developed a short to heat sink, either on the case or one of the leads. It’s happened to me.
Yes TO-3 ( 2N3773 ) and no short, I've checked. What is also really odd is if I flick the power off then on again immediately the current drops instantly to 100mA or what ever I set it to !! ( as long as there is no signal )
HF oscillation, scope the output and see if you can trigger on it.
Try making C3 at least 3.3nF instead.
Try making C3 at least 3.3nF instead.
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Yeah, if it’s not the output accidentally shorted to a grounded heat sink it‘s got to be oscillating. BTW, T7 needs a base stopper resistor. Unrelated to this problem, but you will discover more if you don’t.
That’s the bias resistor for the diodes. The two CCS bases need to be isolated from each other, so that T7 can saturate (clip) and not upset the voltage biasing T4. I‘ve had too many of these latch up and stick to the rail, sending DC to the speaker.
And 10 amps is a lot of cross-conduction current. What kind of “2N3773’s” are you using?
And 10 amps is a lot of cross-conduction current. What kind of “2N3773’s” are you using?
I made a similar amp and had oscillation.
It needed a few ohms from driver to base of outputs to cure it.
I would also add small cap from base to collector of T10.
The lower CFP does often need “compensating” depending on the exact choice of driver and output. And “designers” often make the mistake of also adding cap to c-b of T9 - which does NOT have the same effect of adding one to T10.
Ive sometimes even just given up and stuck PNPs in the bottom half and everything magically works perfectly. After jacking with it to no avail for a couple of weeks. Is it stable if you lower the power supply voltage to something like +/-28V?
Ive sometimes even just given up and stuck PNPs in the bottom half and everything magically works perfectly. After jacking with it to no avail for a couple of weeks. Is it stable if you lower the power supply voltage to something like +/-28V?
Thanks everybody for all your advice, I found the problem around transistors T2 T3, I started testing components by desoldering ( not easy ) and luckily since replacing these 2 tranny's all is well !
Hi Jabiru, what was the problem? Were one of T2/T3 faulty or something else? Just curios.
Cheers
Cheers
You made an amp which is hfe dependent...bc556 usually has too low hfe for the job.try 2sa992 with higher hfe instead.you'd have it more stable.
Hi, to be honest I'm not sure, I just replaced them although they tested ok so it could have been a dry joint on the pcb. I got through a lot of fuses !!
What was happening does sound like a weird symptom of a messed up diff pair. Usually you get DC to the speaker.
Absolutely not on this occasion I was testing with a 5" speaker on the bench and the cone hardly moved ! speaker still working perfectly
Go buy a lottery ticket - you’re on a roll now. Testing an amp that big with no dim bulb, on a 5 inch speaker and a fault that would usually send the cone and voice coil into the middle of next week.
Yup sometimes it's good to just take the fix and move on.
BC556's are fine in that location, provided they are genuine. Can substitute with pretty well any small signal transistor, including the 992.