Where are you taking the speaker output from? Can you draw how you have the amp PCB attached to the inputs/outputs and the psu PCB?
If the shepherd (AndrewT?) can guide me back to the right path, I will have to accept that... In my defense: I used the bulb whenever I could... tested it with bulb, tested it again with bulb, tested again with bulb, tested without bulb, tested again without bulb.... tested with load+source: Smoke.
another update... I only saw smoke on one side, but I am starting with the bulb tester now (omitting the softstart), and on both(!) channels I can't get the power supply higher than +/- 7.5V (bulb stays on).
Power supply unloaded is +/-23.5V
What steps should I follow to determine what's wrong?
Power supply unloaded is +/-23.5V
What steps should I follow to determine what's wrong?
I did...! One channel only, both the bad one and the 'good' one give this result: +\-7.5v!
Strange thing is that i did hear music! Not sure if that was on one ir two channels.
Edit: but hold on.... I now did another test, with a normal IEC cord (without bulb) and tested the 'good' channel:: I get normal input voltages again!! So the bulb tester influences the voltage to the channels (400VA transformer! 60W bulb). This also means that if I had the b*lls, I could test the 'bad' channel without the bulb and see what happens... I don't think I have them though 🙂
EDIT AGAIN... : I think my logic in the first edit was flawed... I have tested both channels before hooking up to speakers with the bulb tester, and the bulb went off like it should....
Strange thing is that i did hear music! Not sure if that was on one ir two channels.
Edit: but hold on.... I now did another test, with a normal IEC cord (without bulb) and tested the 'good' channel:: I get normal input voltages again!! So the bulb tester influences the voltage to the channels (400VA transformer! 60W bulb). This also means that if I had the b*lls, I could test the 'bad' channel without the bulb and see what happens... I don't think I have them though 🙂
EDIT AGAIN... : I think my logic in the first edit was flawed... I have tested both channels before hooking up to speakers with the bulb tester, and the bulb went off like it should....
Last edited:
are the Mosfet's in the wrong holes - seen that before - Check that IRFP240 in N channel side, and IRFP9240 for P-channel side - placed opposite of JFETs.
it's biased = drawing current , so bulb must lit and drop tooo much of mains
ditch bulb tester out and power up - without speakers,please
check bias and offset
ditch bulb tester out and power up - without speakers,please
check bias and offset
Fet's are mounted right next to the PCB, no wired from FET to PCB.
but input phono lead is like 7 meters, the problem i had was the volume control, one of the earths was not connected properly for some reason, also i had the phono cable close to the speaker cable which nelson said is not a good idea since oscilation can happen. all seems sorted now just hope it stays stable, will shortern wires!
R14 is the gate stopper ?
Will sort out the LED later, cheers!
Im using the ebay boards from jims audio. the resistors are actualy just over an inch from the Fets 🙁
No. sounds like a nice idea tho!
Last edited:
Ok... strangest things happening here:
I put down both trimmers P1 and P2 of the bad channel back to 0Ohm --> tested again with my last fuse and normal IEC cord --> Tataa... nice 47V supply power to the channel, of course 0 bias, and a little DC offset...
Then.. I though.. what the heck.. and I started adjusting the P1 and P2 trimmers... and guess what: I could just trim them to 0VDC offset and 0.5xxV bias... like nothing happened...
All of course on the bench with no speakers or preamp attached.
Perhaps the speakers are the problem? Do they work with other amplifier? (Just trying to eliminate unknowns)
I have two 4ohm 20w resistors here... Would it be a good idea to test with those, or are there steps I need to take first?
I guess the question that i have really is:
1. I can re-bias the amp, does that mean that nothing was damaged?
2. It seems its working with no load. What could have caused smoke with load? Too high bias?
1. I can re-bias the amp, does that mean that nothing was damaged?
2. It seems its working with no load. What could have caused smoke with load? Too high bias?
1. if you can rebias its very likely the amp to have survived the bad event (besides possible heat damage most likely on resistors without them being completely faulty)
2.what smoked?? Any damage visible? As long as the heatsinks didn't go beyond 60 degrees Celcius the bias should not have been too high.
2.what smoked?? Any damage visible? As long as the heatsinks didn't go beyond 60 degrees Celcius the bias should not have been too high.
Thanks
1. That's hopeful...There is no visible evidence of damage.
2. I had the lid on, so unfortunately i couldnt see what smoked.
I had already run the amp for 2-3 hours to warm up and bias. Then i unplugged it, carried it to the living room and hooked up the speakers and equipment. It had probably 10mins to cool down. Then i fired it up, started the preamp, and smoke appeared.... Probably within 10 or so seconds. Then i quickly shut it down... The heatsinks certainly were not 60 degrees
1. That's hopeful...There is no visible evidence of damage.
2. I had the lid on, so unfortunately i couldnt see what smoked.
I had already run the amp for 2-3 hours to warm up and bias. Then i unplugged it, carried it to the living room and hooked up the speakers and equipment. It had probably 10mins to cool down. Then i fired it up, started the preamp, and smoke appeared.... Probably within 10 or so seconds. Then i quickly shut it down... The heatsinks certainly were not 60 degrees