I just finished my amp rebuild and i noticed during final testing that the channel i just repaired, oscillates at a very very low level at start up before the protection relay kicks in. Its a clean 3+mhz signal. i did not get a chance to measure the voltage of it, but it is less then 1 volt p-p.
The oscillation goes away the second the protection relay kicks in and the output R-C gets connected. But there is a 10 second delay before that relay kicks in.
Should i be worried about this? Once the relay kicks in the amp runs great. no signs of oscillation. it only happens when the amp comes online, stabalizes and then there is the 10 second delay before the output relay kicks on that you can observe the waveform at the amp output.
Suggestions of what to look for???
Zc
The oscillation goes away the second the protection relay kicks in and the output R-C gets connected. But there is a 10 second delay before that relay kicks in.
Should i be worried about this? Once the relay kicks in the amp runs great. no signs of oscillation. it only happens when the amp comes online, stabalizes and then there is the 10 second delay before the output relay kicks on that you can observe the waveform at the amp output.
Suggestions of what to look for???
Zc
georgehifi said:
I will double check that. I know the input was connected to my sig gen at that time but the level was turned to zero. I will retest and put a shorting plug in the input and see what happens.
I will also test the -3db point.
Square wave response of both channels was not terribly fantastic. and the channel that has the problem seems to drain is caps quicker on power off. Running at 1/3 power for about 20 minutes. the tops of the TO-3 devices were running about 101-103 deg F on the problem channel and about 99-102F on the good channel. Bias is spot on and varies less then .03mv from device to device N or P.
Zc
Hi,
I had not thought about this when I have been re-quoting the general advice to put the RC Zobel at the output terminals.
Would a smaller current version of the Zobel be appropriate at the relay terminals to give the amp a high frequency load at start up prior to switching over to speaker output?
Usually 10r + 100nF is used but how about 15r or 18r + 47nF at the relay and similar at the speaker terminals.
I had not thought about this when I have been re-quoting the general advice to put the RC Zobel at the output terminals.
Would a smaller current version of the Zobel be appropriate at the relay terminals to give the amp a high frequency load at start up prior to switching over to speaker output?
Usually 10r + 100nF is used but how about 15r or 18r + 47nF at the relay and similar at the speaker terminals.
Hi,
using a DPDT will ensure the amp is loaded before switch over and again after switch over, but during switch over it will be unloaded.
What if that brief period was enough to cause a component failure somewhere just as the speakers are about to come on line?
9r1 or 10r + 47nF at both the relay and output should mimic the speaker connected load condition and give a bit of loading prior to and during switch over, maybe just enough to suppress the oscillation.
using a DPDT will ensure the amp is loaded before switch over and again after switch over, but during switch over it will be unloaded.
What if that brief period was enough to cause a component failure somewhere just as the speakers are about to come on line?
9r1 or 10r + 47nF at both the relay and output should mimic the speaker connected load condition and give a bit of loading prior to and during switch over, maybe just enough to suppress the oscillation.
pinkmouse said:
It is this Classe DR-15 amp i just rebuilt.
schematics here:
http://www.audio-circuit.dk/Schematics/CLASSE_DR-15-TM.pdf
pinkmouse said:
I replace all devices with parts from the factory. This was the amp where i noticed that the transitor gains were not what i would have expected. but again i was not testing them at the correct level. I had made several posts asking about that and asking if the missmatch could cause oscillations?
The transistors are MJ150024/25 devices according to the factory. Hfe for the Npn's were very low, 7-12 where the devices i removed were in the 30's. the P devices were in the high 80's low 90's where as the devices i removed were in the 60's i think if i remember correctly. a large variation fro the spec sheets and from the devices removed.
I was concerned about the N devices with the Hfe's so low. but again i was only using my little M3 tester that test's devices at a very low current and i dont know what the real HFE numbers are. only that they are different as compared to the devices removed and a batch of new onsemi factory parts.
I only replaced the outputs, Bias and the driver transistors as all the other devices tested good. The Bias transistor was still good but the factory reccomended replacement anyway. and i did varify that it was the same device. the drivers and outputs i cannot verify they are the same as classe polishes off the tops of the transistor cases so i have no idea what really is in the other channel.
No other parts were replaced. but i did check every diode, resistor and capacitor.
Zc
Ok so now i am starting to suspect my setup. I took the cover back off the amp tonight and ran it on the bench with the output relay disconnected and no matter what i did i could not get the oscillation back! the amp ran perfect.
The difference tonight is that i did not use the variac to bring the amp up. i just flipped the switch and let it come up as normal and no problems. i tried everything and nothing would make it break into oscillation.... Not that i wanted it to anyway....
I need to measure -3db down as suggested but i dont think my Fluke 189 will go high enough to measure -3, I think my meter is only good to 40Khz. and i think this amp will go up to 100khz or so according to specs.
Zc
The difference tonight is that i did not use the variac to bring the amp up. i just flipped the switch and let it come up as normal and no problems. i tried everything and nothing would make it break into oscillation.... Not that i wanted it to anyway....
I need to measure -3db down as suggested but i dont think my Fluke 189 will go high enough to measure -3, I think my meter is only good to 40Khz. and i think this amp will go up to 100khz or so according to specs.
Zc
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