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Old 6th July 2004, 04:29 PM   #1
Bensen is offline Bensen  Belgium
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Unhappy I'm going crazy, please help, Pic's

Hello diy'ers,

I've build two crecende ME amplifiers. In the last few months, I had many problems with these amps, oscillation and stuff.
I rebuild them a few times on totaly different pcb's, other components, etc.

The last version work good for about 2weeks. Than suddenly, one channel was heatig up very fast. I measured the biascurrent, it was 3amps
I turned the bias pot to decrease the current, but the bias current stays the same??
IMO, there was heavy oscillation that destroyed my 4 FET's.

I added a few pic's. Maybe you can see any changes I have to make to minimalize the problem of oscillation.

Greets

Ben
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Old 6th July 2004, 04:31 PM   #2
Bensen is offline Bensen  Belgium
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A pic off the underside off the PCB with the output devices on it.
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Old 6th July 2004, 04:33 PM   #3
Bensen is offline Bensen  Belgium
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The PCB with the two input stages and VAS from the two ME amps.
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Old 6th July 2004, 04:34 PM   #4
Bensen is offline Bensen  Belgium
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The onderside off this PCB.
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Old 6th July 2004, 04:37 PM   #5
Bensen is offline Bensen  Belgium
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The power supply. 625VA with dc filter and soft start.
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Old 6th July 2004, 05:39 PM   #6
joensd is offline joensd  Germany
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The older crescendoŽs seem to oscillate when using wirewound resistors on output but the newer ME is not that sensible.
IŽm even using a wirewound with the ME and donŽt have any problems.
You might wanna have a look here .
See Samuels post.
At the time I built mine I had email-comtact with wuffwaff as he built quite a few of those. You might try to drop him a mail.
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Old 6th July 2004, 05:49 PM   #7
Bensen is offline Bensen  Belgium
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Thanks Joensd,

That is a good link that I didn't found yet.

I've contacted Wuffwaff en several others, but none off them was able to give a good solution. I've tried eveything they advised.

Ben
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Old 6th July 2004, 06:13 PM   #8
joensd is offline joensd  Germany
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What about the cap Samuel mentioned in his post?

BTW: Wanted to say "sensitive" which is "sensibel" in german
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Old 6th July 2004, 06:24 PM   #9
Bensen is offline Bensen  Belgium
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The meaning off these caps is to equalise the input capacity between de P and N FET's. I didn't do that on my original Crescendo which I build also with much less problems as the crescendo ME. If I'm not mistaking.

What I can try is to put the Zobel network after the output inductor?

When you see the back off my PCB's, I have many empty spaces. Now, if I fill these spaces up with ground plane, should this improve the stability??

I use long (20cm) wires between the PCB with the input stage and the PCB with the output devices, is this maybe also a thing I should consider to be a cause off oscilation?

Thanks
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Old 6th July 2004, 07:54 PM   #10
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Your problem is probably in too long all wires, mainly in gate circuits. Mosfet can oscilate very easy on frequencyes up to several hundred megaherz, so you can't see it easy on scope. Try to give there litte bit higher value of serial resistor in gate. You can also give directly on gate pin an ferite pearl.
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