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Old 21st May 2007, 03:40 AM   #11
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Quote:
Originally posted by Workhorse
Also show us your opamp frontend, and check for series connected back to back 2 zeners of value around 6.8V from inverting pin to output pin of opamp. if not there then connect them. They will solve your unstable clipping behaviour.
Thanks, I will have to work on the scans of the front end. There is a whole pre-amp section in front of this and I need to find a better way to scan and convert the drawing. I will have to work on it monday.


Thanks


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Old 21st May 2007, 04:12 AM   #12
Eva is offline Eva  Spain
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Have you already discarded a plain relay drive problem?
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Old 21st May 2007, 11:48 AM   #13
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Quote:
Originally posted by Eva
Have you already discarded a plain relay drive problem?

Well, the relays work fine for a while. for example, after i replaced the relay for the first time. the amp ran fine on the bench at low levels no problem. But when the amp clips, the relay contacts all melt together. they are literally ultrasonicly melting. the arm heats up and bends and shorts against both the NO and NC contacts and then shorts the amp out.

I am definitly changing that relay to something a little stronger but i have to fix the cause first!


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Old 21st May 2007, 02:13 PM   #14
Bonsai is offline Bonsai  Taiwan
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Try a 2-3uH coil in parrallel with a 2.2 Ohm resistor in series with output and the relay contacts.
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Old 21st May 2007, 03:18 PM   #15
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cool i will try that as well.


My concern is that, this amp didn't do this before!

It developed a problem and blow up for the previous owner. he had it repaired and it died again. then i acquired it. When i got it, i found the output relay shorted but the amp section worked, but had this problem. so there is something wrong i haven't found yet.


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Old 21st May 2007, 03:22 PM   #16
Bonsai is offline Bonsai  Taiwan
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Zero Cool, you say the amp was repaired:-

1. were the output devices replaced (with or without the same types as the original amp)?
2. How old is the amp?
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Old 21st May 2007, 05:51 PM   #17
jaycee is offline jaycee  United Kingdom
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What kind of opamp is in the front end ? The TL072 for example, will invert when clipping. It's also possible the opamp couldve been damaged by some abuse.
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Old 21st May 2007, 06:10 PM   #18
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the amp was repaired with parts from the factory by the previous owner. the exact same brand and model output mosfets were used and have colored coded matching dots on them. they came as a set.

This is an Ampeg SVT-350H bass amp and it has a pre-amp section in front of this power amp. I don't know where the mention of an op-amp came from? I will test the power amp without the pre-amp connected tonight.


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Old 21st May 2007, 10:53 PM   #19
Eva is offline Eva  Spain
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Poor relay drive when the supply rails sag is causing the contacts to arc at high current. Just my two cents.
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Old 21st May 2007, 11:40 PM   #20
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Quote:
Originally posted by Eva
Poor relay drive when the supply rails sag is causing the contacts to arc at high current. Just my two cents.
Thanks Eva, I removed the relay and put a jumper wire in its place during testing until i can buy some more relays. and the amp still has the problem. goes crazy close to clipping. I will do some more research on this and see whats going on. I need to figure out if the amp section does this when the pre-amp stage is disconnected and go from there.

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