Mosfet amp goes crazy at clipping

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Zero Cool

Zero Cool

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Joined 2004
I am working on a power amp that uses the 240/9240 mosfets. the amp works fine at all levels until you get to clipping then it goes nuts.

I couldnt figure out why this thing was burning up output relays. the relays are rated for 10 amps.

I replaced the relay and the amp ran fine. In fact it ran into a dead short until i figured out that the output relay had disintegrated and shorted all the contacts together (one being ground). I removed the relay and the amp ran fin on the bench. nice clean signal with no load up to clip.

So i replaced the relay, put it back together again and tested it. sounded great. so i cranked it up a little but and the power light flickered and then dimmed and then no sound. power light still on.

Back down to the bench i had no output. after a bit of checking, output relay melted again. this time i removed the relay and re tested the amp. works great. hooked up a 8 ohm resistive load and cranked it up. looks good until you get close to clip, then a little bit of what i would describe as noise on the wave form. Hook up a 4 ohm load and NOW the problem becomes clear!!!

Just before clip with a 4 ohm load, lots of noise appears on the waveform. then it starts to look like ripple, then the amp goes massivly into clipping and the scope screen is a scrambled mess of massivly clipped waverform. the amp starts mechanically screaming a very high pitched squeal.

I think it was ultrasonically welding the relay contacts together!

back the input signal down and the amp works fine. its only when the amp gets near full power does the problem happen.

So now i am wondering if it is something in the IV limiter, or if the mosfets are bad???


Any ideas???


Zc
 
H

HDTVman

Member
Joined 2001
It sounds to me like the amp is starting to oscillate as it goes into clipping. Try adding some bypass caps from the PS rails to grd. @ the outputs. .1uf might to the trick or try .1 ohm series resistors in the power supply lines to the outputs.

My guess is that the clipping is causing the power supply to ring like a bell and that the ringing noise is getting into the drive circuits causing the oscillation.

Just a thought, what's it cost to give it a try?

BZ:geezer:

PS what are you using for a scope?
 
Zero Cool

Zero Cool

Member
Joined 2004
HDTVman said:
It sounds to me like the amp is starting to oscillate as it goes into clipping. Try adding some bypass caps from the PS rails to grd. @ the outputs. .1uf might to the trick or try .1 ohm series resistors in the power supply lines to the outputs.

My guess is that the clipping is causing the power supply to ring like a bell and that the ringing noise is getting into the drive circuits causing the oscillation.

Just a thought, what's it cost to give it a try?

BZ:geezer:

PS what are you using for a scope?
Click to expand...


I will try that. I do notice that when i bring the amp up on a variac slow, it oscillates until the rails get closer to there rated voltage.

I have a sencor SC-61 for a scope. 60Mhz usable to 100Mhz it says. not a great scope but i like the built in DVM and Freq Counter plus the delta T functions. makes some calibrations really easy.
 
Zero Cool

Zero Cool

Member
Joined 2004
Workhorse said:
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.
Click to expand...

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


Zc
 
Zero Cool

Zero Cool

Member
Joined 2004
Eva said:
Have you already discarded a plain relay drive problem?
Click to expand...


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!


Zc
 
Zero Cool

Zero Cool

Member
Joined 2004
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.


Zc
 
Zero Cool

Zero Cool

Member
Joined 2004
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.


Zc
 
Zero Cool

Zero Cool

Member
Joined 2004
Eva said:
Poor relay drive when the supply rails sag is causing the contacts to arc at high current. Just my two cents.
Click to expand...

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.

Zc
 
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