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Old 22nd June 2004, 03:10 AM   #1
Duo is offline Duo  Canada
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Default Devastated with amplifier.

Well, my work in the audio amplifier field so far has left me devastated.

My latest design was working happily today until when I went to bias it properly, it exploded in my face. Blowing all eight of the output transistors, which are mj21193, and 21194. And it blew the drivers. I have to find out 'why' it did this.

I'm just so disappointed after I spent months working on this and it sounded so good.

Maybe I'll post a schematic soon and see if anyone can help me with the autopsy.

Anyone here who's had this happen?
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Old 22nd June 2004, 03:20 AM   #2
paulb is offline paulb  Canada
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Yeah, I blew a whole bunch of $100 power op amps one day (not an audio application). It sucks.
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Old 22nd June 2004, 03:25 AM   #3
Duo is offline Duo  Canada
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It's just awful.

It's not so much the value of the parts, but the amount of work that went into building it.
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Old 22nd June 2004, 03:37 AM   #4
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I've had it happen to me, not entirely sure why. I think under one circumstance I was using a compound configuration in the output and just turning up the bias sent it into oscillation, something sort of easy to do with that circuit. Aother time I think I remember using a metal bladed screwdriver to adjust the pot which maybe sparked something, but also maybe the output was unstable anyway and just waiting to be turned on. These things happened with first time experimental circuits too, and I've long since used plastic tuning tools.
Since your circuit was already up and working, it's a head scratcher to me. Maybe you brushed up against something unintentionaly. If you used a metal blade to adjust, maybe ESD got you somehow. I have heard of amplifiers with certain output configurations behaving ok until one day, they blow, perhaps due to transistor(s) properties altering over time and thermal cycling. I'd be interested in knowing more myself.
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Old 22nd June 2004, 03:38 AM   #5
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So you accidentally cranked the bias too high and a part blew
and took the rest of them with it. Happens to all of us.
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Old 22nd June 2004, 05:36 AM   #6
Duo is offline Duo  Canada
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Well, this was by adjusting a pot with my bare hands. So no metal in there.

It has been working properly.

But I think the reason it blew was because of a few possibilities.

One of the most probable was that I had the wires from the board to the output transistors too close and I think two of them shorted because there was an amazing spark there. It had some real current behind it and was very loud.

Next, I know now that there is an inherent current sharing problem in my topology. I didn't implement the CFP properly. The way I had it caused feedback only from one of the transistors on each rail, instead of all of them.

And next on the list. My LTP didn't have the most matched transistors around, so the amplifier had a small offset of almost half a volt. I think that might be a real problem when rising the bias current don't you?

Anyway, I'm completely re-designing it with more output transistors (10 per channel instead of 8) and more stability, lower distortion, and power management for extra safety and clean power up and down.
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Old 22nd June 2004, 06:58 AM   #7
wytco0 is offline wytco0  United Kingdom
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Duo what a shame but Its good to see that its not just the amateurs and complete beginners like me, that have problems from time to time. Nelson thanks for admitting that the professionals do sometimes as well.
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Old 22nd June 2004, 07:18 AM   #8
karma is offline karma  Canada
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better yours than a customers than it really sucks

my amp died last week

sorry for your loss
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Old 23rd June 2004, 01:53 AM   #9
Duo is offline Duo  Canada
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Well, I do believe I've found out how my amplifier fried.

After some serious autopsy, and then a look at the schematic, I've realized that my Vbe multiplier bias circuit was not implemented correctly.

By turning the pot enough, you can easily cause the multiplier to lose bias and open wide. This in turn causes the output transistors to conduct as hard as they can as the current source in my VAS causes one rail to be pulled, and the VAS itself pulls the other.
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Old 23rd June 2004, 02:01 AM   #10
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one thing I find useful in experimenting with designs is to put a power resistor in serial with the rail(s). it acts like a current limiter. it worked like a charm for me.
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