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Old 14th July 2009, 10:47 AM   #1
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Default Class A project with some smokage

Hello users of DIY, my class A project is coming along very well but I have run into some issues in the final stretch. Here is the schematic of the amp:
Click the image to open in full size.

The power supply runs perfectly and so does one channel. I completed the other channel and upon the first powered run the 10R resistor (farthest right in schematic) overheated and started to smoke. I thought this was strange so I checked all the wiring and circuit and it seemed to be fine, identical to the working one. The output transistors are also all working fine. So I turned it on again, and now my inductors in the power supply are emitting a high pitched squeal. The squeal isn't there with the other channel, it still works fine.

The speaker output reads about 30V AC and 10V DC. The squeal is also quite alarming and makes me think something is going to violently explode. I'm not too sure what to think what the cause would be.
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Old 14th July 2009, 11:12 AM   #2
AndrewT is offline AndrewT  Scotland
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hi,
the amp is oscillating.
It will overheat and eventually some/all of the devices will explode.

Did you ask ESP for permission to post their schematic?
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regards Andrew T.
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Old 14th July 2009, 11:29 AM   #3
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You know your stuff don't you. I did noticed the other channel amplifying some repetitive noise when both were running. Before I go on the hunt for oscillation knowledge, any suggestions on eliminating it?

Oh and I guess I didn't read the ESP disclaimer close enough, I'll take the schematic down.
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Old 14th July 2009, 01:05 PM   #4
paulb is offline paulb  Canada
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I'm still seeing the schematic...
ESP has a technical forum, you might try there. You can refer to the schematic by ESP project number and everybody will know what you mean. Chances are that it's related to your layout and/or wiring.
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Old 15th July 2009, 10:28 AM   #5
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It's Project 10 from the ESP site.

I went ahead and reheated all the solder joints and even tested the individual paths with an ohmmeter. The output transistors all work fine, and as far as I cant tell all the resistors on the board are working fine (might as well be sure). I'm positive none of the smaller transistors are oriented incorrectly and use plastic casings so no chance of shorting each other on the heat sink. The layout is about 98% the same as the working channel, the difference being I cut some leads shorter. The wires going to the output transistors which use snap connectors are also making good contact.

I think I'm going to start testing the transistors on the board to see if by fluke one of them is bad even though all parts are brand new. Once I'm certain all the parts are working perfectly I'm not sure what I'll do next...

I want to rule out layout and wiring because of it being an almost exact mirror of a working channel.
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Old 15th July 2009, 12:09 PM   #6
DRC is offline DRC  United Kingdom
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Quote:
I want to rule out layout and wiring because of it being an almost exact mirror of a working channel.
I don't think you can rule out oscillation so easily, as individual component variations, especially in the semiconductors, will have a significant effect.

dave
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Old 15th July 2009, 10:18 PM   #7
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Link to the project -> http://www.sound.westhost.com/project10.htm

I can't believe my first circuit worked first try. I thought the next one would be just as simple. I'm going to start tearing it apart then. I'm sure I'll find something eventually.

Just a question before I go, will a DC line crossing a signal path potentially create noise or oscillation? This is point to point wiring so in some spots different wires cross or run beside each other or other components.

Finally, is it possible to edit old posts on this site?
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Old 21st July 2009, 07:18 AM   #8
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Well I'm officially a tool. After rebuilding half the circuit going through all the transistors and parts and finding nothing wrong I set my voltmeter across some other smoking resistors only to find the voltage to be about 3 times what it should. Well the voltage across those resistors are controlled by a multi turn pot which I was positive I had set to it's max resistance. It turns out it was at 0Ohms. I thought I had fried it, but after about 4 rotations it finally started to increase and about another 10 turns and it was at 1K. I thought I had these pots figured out. Guess not. So basically problem solved, all due to my dumb-assery.
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Old 21st July 2009, 07:31 AM   #9
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That's how we learn best...from our mistakes.
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Old 21st July 2009, 08:15 AM   #10
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I thought I spoke too soon, but well....not really.

With the source and load unhooked, the amp is silent when you turn it on. When you turn it off however, the inductor makes a noise like really small turbine spinning down. With the source and load hooked up everything is silent but that 10R still gets hot as hell. It'll burn you pretty good if you as much as poke it. Good news is it doesn't smoke anymore. I've yet to see if the other channel's resistor does that. If so, I'll just get some physically bigger ones.

The thing with the inductors making the noise upon power down is quite strange indeed, at least to me. The power supply is simple enough being rectifier > +- rail > several caps > inductor > amp channel. The original plans placed the inductors farther back in the chain, between some of the caps but this caused a bad hum so I moved them to the end and problem solved.

It now produces sound just like the other channel so I want to say all is well but I feel as if something is not totally fixed. I'll hopefully abolish my suspicions as I get it all back together again.
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