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Old 21st March 2004, 04:36 AM   #1
Jamh is offline Jamh  United States
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Default GC amp problem

Hi everyone,

I built my first GC, based on the LM3875T, and verified everything, but when I plugged it in with no load, and let it sit for 2 minutes, the 1000 uF caps blew! The fuses didn't go off though..

Can someone shed a light as to what I did wrong?

The 1000 uF caps were NTE, rated at 35V at 85 degrees.

Basically, here's what I did:

pin 1 -> + of the bridge (full wave, 100PRV 30 Amp), and + of the 1000uF cap. Then from the - of the cap to the ground.

pin 3 -> 20K into pin 8, also to speaker +

pin 4 -> - of the bridge, and - of the 1000uF cap. Then from the + of the cap to the ground.

pin 7 -> 1k resistor -> variable pot (50 K) -> input

pin 8 -> (20 K from pin 3), then to 1k, then to 22uF, then to ground.

Single heat sink for both chips, but not grounded. The transformer/bridge make 16.53Volts (plitron dual 15V) that I verified before hooking everything up.

Three of the the 4 1000uF caps blew up at the same time. My circuit must be screwed up somewhere.

BTW, do you think the chips are toast?

Thanks for the help!
Jam
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Old 21st March 2004, 08:31 AM   #2
Nuuk is offline Nuuk  United Kingdom
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Difficult to say but it looks like the 1000 uF caps were wired the wrong way around. Are you sure that you connected them the right way?

Did you test your PSU before connecting it to the amp circuit?
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Old 21st March 2004, 12:02 PM   #3
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ya the same happened to me. I had 4800uf on the ps board of a 3886 amp, and a 330uf on the actual board. They were all rated at 35V, and none on the ps board blew, but the second i plugged it in, the lights dimmed, and the 330s started spraying out this horrible smelling smoke. I think they were in backwards.

-Mike
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Old 21st March 2004, 03:54 PM   #4
Jamh is offline Jamh  United States
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My first thoughts were reversed caps too. It wasn't clearly marked on these caps, but they had a black band on one side, I assumed that was the positive (no other markings of any sorts).

Everything else looks right to everyone? In particular, was it ok to have the uninsulated LM chips on the same heat sink if it's not grounded? Or the mica is a must?

It broke my heart that it blew, the housing came out really nice.

Thanks much.
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Old 21st March 2004, 03:57 PM   #5
Jamh is offline Jamh  United States
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To answer the questions, I followed the schematic I supplied with respect to the caps' orientation, + of the cap to the + of the bridge, - of the cap to the - of the bridge.

I did test the PSU, and it was very steady at 16.53V.
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Old 21st March 2004, 07:34 PM   #6
Nuuk is offline Nuuk  United Kingdom
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Quote:
In particular, was it ok to have the uninsulated LM chips on the same heat sink if it's not grounded? Or the mica is a must?
I would say not. As I remember, the case of the chip is at the negative voltage level but of course it is the same for chips. However, I would not think that it is good practice to have them uninsulated on the same piece of metal.
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Old 21st March 2004, 08:43 PM   #7
Morse is offline Morse  United States
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Hi Jamh;

>>>...In particular, was it ok to have the uninsulated LM chips on the same heat sink if it's not grounded? Or the mica is a must?...<<<

I'm just guessing here, but could it be that the amp went into ultrasonic oscillation because of this? In which case, you'd have lots of current draw, even if it was not receiving an input signal.

If you did not allow a return path directly to the PS trafo, then the PS caps are the only return path - and all the extra current of oscillation might have been enough to smoke 'em. I made the same mistake with a virtual ground on my first GC - it worked fine, but as was pointed out by another contributor the PS caps were the return path for all current drawn, which offers some real possibilities for disaster.

Good luck and all the best,
Morse
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Old 21st March 2004, 08:46 PM   #8
adx is offline adx  New Zealand
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Quote:
Originally posted by Jamh
My first thoughts were reversed caps too. It wasn't clearly marked on these caps, but they had a black band on one side, I assumed that was the positive (no other markings of any sorts).
The black band is negative! (unless it's a tantalum...)

Your chips are probably fine. Wash off the stinky oil and stick in some new caps!
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Old 22nd March 2004, 12:30 AM   #9
Jamh is offline Jamh  United States
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Ok, the problem is definitely with the heat sinks, as I double verified the caps and all. So here are some questions:

1. When you put the mica behind the chips, won't the screw make contact with the heat sink and the chip anyways? Or you have to put 2 layers, one in front, one in back, and make sure the screw doesn't touch the sides? That seems pretty hard.

2. How thick should it be, and where can I get them.

3. What is the purpose of the LM uninsulated chips if they're not meant to be used with ungrounded heat sinks?

Again, thank you all for helping out.
Jam
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Old 22nd March 2004, 12:38 AM   #10
Jamh is offline Jamh  United States
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Here's a picture of the thing.
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