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Old 15th March 2007, 07:54 PM   #1
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Default LM3875 gainclone help!!!

Hello all,

I recently put together a gainclone amplifier from Peter Daniel's website. Obviously it doesn't work. I did everything exactly as instructed, except the chassis ground is not connected (CHG). There is no sound whatsoever coming from the amp and the 4 inner diodes in the power supply are extremely hot compared to the other diodes. When I disconnected the PS from the amp, 35 Volts were passing between V+ and V-. I have attachted (high) quality pictures of the front of the amp. If anyone knows what is wrong with it, please help!!!
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Old 15th March 2007, 07:56 PM   #2
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The back picture is attachted...
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Old 15th March 2007, 10:23 PM   #3
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If you have a meter, check the supply rails are where they should be first. Then we can move forward from there.

-SL
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Old 15th March 2007, 11:30 PM   #4
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Quote:
If you have a meter, check the supply rails are where they should be first.
SpittinLLama, how do i do that?
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Old 16th March 2007, 12:15 AM   #5
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Quote:
how do i do that?
Make sure you're getting +35 or so V on V+ and -35 or so V on V-.

Does your transformer have dual secondaries? Is your amp set up as a monoblock or stereo? I went through some problems with my second gainclone aswell so I might be able to help you out...

Tristan
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Old 16th March 2007, 12:30 AM   #6
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I checked and made sure that there is about +35 Volts on V+ and -35 Volts on V-. I don't know what you mean by monoblock...see pictures...they may help...As far as the transformer, all I know is that there are 2 wires for the input ~120 VAC and 2 wires for the output ~27-28 VAC.
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Old 16th March 2007, 12:50 AM   #7
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Carefully measure pin 1 on the lm3875 from ground and you should get +35, then measure pin 4 you should get -35 Volts there, careful not to slip and short anything though, it's easy to slip as I found out....

Also, do you have a heatsink connected? That was my problem everything was hooked up correctly, but the heatsink I had was not up to the task, so all i'd get was very quiet distortion, that would cut in and out, and no sound whatsoever if I had no heatsink hooked up, which was evidently the chips protection kicking in. Might be the problem, I didn't see a heatsink in the pics.

Good luck!
Tristan
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Old 16th March 2007, 01:34 AM   #8
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Kindly clean your PCB track side with a brush, in your picture i can see small wire burrs/ pices lying here and there..These may cause short circuit or heavy leakage currents.
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Old 16th March 2007, 01:43 AM   #9
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Here is what I did: I checked the Voltage between pin 1 and ground and got 1 Volt. Then I disconnected PG+ and PG- and got 35 Volts. What is PG??? Does the amplifier need PG???

I will clean PCB when I get a chance...
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Old 16th March 2007, 04:17 AM   #10
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Hi DavidWardlaw,

Your transformer should have 4 wires on the output side.

PG = Power Ground, there are 2, PG+ and PG-

Did the fuses blow?

You might want to have a look here at pictures of the BrianGT version of this amp for wiring examples. http://www.briangt.com/gallery/nigc

regards
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