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-   -   Running two LM1875 mono amp from single psu (https://www.diyaudio.com/forums/chip-amps/256566-running-lm1875-mono-amp-single-psu.html)

si82 20th May 2014 12:07 AM

Running two LM1875 mono amp from single psu
 
Hi
I'm new to chip amps although ive built many car audio systems.
I've Wired two lm1875 mono amplifiers to a single power supply only to have the left amplifier have no output.
Ive checked the resistance of the input and the other wiring for good conductivity and still the left amp doesn't output.
Is it wired incorrectly?

https://i1106.photobucket.com/albums...519_232745.jpg

DUG 20th May 2014 01:03 AM

The power looks OK. (Except I would have installed a fuse and switch in the AC feed...covered for safety, of course)

If you are using a common ground source and a 4 wire feed (ipod or computer) and the "in" and "gnd" were reversed on the left channel amp connections then the left would provide no output.

Connect to one amp at a time to test.

Is the left amp the top or the bottom?

The top amp IC pins look a little bent. Might have shorted or twisted the copper pads on the bottom of the board.

Just a guess.

:)

AndrewT 20th May 2014 03:05 PM

Mains power and non insulated terminals combined with non-earthed metal work !!!
Or is that an earth tag attached to the transformer bolt?

You live very dangerously.

I hope we continue to hear from you.

Fast Eddie D 20th May 2014 03:43 PM

Quote:

Originally Posted by AndrewT (https://www.diyaudio.com/forums/chip-amps/256566-running-lm1875-mono-amp-single-psu-post3933916.html#post3933916)
Mains power and non insulated terminals combined with non-earthed metal work !!!
Or is that an earth tag attached to the transformer bolt?

You live very dangerously.

I hope we continue to hear from you.

That's the first thing I thought. I don't even see a grommet for the mains wire.

Sloppy, careless wiring like that has started a few car fires too, so the OP might not be as familiar with car audio systems as they claim.

Mark Whitney 20th May 2014 04:45 PM

The boards are not connected in the same way. One board has the two white wires connected on the left side, the other board on the right side.

DUG 21st May 2014 12:11 AM

Quote:

Originally Posted by Mark Whitney (https://www.diyaudio.com/forums/chip-amps/256566-running-lm1875-mono-amp-single-psu-post3934051.html#post3934051)
The boards are not connected in the same way. One board has the two white wires connected on the left side, the other board on the right side.

I missed that.

:)

That looks like a definite problem.

johnr66 23rd May 2014 01:31 PM

Yes, one side has input connected to output and output to input! Transformer looks small-ish. Perhaps no more than 25va??

KatieandDad 23rd May 2014 01:38 PM

I can't see any insulation between the boards and the chassis.

danielwritesbac 24th May 2014 12:54 AM

Check with ohmmeter between LM1875's tab and the heatsink. There should not be any conductivity between those two points. LM1875 needs thermal pad and plastic shoulder washer.

P.S.
One of the boards is reversed with speaker cable at input side.

sreten 24th May 2014 01:08 AM

Hi,

Its wired up very wrong, and the transformer looks too small.
The top looks right, the bottom *rse about tit, simple mistake.

The transformer looks very inadequate, to do the job required.

rgds, sreten.


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