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Chip Amps Amplifiers based on integrated circuits

Running two LM1875 mono amp from single psu
Running two LM1875 mono amp from single psu
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Old 20th May 2014, 12:07 AM   #1
si82 is offline si82  United Kingdom
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Default 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?

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Old 20th May 2014, 01:03 AM   #2
DUG is offline DUG  Canada
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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.

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Old 20th May 2014, 03:05 PM   #3
AndrewT is offline AndrewT  Scotland
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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.
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Old 20th May 2014, 03:43 PM   #4
Fast Eddie D is offline Fast Eddie D  United States
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Quote:
Originally Posted by AndrewT View Post
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.

Last edited by Fast Eddie D; 20th May 2014 at 03:45 PM.
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Old 20th May 2014, 04:45 PM   #5
Mark Whitney is offline Mark Whitney  Netherlands
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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.
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Old 21st May 2014, 12:11 AM   #6
DUG is offline DUG  Canada
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Quote:
Originally Posted by Mark Whitney View Post
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.
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Old 23rd May 2014, 01:31 PM   #7
johnr66 is offline johnr66  United States
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Yes, one side has input connected to output and output to input! Transformer looks small-ish. Perhaps no more than 25va??
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Old 23rd May 2014, 01:38 PM   #8
KatieandDad is offline KatieandDad  United Kingdom
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I can't see any insulation between the boards and the chassis.
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Old 24th May 2014, 12:54 AM   #9
danielwritesbac is offline danielwritesbac  United States
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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.
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Old 24th May 2014, 01:08 AM   #10
sreten is offline sreten  United Kingdom
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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.

Last edited by sreten; 24th May 2014 at 01:11 AM.
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