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Overdream 3rd July 2012 03:30 PM

HUge dc offset

Im currently building a lm1875
Im building a dual mono... And im using toroidal transformer 160vac, with secondary output 2x18v

First of all, i was wiring the PSU...and i've got the output is 25V and i think it is great....

The im wiring up my amp.... Without input an output wired on my amp board, i was checkin the dc offset....
And i've got 22V !!!

The im browse any internet, and ive got 2 ways to measuring the dc offset

1. Short the input
2. Put 10 ohm resistor across the output

I did the first way, and got the same result 22v, but not do the second way yet...

Did i just broke my ic ????

Arty 3rd July 2012 04:01 PM

please, schematic of your build, and nice high resolution pictures.

Minion 4th July 2012 01:16 AM

You should measure DC offset with a load attached and the input grounded ......

Peter Daniel 4th July 2012 01:33 AM


Originally Posted by Overdream (
Did i just broke my ic ????

Not necessarily, you could be missing input ground connection.

Also, adding a load will not affect offset reading.

Overdream 4th July 2012 04:57 AM

thank you for replying....
is it same thing between loading 10ohm resistor across the output and put a speaker on the output ???

coz i think i just blew up my speaker ( lucky for me, its an unused speaker )
but when i load a speaker... i still got 22v on my output ( measure from + and - at the speaker )

but when i saw peter daniel thread, he has a huge dc too

but my question is.... i dont know what the function of 10ohm resistor... sorry i'm noob here......

and please find the schematic at the attachment.... i will attach my amp tonight
thank you very much

Overdream 4th July 2012 05:28 AM

but when i measure the output... and i dont use the ground from the amp board but from the wall ( nail on the wall :D )
i've got a tiny dc... its 10mv

gootee 5th July 2012 01:51 AM

Sorry about your speaker.

Voltages only exist between two points. For the output voltage, your wall ground is not one of those points.

Output voltage must be measured only between the two output connection points (where a speaker would be connected).

NO load should be used (no resistor and no speaker), when measuring output offset voltage.

The signal input should be shorted (connected) to the signal input ground, when measuring output offset voltage. (But I would also measure it when the source device is connected and turned on, but with no signal.)

You should NEVER connect speakers until AFTER you have checked the output offset, and verified that it is low-enough.

Output offset should typically be a few 10s of millivolts, or less.

gootee 5th July 2012 02:01 AM

Have you connected the G- and G+ grounds of your power supply together, at the power supply output, and then connected all of the amplifier grounds to that point?

godfrey 5th July 2012 02:17 AM


Originally Posted by Overdream (
First of all, i was wiring the PSU...and i've got the output is 25V and i think it is great....

The power supply should give +25V and -25V, total 50V from plus to minus.

Overdream 5th July 2012 04:52 AM

@gootee : no not yet.... i was wiring v+ g- g+ v- from power supply to amp board.... haven't do the other wiring... is this the problem ??

@godfrey : yes i mean 25V

i decided to buy a new ic today... and decided to switch the old one tonight.... hope everything going well

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