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spooney 29th February 2008 11:06 AM

What is DC offset?
I've read many threads on here about repairs that talk about dc offset but I am still confused as to what exactly dc offset is and what it means as far as amp repairs.It would be greatly appreciated if somebody could shed some light on the subject for me.

richie00boy 29th February 2008 11:11 AM

Its a DC voltage at the amp output. Typically if it's over 100mV that's not very good.

Perry Babin 29th February 2008 11:19 AM

Ideally, there will be no DC voltage (offset voltage) across the speaker terminals of an amplifier. Most amplifiers have a small amount of DC. Most people (those building or repairing amplifiers) consider 0.025v or less to be acceptable. 0.1v may be acceptable for others. Most ampliifers in good working order will have significantly less than 0.1v of DC offset.

Generally, when an amplifier fails and has DC offset, the DC voltage across the speaker terminals is approximately equal to the rail voltage.

1moreamp 29th February 2008 06:26 PM

I look and test for it. I also try to repair its causes. I like to see Like new spec's for this tech spec. I use the DC offset figure as a gauge of amp abuse, and shortened life span. Sorta like a gauge as to a amps overall health indication.

Just goggle DC offset and you will see others info posts about it, and its links to THD, and noise figures. I feel its a important thing to look at, But this is only my opinion,,,:)

spooney 29th February 2008 07:10 PM

what are some things that can cause the dc offset to rise?

1moreamp 29th February 2008 07:39 PM

Diff input pair damage, I find DC offset here, and its thermally enhanced by unequal temperatures of this group of transistors. Leaky outputs and drivers, bad caps. Resistors that have overheated and altered value especially in the input section. Leaky diode also can be a fault for this issue. I have also seem power supply imbalance picked up and sent to the output as DC offset due to bad filter caps in older amps.

PPI used a simple two resistor with a small value pot in the middle across the 15 volt rails to make a low level DC signal to insert in the invert input so any normal offset could be Nulled out at the input stage. I find this a simple method to the possible mismatch and or aged components of the amp circuitry.

All in all its a sure sign of many things. As most amps today are balanced complimentary design and with that comes DC offset related issues when the semi's have been aged by heat, abuse, and just plain old age and use. Nothing lasts forever, if it did we would only buy one of anything, and that would hinder world commerce....Especially Chinese imports :)

jol50 29th February 2008 08:20 PM

If that does not make sense, the job of the amp is to make the speaker go in/out with AC current....+ voltage one way and - voltage the other. Sitting there idle with no input it should have 0 current to the speaker AC and never have DC....but few amps are perfect and a little makes no difference. Ones I see usually are under 50mv, better ones under 30 or 10mv even. Some amps are 0 depending on how they are made. I have one with 250mv I am trying to fix, if I would get to work in it again and put the new caps in :) but not sure what is wrong.

1moreamp 1st March 2008 12:02 AM

with experience you will see what will pass for a reasonable offset spec. Many class D amps have like 250 to 500 MVDC offset at idle.

I have seen many new amps fresh out the box with 25 MVDC offset. But this was a imbalance in the power supply causing this, as when bridged the offset was 3 MVDC, so it was common to all channels and was inverted on the bridge channel so it canceled when bridged.

I generally shoot for the lowest possible offset figures, but sometimes that requires way too much time to be worthwhile so I generally talk to the customer about these, so a consensus can be drawn on the amp value, as opposed to repair cost.

On PPI's I generally am able to reduce the offset by adjustment to less than 1 MVDC without issue. Just a few minutes to adjust and is easily verified on burn-in final test.

Lower is better...:)

spooney 1st March 2008 12:29 PM

I have an amp on the bench right now that has .003 volts on the channel I just repaired and .027 volts on the channel that was original to the amp.Is this difference going to cause any major issues?

richie00boy 1st March 2008 03:09 PM

None whatsoever. It's not the difference that matters really unless you are running it bridged.

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