What is DC offset?

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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.
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,,,:)
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 :)
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.
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...:)
audiobahnkid592 said:
both class D amps i repaired were really low one was 0.003V and one was 0.005V. So this means my amps are in great shape? or how does it work?

Lower DC Offset is better...:)

Low is good....

Zero actually exists in the real world. Positive or negative means nothing. It's the number itself and how far from the prefect world of zero offset that counts IMO.
The further away from the perfect zero the more I study the amp wondering if the amp is even worth fixing.

Also consider this, Most high quality amps have some sort of DC output sensing for the built in protection circuitry. The higher the DC output the closer you are to tripping the amp off line before the amp gets to full unclipped power.
I have a sweet looking Zapco 500 sitting here. It has just under 60 MVDC out at idle no signal no load. Customers complaint it was cutting on and off while playing. Well DUH ! I wonder why with 60 milli-volts DC just leaking out without a signal applied.

So you see DC OFFSET is important, and it does effect the proper operation of many amps. Bridged or not. It is something worth paying attention to in most cases.

This Zapco has a small fan located underneath it, and it was mounted into carpet so it got no cooling air inside it until it was way too late. The amp baked on itself so pretty much everything inside sits and leaks DC out from stage to stage to output.

I use it to hold the door open now....:)
Sorry to bring this back from the dead, but I have a question regarding DC offset. Am working on my PG 600.2 TI amplifier and notice that the dc offset for the left was around 55-60mv and around 35-40 for the right. Well I notice that one pre-driver in each channel was causing the dc offset to be reduce by simply cooling it or touching it. So I added heat sinks to them and now the dc offset for the left is 20mv and 5mv for the right.

So could I assume those pre-drivers are bad/faulty or should I be checking something else? Or is it safe to leave the heat sinks on since dc offset is now pretty low at an acceptable range.

Here a pic of the pre-driver that I attached the heat sink to. This is for one channel but both pre-drivers where in the front side and outer sections.
Seen this before on PG, on older M series amps. I just remove and flipped the transistor so it rested against the bottom plate as a heatsink once the board was reinstalled. I saw PG add clip on sinks to some older M series amps so this is where i got the idea to just flip them to the bottom of the board and rest them against the metal bottom plate. You can replace them but its a heat related issue and will eventually come back. This is why I opt for the bottom of the PC board mounting...;)
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