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Old 1st March 2008, 10:46 PM   #11
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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?
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Old 2nd March 2008, 12:14 AM   #12
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Quote:
Originally posted by audiobahnkid592
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....
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Old 2nd March 2008, 07:30 PM   #13
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i just tracked a scratchy channel at low volume to an opamp, the tantulum connected to the supply leg blew and caused the opamp to become defective....Dc offset was 0.579, but the higher i turned the volume the lower the offset would be, it would return to normal level at a certain volume, but at low volume sounded horrible.
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Old 2nd March 2008, 10:35 PM   #14
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If the cap failed (opened), it could have caused the op-amp to become unstable. It may have been oscillating at a low level.
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Old 8th September 2013, 11:03 PM   #15
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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.
Click the image to open in full size.
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Old 9th September 2013, 12:21 AM   #16
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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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Old 9th September 2013, 12:32 AM   #17
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Thanks just what I thought. So as long as my heat sinks are attached well and dissipate more heat than w/o the heat sink I should be okay. But I do see your method working as well, thanks for the reply friend.
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