How to check if there is a DC offset in the Amp Output

shankarcams

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Hi. I have been having some issues with my amp blowing speaker fuses at high levels (but not at too high a level) and I have been requested to see if the preamp or amp are leaking DC.

Would like some advice on how to go about doing that.

Thanks a lot
 

jls001

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You'll need a multimeter, preferably a model that has 2V range. You should be able to buy a decent multimeter for less than Rs 2000.

A DC offset means the amplifier generates its own DC voltage even when there is no external audio input. In an ideal situation it should be zero, but it's never zero and may vary between +/-2 millivolts. I've lived with about +30 mV DC offset on one channel as I could not reduce it further. It didn't blow my speakers but it was not comfortable.

How to test:
1/ with amplifier powered on, unplug all inputs, or select a source with no input.
2/ Unplug the speaker cables too.
3/ set the multimeter in DC Voltage mode at 2V range.
4/ touch red probe of meter to + terminal of speaker binding post of amplifier (any channel).
5/ touch black probe of meter to - terminal of speaker binding post of amplifier (same channel as red probe).
6/ the reading you see is the DC offset. If the value is >2V, set the range to the next higher range (usually 20VDC). Hopefully you should not need to do this.

Some people prefer to loop (also called short) the inputs. Plug RCA-RCA cable between - and + sockets of left channel (or XLR-XLR, as the case may be). Ditto for right channel.

To add: some preamps and amps have a DC blocking capacitor so that no DC passes to the output, while there are others that are DC-coupled (without DC blocking capacitors in the signal path).
 

frend2001

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Great Info jls001.

If I might add a few things :

1. DC offset could be negative as well. No problem if it's within suggested range as per the service manual.

2. I insist you look up the service manual about the suggested range in SM and if you're feeling confident enough, how to set it.

3. You may encounter no DC at speaker binding posts, in case of a Capacitor coupled amplifier, so look up about the model you want to check.

Searching GOOGLE baba usually is a great help and preliminary of doing anything with your gear if you aren't sure.

Hope this helps
 
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