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Laptop Audio DC Offset

post #1 of 3
12/26/10 at 12:51am
Thread Starter 

One of the audio jacks on my ASUS laptop gives me a large "pop" when plugging in headphones.  So as a first check, I measured the DC offset between ground and the left and right channels, and found it was on the order of 1400 mV.  I'm not sure what an acceptable offset is for laptops, but this seems a bit off the charts to me.  For comparison, I'm not able to measure an offset with my voltmeter on my M^3 headphone amp.

 

Should I just strip out the DC offset with caps?  Or, am I better off sending in my laptop for repairs and loosing it for a month or two?  I'm not that confident they'll fix the problem.

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post #2 of 3
12/26/10 at 3:15am

That's around 1.4V, so it's definitely not safe for any of your headphones.

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post #3 of 3
12/26/10 at 4:06am
Quote:
Originally Posted by duanewhitney View Post

One of the audio jacks on my ASUS laptop gives me a large "pop" when plugging in headphones.  So as a first check, I measured the DC offset between ground and the left and right channels, and found it was on the order of 1400 mV.  I'm not sure what an acceptable offset is for laptops, but this seems a bit off the charts to me.  For comparison, I'm not able to measure an offset with my voltmeter on my M^3 headphone amp.

 

Should I just strip out the DC offset with caps?  Or, am I better off sending in my laptop for repairs and loosing it for a month or two?  I'm not that confident they'll fix the problem.

Make sure you aren't plugging into the microphone jack as that will have large D.C. offset by design in order to provide a voltage for the capcitative mike pickup to work.

 

If you are sure you are pluging it into the headphone jack then you need to have it serviced by a qualified computer electronics tech. Yes definately off the charts. Will blow any headphone attached to it in short order. D.C. blocking caps have apeared to fail. If dual rail supply & direct coupled you may have a failed DAC or input on Differential to single ended converter/buffer/filter. Most laptops though I believe already have coupling caps as most would have single rail supplies. In other words they would only have a plus power rail & no minus rail to balance it. That would in most cases demand a coupling cap at the output of each channel. Also most lappies would use the DACs inside the CODEC directly to the output though a coupling cap often with no external buffer at all. Only way that you could direct couple with a single ended power supply is if you had a ground reference amplifier & I can pretty much assure you that is not the case with a laptop. It is an extremely rare circuit as it is.

 

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