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nico 27th February 2013 02:37 AM

Headphone amp noise. Help!
Hi, Headphone amp inside the Xitel Soundaround SRS unit ($10 USD) is very noisy. LM4881 headphone amp is used. When I remove the 20k volume pot output is dead quiet. With pot in the noise is at max when the volume is at minimum and decreases when volume is at the maximum but still noisy. I disconnected the signal going in to the pot, also tried 10k alps pot but no luck.

Question: non-inverting circuit with feedback resistor 120k/22k (gain of about 6), is the resistor value too high? The circuit is so simple, I can not think of anything else.

Any help is appreciated. I wanted ask before messing with the SMTs.

Thanks, Nico

kroto 28th February 2013 02:00 AM

I think 120k is too high. higher value = higher noise.
try this : Ci=2u2, Ri=4k7, Rf=22k.

nico 7th March 2013 10:04 AM

Kroto, thanks for the input. I finally had time to work on it. I changed Ri=3k Rf=10k also Ri=10k Rf=47k during second try. It definately solved the problem with both set but now the 20k pot makes scratching noise when it is turned. I heard about this problem but this is the first time I am experiencing it. No big deal. Now the head amp is quiet when the volume is at minimum.

Thanks again!

weslito 9th March 2013 07:54 PM

Sure it's non-inverting? Figure 1 of the LM4881 datasheet looks like it can only be used in inverting topology.
The scratching noise is DC flowing through the pot, usually caused by the device's input bias current. If it were non-inverting I wouldn't have thought changing the feedback values would have any effect on the pot. Inverting, however, Ri is in series with the pot, and in theory would affect the bias current, though I wouldn't have thought 22K would be enough to do this.
You'd usually use a coupling cap between the pot and amp input to stop it altogether.

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