Sources of 120hz hum?

rntlee · 30th January 2013, 05:52 PM

Hi,
I have a Chinese amp (yaqin ms-20l) that has recently developed a slight 120 Hz hum in the right channel. What would be the likely source of the noise?

simon7000 · 30th January 2013, 05:54 PM

Loose screw on a mounting lug, bad connector connection. Be sure to swap inputs.

rntlee · 30th January 2013, 06:03 PM

Hum remains if I remove the preamp tubes, so I guess that narrows it to the power amp side of the circuit? Bias is fine as well.

TheGimp · 30th January 2013, 06:14 PM

Possibly shift in bias of the output tubes. This causes the PS ripple rejection to go down and you get more hum.

How did you check the bias?

Otherwise, it could be a bad output tube. Swap them between the two channels.

rntlee · 30th January 2013, 06:56 PM

Looks like it's a noisy tube, thanks Gimp!

30th January 2013, 05:52 PM	#1
rntlee diyAudio Member Join Date: Jan 2012	Sources of 120hz hum? Hi, I have a Chinese amp (yaqin ms-20l) that has recently developed a slight 120 Hz hum in the right channel. What would be the likely source of the noise?

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30th January 2013, 05:54 PM	#2
simon7000 diyAudio Member Join Date: Nov 2008 Location: Oakmont PA	Loose screw on a mounting lug, bad connector connection. Be sure to swap inputs.

30th January 2013, 06:03 PM	#3
rntlee diyAudio Member Join Date: Jan 2012	Hum remains if I remove the preamp tubes, so I guess that narrows it to the power amp side of the circuit? Bias is fine as well.

30th January 2013, 06:14 PM	#4
TheGimp diyAudio Member Join Date: Aug 2009 Location: Johnson City, TN	Possibly shift in bias of the output tubes. This causes the PS ripple rejection to go down and you get more hum. How did you check the bias? Otherwise, it could be a bad output tube. Swap them between the two channels.

30th January 2013, 06:56 PM	#5
rntlee diyAudio Member Join Date: Jan 2012	Looks like it's a noisy tube, thanks Gimp!