Hum in one channel only

mjknight · 25th November 2011, 09:22 PM

I have an Almarro 318B. I have removed volume and input switching, using a Placette passive, and I have installed new coupling capacitors. I have a hum in the right channel only. Not affected by volume or tube switching. I have carefully looked at the point to point wiring, and nothing seems to be remiss. I also tried shielding the right channel because it is near the transformer, but with no effect. Any ideas?

gingertube · 28th November 2011, 10:52 PM

When you removed the Volume Control and Input Switching you most likely either:
1) left a shield disconnected
OR
2) introduced an extra shield to ground connection unitentionally, thus creating a ground loop

The place this often happens is right at the input connector, The connectors themselves should have their "ground" sides insulated from chassis a be connected to the amps 0V point via the shield of the lead from that connector.

A few things for you to check.
Cheers,
Ian

mjknight · 29th November 2011, 12:44 PM

Thanks Ian. A ground loop is certainly lurking somewhere. My wiring looks correct, but I didn't think to check the insulation of my new phono plugs from chassis ground. I'll do that. If no luck, I'll disconnect the inputs at the tube sockets to isolate the ground loop, one stage at a time.
Regards,
Michael

25th November 2011, 09:22 PM	#1
mjknight diyAudio Member Join Date: Mar 2011	Hum in one channel only I have an Almarro 318B. I have removed volume and input switching, using a Placette passive, and I have installed new coupling capacitors. I have a hum in the right channel only. Not affected by volume or tube switching. I have carefully looked at the point to point wiring, and nothing seems to be remiss. I also tried shielding the right channel because it is near the transformer, but with no effect. Any ideas?

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28th November 2011, 10:52 PM	#2
gingertube diyAudio Member Join Date: Oct 2004 Location: Adelaide South Oz	When you removed the Volume Control and Input Switching you most likely either: 1) left a shield disconnected OR 2) introduced an extra shield to ground connection unitentionally, thus creating a ground loop The place this often happens is right at the input connector, The connectors themselves should have their "ground" sides insulated from chassis a be connected to the amps 0V point via the shield of the lead from that connector. A few things for you to check. Cheers, Ian

29th November 2011, 12:44 PM	#3
mjknight diyAudio Member Join Date: Mar 2011	Thanks Ian. A ground loop is certainly lurking somewhere. My wiring looks correct, but I didn't think to check the insulation of my new phono plugs from chassis ground. I'll do that. If no luck, I'll disconnect the inputs at the tube sockets to isolate the ground loop, one stage at a time. Regards, Michael

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