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Old 3rd September 2004, 12:46 PM   #1
jives11 is offline jives11  Europe
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Default hum loop advice

Hi,

This is not specifically a tube issue but this seemed the best place to kick off with my hum problem


I have a Radford STA15 tube amp powering LS3/5a speakers. Sound is good. I use a Quad 44 preamp which I like for it's slope controls and a Cambridge CD player.

Normally I get very little hum from the amp and have to place my ear against the speaker grill to hear any. Mechanical hum from the transformer is actually louder, but she is an 40 year old amp and I forgive her that. Components are replaced and input and phase splitter tubes have a DC supply.

However when I connect the amp to the preamp and turn on I get noticable hum through only one channel (left). This is independent of volume, but annoying in quiet passages.

The Quad 44 provides mains sockets from the back which are slaved off of the pre-amp power switch. Again I feature I like. This way turning on the pre-amp turns on the power amp.


My guess is I have a hum loop as I have both a mains ground link between the pre and power through the slave socket AND a ground connection via the interconnect grounds.

The Quad mannual suggests , under these circumstances, disconnecting the power amp ground at the plug, relying purely on the interconnect ground. Not sure if this is a wise thing to do in case of a catastrophic failure. Presumably the alternative would be to break the interconnect ground connection in either a) the offending channel or b) both channels.


Any thoughts or similar experiences ?


many thanks
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Old 3rd September 2004, 08:38 PM   #2
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Hi Jives,

do NOT break the interconnect ground connection.
If you insist on using the main ground for safety reasons, you can break the ground loop by adding a resistor in this current path.

Guess the easiest way to do so is in the Quad, remove the internal ground wire and connect a 10 ohm resistor to restore the connection.

However in my opion it shoud be quite safe to remove this ground connection on the Quad.

In some countries this connection is even omitted.

Dick.
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Old 3rd September 2004, 09:10 PM   #3
jives11 is offline jives11  Europe
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Many thanks Dick,

Assuming I did this at the amp, do you mean float the main "safety" ground connection with the 10Ohm or float the interconnect ground ?
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Old 3rd September 2004, 11:37 PM   #4
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Quote:
Assuming I did this at the amp, do you mean float the main "safety" ground connection with the 10Ohm or float the interconnect ground ?
While both options should work, I meant the main safety ground as this will have no sound degradation.

Dick
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Old 6th September 2004, 02:54 AM   #5
EC8010 is offline EC8010  United Kingdom
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Your problem is that both the Quad and the Radford have their signal earth connected to the chassis. For safety, the chassis is bonded to mains earth, so when you connect them together, you get a hum loop. The safe solution is to find the connection between the signal earth inside the Radford and the chassis, and break it. It will almost certainly be close to the input valves, and may even be at the input sockets.

Never, ever, remove the earth bond at the mains plug.
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