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Old 3rd November 2009, 05:09 PM   #11
waltube is offline waltube  Italy
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Hi,

try to put a 1/2w resistor- 10 ohm - in series to each ground of the input pin jack.
This will break the ground loop.

Ciao

Walter
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Old 3rd November 2009, 05:32 PM   #12
pjanda1 is offline pjanda1  United States
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This is a common occurrence when implementing plate amps. Posts about it appeared frequently back in the days when I visted the Parts Express boards. I must admit that in my plate amp days, I used an IEC cord with the earth prong sawed off. Sheldon's advice is considerably safer. Wouldn't it also be reasonably safe to disconnect the signal ground from the chassis, but still leave the chassis connected to earth?

I later battled a ground loop with my CATV and chip amp. I ended up with a resistor between the signal and chassis/safety grounds. I tried such a solution on my tube amp, but any resistance between the two (with the cap to short HF) caused a hum. I'm not smart enough to know why, but I no longer have cable TV (not as a result of the hum). As long as the tube amp is the only thing with a third prong, I'm fine.

I suspect that once you do anything to change the potential between the signal and safety grounds on your plate amp(s?), you'll eliminate your hum.

Paul
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Old 3rd November 2009, 06:26 PM   #13
waltube is offline waltube  Italy
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Quote:
Originally Posted by waltube View Post
Hi,

try to put a 1/2w resistor- 10 ohm - in series to each ground of the input pin jack that must be insulated from chassis.
This will break the ground loop.

Ciao

Walter
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Old 3rd November 2009, 09:01 PM   #14
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With my audio systems my earthing depends on what is connected.

1/ WIth a floating input I switch the zero volts on the amp to earth.
2/ WIth an earthed input I unswitch the zero volts on the amp from earth.
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Old 4th November 2009, 04:10 AM   #15
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Yes, the cheater plug eliminated the hum.

I think (?) Paul and Nigel are saying the same thing.
(Disconnect the signal ground from the chassis but still leave the chassic connected to earth / with the earthed input unswitch the
zero volts on the amp from earth.
By this do you simply mean to cut the shield on one side of the interconnect.
Otherwise, of the two previous replys, I will probably try Waters fix first, as I can get the resistor easily from Radio Shack.
I take it that I would wire in a resistor in series with the ground in my pre-amp for the left and right connections for the sub amp (pre-out) connection only, and don't do it for the pre-out to the full range amplifier
Thanks again
Paul
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Old 4th November 2009, 02:24 PM   #16
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by pforeman View Post
Yes, the cheater plug eliminated the hum.

I think (?) Paul and Nigel are saying the same thing.
(Disconnect the signal ground from the chassis but still leave the chassic connected to earth / with the earthed input unswitch the
zero volts on the amp from earth.
By this do you simply mean to cut the shield on one side of the interconnect.
Otherwise, of the two previous replys, I will probably try Waters fix first, as I can get the resistor easily from Radio Shack.
I take it that I would wire in a resistor in series with the ground in my pre-amp for the left and right connections for the sub amp (pre-out) connection only, and don't do it for the pre-out to the full range amplifier
Thanks again
Paul
The basic idea is that you'd like to have a single point where the signal common (zero volts) and earth are connected. It's best to have this point at the preamp. I wouldn't lift this connection. If it is lifted, I would make sure that the lifting resistor value for all downstream components is at least as high as the preamp resistor.

For safety reasons, you want all chassis earthed through the mains plug.

So, you can lift the signal from earth at your amps by disconnecting the signal common from the chassis (or putting a switch in series between the earth and signal common).

Or you can try, as waltube suggested, and put a resistor between signal common and earth on the amps, so that any noise on the earth side will be somewhat isolated from the signal commons.

Or you can diode lift the earth connections, as I suggested.

I'd try the easiest thing first, which will depend on what you can get at and how connections are made in the amp.

Cutting the shield connection at one end won't typically affect low frequency hum. It's usually more of an RF issue. If you are picking up radio stations, or see signs of oscillation in a component, you might try looking at it. I think the most important thing is to have a low impedance path to earth at least at one end of the interconnect. In environments with lots of RF, it may be better to have it only at one end - probably the preamp end again.

Sheldon

Last edited by Sheldon; 4th November 2009 at 02:30 PM.
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