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Old 14th April 2007, 04:29 AM   #1
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Default stopping a ground loop through the power cord

What's the best way to stop a ground loop through the power cord? I plugged my amp into the outlet with a 3 blade to 2 blade adapter and the hum went away. I still want to keep the safety ground intact for obvious reasons.
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Old 14th April 2007, 06:09 AM   #2
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is there anything else connected to the amp that is grounded? if so, you can leave the ground lift on...... or put the ground lift on the other device and ground the system through the amp
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Old 14th April 2007, 05:31 PM   #3
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My computer, turntable, and AM/FM tuner are wired through a mixer to the input of the amp. The computer has a 3 blade power cord. The turntable has a 2 blade power cord. The mixer is powered from a wall wart. I made a modification to the amp a couple days ago. It's a homemade amp. I mounted some 1/4" phone sockets in it for speaker outputs so I could use professional speakers with it. The ground (sleeve) of these is metal and is mounted in a metal chassis. The original speaker terminals are the spring clamp type. The phone sockets are paralleled off the original speaker terminals. The wires are maybe 5 inches long. Would that make a ground loop?
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Old 14th April 2007, 05:56 PM   #4
jcx is online now jcx  United States
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It is illegal and unsafe to rely on signal interconnect to provide chassis/safety gnd

The chassis/safety gnd must always be connected when the amp is powered and must be able to carry enough fault current to reliably blow the fuse in the event of a internal live to case short circuit neither may be the true with signal IC providing chassis gnd

If the rest of the safety gnd is correct then using isolated output jacks should be fine
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Old 14th April 2007, 06:22 PM   #5
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I've had that problem with when hooking the PC as a signal source because the soundcards signal ground is usually grounded instead of floating. What happens is you connect the PC to the amp which both have a grounded audio grounds. So they how have two paths to ground through the RCA cable sheild.

there are a couple things I'd try before removing the ground pin from the amp as that is probably the least safe of options.

The simplest solution:
-Hack the shield off one end of the RCA's that run from your sound card to your selector box(IF they are RCA's). If it's 1/8 the inch connectors at both ends, find a way to isolate the ground conductor at one end.

Less jimmy rig solutions that are more difficult:
-Tie your amplifiers audio ground to chassis ground at the RCA input jacks. Detach the existing audio- to chassis ground point. usually soundcards are grounded at the output, so if you ground your amp at the input this will take the circuitry out of the loop. Depending on the ground layout of the amp this option may not be suitable.
- Or, Tie a resistor between the Input jack grounds and audio ground(some where around 100 ohm). This resistance should be high enough to eliminate the loop effect, but low enough to pull down the the audio ground of floating sources.
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Old 14th April 2007, 06:48 PM   #6
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My star ground in the amp is at the power supply center tap return. The input leads run from the rca jacks in the amp to the volume controls then to the each channel. Do the resistors go between the ground side of the input jacks and power return? Here is an old picture of the amp and its layout and wiring.
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Old 14th April 2007, 07:20 PM   #7
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If your phone plug output jacks are grounded to the chassis, that's probably bad. It may not be the source of your hum, but the original spring clip connectors probably ran a wire back to the star ground at the filter caps. The phone plug jacks should be isolated from the chassis, and grounded back at the star the same way, lest the speaker current have to find its way back to the caps through the chassis.
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Old 14th April 2007, 08:12 PM   #8
Jeb-D. is offline Jeb-D.  United States
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It appears you have two mono amp modules running off one power supply. In this case you have it grounded properly, at the star point. If you were to change the ground point to the inputs in your case, would cause an internal ground loop in the amp. The resistor idea would more than likely cause a ground loop in the amp for this circumstance as well. Unless you were to wire it in a way which would cause a noise compromise in one of the channels.

Your best bet is to isolate the PC's ground from the amps ground by causing a break in the signal cables ground conductor. I have two do this in one of my amps as well. Since it has a dual-mono type audio circuit sharing a power supply.

Also, in your config the input jacks should be isolated from the chassis and each input jack should have it's own ground conductor. The two channels grounds must only connect together at one point, which is at the star point. It appears in your amp the two inputs are sharing the ground conductor then splitting to the two modules. That will cause a problem. But if it's one cable and the ground conductors are separated that is ok.

Make sure the star point is not physically located between the transformer CT and reservoir caps. The heavy charge currents going between the two make a noisy ground. Power should go from transformer CT to reservoir cap grounds. Reservoir cap ground tied directly together. Then from the reservoir cap ground make a T -off and that should be your star point for the amp modules current return ground. Connect the chassis ground to the Star point, not to the transformer CT. It looks like you for the most part got it right.
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Old 14th April 2007, 10:01 PM   #9
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It's good to know I wired the amp correctly both in the 120 volt section, signal, speaker, and DC sections. The speaker returns actually tie into the amp module PC boards and come back to the supply via the DC neutral lead from each amp module. The signal cable from the input RCA jacks is a separated cable with two signal leads and two ground leads. The amp was quiet before I installed the phone sockets. It's dead quiet with open circuit inputs and ground isolated sources (i.e. a Discman). I have some rubber bushings I can put in the chassis holes to isolate the phone jacks from the chassis.
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Old 11th May 2007, 12:08 AM   #10
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Default It's fixed

I drilled out the holes for the phone sockets and put some rubber bushings in. That broke the loop. Now the amp is silent at idle. Thanks for all your help in fixing my problem.
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