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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => AC Power and Grounding => Topic started by: Nitin Sidhu on October 31, 2014, 02:50:45 pm

Title: ground loop off same drop
Post by: Nitin Sidhu on October 31, 2014, 02:50:45 pm
Hello!

What could be causing the following :

Stage left Power, single drop, powering the following :

1. 2 x Fender twin guitar amps
2. guitar tote with misc stomps.

If the stomps plug into both amps (stereo delay/chorus) hum in both amps, disconnecting the input on any one amp takes care of it.

Thank you for any insight,

Regards,
Sidhu
Title: Re: ground loop off same drop
Post by: Steve M Smith on October 31, 2014, 03:03:21 pm
The amps are both connected to ground via their power leads.  I am assuming that the effects also share a common ground but are not actually connected to a power ground as they usually use ungrounded power supplies or batteries.

Plug into one or the other amp and all you are doing is connecting the effects to ground.

Plug into both amps and you are connecting the grounds of the amps together via the effects.  As the amps already have their grounds connected via the power leads, this will create an earth loop.

The solution is to disconnect the ground in the 1/4" plug of one of the effects to amp signal leads at one end only.


Steve.
Title: Re: ground loop off same drop
Post by: Jerome Malsack on October 31, 2014, 08:48:12 pm
Try something like this on one of the 1/4. 

http://www.sweetwater.com/store/detail/Hum?adpos=1o1&creative=54989263441&device=c&matchtype=&network=g&gclid=CPLN2JGX2MECFWVp7AodmU0Agg

you will need a second 1/4 to 1/4 to connect to the amp. 

Connect the stomp box normal to one side with amp.  Then take the second output that when added causes the ground hum and add the ebtech box to transformer isolate the second amp.  Insert the second cable into input 1 and output 1 goes to the amp on the added 1/4 to 1/4 cable. 
Title: Re: ground loop off same drop
Post by: Steve M Smith on November 01, 2014, 03:22:51 am
You can prove that it's a simple ground loop caused by connecting the input grounds together quite simply:

Set it up so that the hum occurs with the effects connected to both amps.
Disconnect the inputs to both amps and check that the hum stops.
Connect the amps' inputs together with a single cable and if there is an earth loop, the hum should be present again.

Either the method I suggested of disconnecting the ground in one signal lead or the isolation transformer linked to above should work, but before modifying a cable or buying an isolator, make sure your signal leads are o.k.

When a ground loop occurs, the hum present in the shielded conductor of the lead is in series with the guitar or effects output and presents itself as a signal to the amplifier's input. If the shielded conductor has a higher than normal resistance, then the hum level will be higher.

Try the above test with a variety of leads and you might find that you have one which causes the hum to be worse than the others.  If this is one of the two leads which are causing the problem, then try replacing it.


Steve.
Title: Re: ground loop off same drop
Post by: Nitin Sidhu on November 01, 2014, 05:00:33 am
Understood. Thank you all.