Buzz when second amp is plugged in

travis_38

travis_38

Active Member
Tonight I noticed a pretty loud hum when I had two amps plugged in. I tried using the B isolation but it didn't make any difference. I'm using loops 9 and 10 in stereo, and going out into a passive junction box then to two different amps. Any ideas?
 
Basically if you have multiple devices plugged into different outlets and the audio ground is connected between it makes 2 or more paths to ground potentially with different voltages. That causes the hum.
 
Had the same issue again at rehearsal last night. Ground lifting one amp (the not-safe way) fixed it. Since my PBC outputs are kind of buried I had to use the junction box still. If the junction box is the issue what's the best way to isolate the two channels? Will I need two separate boxes for left and right?
 
I had a similar problem with mine. An isolation transformer solved it for me. I think the RJM tone saver would do it. ( http://www.rjmmusic.com/products/tone-saver/ ). In my case, because I'm in the UK I just went for a cheap and cheerful ART version ( http://artproaudio.com/isolators/product/dti/ ) with two separate channels from a local music shop who said they would take it back if it didn't do what I needed. Having two separate channels means that, if required, I can put one on the Amp A output from PBC and another on the Out 5 - 6 going to the front end of my amp for the 4 cable setup. It turned out that I only needed the Out 5 - 6 but I still have another channel available in future if required.
 
You'll have to do some testing to see where it's coming from, but if you remove the junction box and the hum goes away, then doing some modifications on the junction box should fix it. Replace at least one pair of jacks with plastic bodied jacks so they don't connect to the chassis and short to the other jacks. Neutrik and Cliff make good ones.

Or, just add an isolation transformer as mentioned. If you're in North America, the Hum-X is a safe way to lift the power ground on one amp and has less effect on tone than an audio isolation transformer would. (They only make a 120V version as far as I know)
 
rjmmusic said:
You'll have to do some testing to see where it's coming from, but if you remove the junction box and the hum goes away, then doing some modifications on the junction box should fix it. Replace at least one pair of jacks with plastic bodied jacks so they don't connect to the chassis and short to the other jacks. Neutrik and Cliff make good ones.

Or, just add an isolation transformer as mentioned. If you're in North America, the Hum-X is a safe way to lift the power ground on one amp and has less effect on tone than an audio isolation transformer would. (They only make a 120V version as far as I know)
Click to expand...

Thanks for the info, I think I could handle the mod to my junction box. Would the in and out that aren't connecting to the box need to be grounded together since they're not connected via the box?
 
Yes, connect the in and out pair so they're grounded together. Just make sure they don't connect to anything else.
 
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