Diagnosing source of hum in my rig.

Posted: 2/20/2020 7:08:14 PM

Joined: 1/29/2020

I had a bummer of a night at an open jam session last night. Set up to play on a tight stage with two guitars, bass, keys, and a vocalist. There were lights, monitors, amps, PA speakers, and each guitarist had an effects rig.

I’ve regularly played this venue with this setup. But last night there was a pronounced hum in my signals chain, from the theremin, through my effects, to my amp. Had to quit early—sound from my instrument was crap.

All gear worked FINE when I got it home—the problem was with something at the venue.

Other than checking my own gear, what are other common sources of this kind of interference on a tight stage at a rowdy rock show?

Can cables to mics/monitors/PA create the kind of interference I was experiencing?


Posted: 2/20/2020 7:09:33 PM

Joined: 1/29/2020

Can amps next to other amps create a hum?

I had my keyboard amp next to a Vox tube amp and a solid state fender bass amp.

Posted: 2/20/2020 7:29:12 PM

From: 60 Miles North of San Diego, CA

Joined: 10/1/2014

Amp next to Amp would not be my first choice.

I would check the quality of earth ground to the theremin. I wanted to make a gadget for this people could check earth ground before they set up or what receptacles were best to use as it is so critical to public theremin performing. There was no interest here. Earth ground has three main purposes few understand.

This is different than a ground check plug in, that also is not a bad place to start. Ground quality could be done with a mini oscilloscope looking at the distortion of an RF wave, which needs earth ground. This is important in my getting a nice sound.

There may have been slight hum and then someone started using ground loop isolators on the main power cord which really screwed you up.

All we can do is speculate from our sofas.


Posted: 2/20/2020 9:13:49 PM

From: earth

Joined: 5/8/2017

LED lights with cheap switching transformers. Wall warts with with cheap switching transformers.

For best grounding, use a long instrument cable and lay it across the floor and if you really want to you can tuck some of the wire into your sock or take off your shoes and stand on the cable in stocking feet.The Big Briar Etherwave was very susceptible back in the day.

Since I got the Model 302 theremin I do not have to worry about such things. With  the battery, it is never connected to power. Its ground is the stand and the instrument cable. Once I picked up radio station in SF - the only anomaly.

Big Bow - Rupert Chappelle, Theremin

Posted: 2/22/2020 12:07:38 AM

Joined: 1/29/2020

Thanks for the speculation.

Last time something like this happened, I got an amp stand (although I didn’t use it this time). Lifted the amp up and away from the other amps, maybe my amp’s proximity to the other gear was the reason.

We’ll see ahead of my next gig ( in a tighter space!)

Thanks for speculating.

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