Handling ground loops without chasing ghosts

[Alan]

You finish up the wiring, hit the power switch, and suddenly, there it is… that annoying hum you never asked for. If you’ve been in this situation, you know what I mean.

Let’s dive into some practical tips for managing ground loops in audio and control setups with multiple power supplies.

 

[Circuitsmith]

The first time I heard that hum, I was convinced my speakers were haunted. Turned out, a rogue ground loop was messing things up now I always check my power paths first!

 

[cap&resistor]

I spent half the day dealing with a buzzing console, only to discover two power supplies were clashing. Now, I label everything and keep my cables organized life without hum is wonderful.

 

[barfle]

Back in the day, I worked for a company that installed video cameras, and ground loops were a common bugaboo. Isolating the cameras from local ground usually cleaned it up. It’s important to just have a single ground reference, because “ground” (dirt) sucks as a conductor, and often actually generates noise, just because of the various and undefined chemical reactions that can happen.

 

[BrianB]

Hums are not all caused by ground loops. I was called in for a hum in a rack mounted commercial sound reinforcement system. Just a few mic channels out of about a dozen. Spent some time troubleshooting before I had the "Duh" moment. Turns out one of the installers had set a wall wart inside the rack on top of the multi channel mic level preamp/processor.

Which reminds me of the time I went to a local AES chapter meeting and the presenter couldn't get his oscilloscope to display the signal. He spent about 5 minutes messing with knobs and connections before someone finally walked up and removed the small woofer that was sitting on top of the scope. This was 40 years ago when scopes used CRTs and that woofer magnet was pulling the scope trace right up through the top of the display.

 

[barfle]

Hums are not all caused by ground loops. I was called in for a hum in a rack mounted commercial sound reinforcement system. Just a few mic channels out of about a dozen. Spent some time troubleshooting before I had the "Duh" moment. Turns out one of the installers had set a wall wart inside the rack on top of the multi channel mic level preamp/processor.

Which reminds me of the time I went to a local AES chapter meeting and the presenter couldn't get his oscilloscope to display the signal. He spent about 5 minutes messing with knobs and connections before someone finally walked up and removed the small woofer that was sitting on top of the scope. This was 40 years ago when scopes used CRTs and that woofer magnet was pulling the scope trace right up through the top of the display.

That’s definitely true. I remember getting called to a site because the rack was “buzzing,” even when it was off. It turned out to be a fluorescent ballast in the fixture over the rack!

 

[BrianB]

CATV from incoming service can be notorious. Not just low level hum. Had to fly back to a large commercial job once where they hooked up the cable TV after I left and one of the audio power amplifiers blew up. The cable tuner box had a 2-wire ungrounded power cord. There was almost 20VAC between the coax shield and system ground. You should have seen the spark when I earth grounded that shield!

Had another one where I couldn't get the entertainment lighting console to work. Turns out they had pulled power to the booth from two separate power panels in different parts of the building. Back in the days when electrical safety ground was provided solely by the conduit (maybe still is in some jurisdictions?). There was about 12 - 13V potential between the controller power supply conduit and the load power supply conduit. The 0 - 10V lighting logic control signals were getting swamped. Getting rid of that ground potential difference solved that one.

 

[Alan]

Thanks for sharing. Those stories really show how tricky grounding issues can be and why it’s so important to pay attention to detail during complex installations.