Ground Loop Hum using Qlab on Macbook Pro for Lights and Sound

Alex Smith

Member
Of course, this is a show in a space not intended for or designed for theatrical productions, which should have made me shy away but I never learn.

Running Qlab 4 for lights and sound on a 2016 MacBook Pro, into a GigaPort via USB, and out the GigaPort into a long 3.5mm cable (I know, I know) that runs to an amp and speakers that are part of the venue.

Also running a USB to 3pin DMX USBKing adapter out of the same laptop and to the DMX run for the lighting equipment (3 shoebox dimmers and 4 cheapo LED pars), and the run is terminated.

Classic Ground Loop hum (buzz) that's exacerbated when the conventionals dim. Ran Amp power to a new circuit on a different breaker. Also tried running with the laptop not plugged in to power. Buzz of course recedes only when DMX is unplugged from MacBook.

I've had ground loop issues when sound and lights share a circuit before, but this is my 3rd time in 3 separate venues (one designed for theatre, one not, and this one) running this Qlab LX/SX configuation and this is the first time I've encountered this challenge. I could run Wireless DMX from the Mac to the first dimmer pack if I had to, but I of course trust wired connections more.

Any thoughts? Research only turned this up: https://www.amazon.com/dp/B09BPZXF5Q/?tag=controlbooth-20

But don't quite have the time for it to make it to me before we go up.

Thanks for your time!

Alex
 
Adding a ground loop isolator to the USB or to the RCA connectors on the Gigaport will likely do the trick. Both of which are fairly inexpensive solutions.

Adding a ground loop isolator on the power to your Macbook like the one you linked to probably won't do anything. It seems like the ground loop is not intruding from the AC power but from the DMX line. Could be any number of reasons for that -- especially when you get into the realm of budget community theatre-type spaces, but basically the Macbook's USB connections are bridging the ground loop through your Macbook from the DMX cable over into the Gigaport's output cables.

Another option, little more expensive but more flexible long-term, is to switch to a Dante interface you can park near the amps and drive Qlab over Dante Virtual Soundcard. Then whatever venue you land in, you just need a network cable to get to your amps/speakers, wherever they may be. Could be Audinate's AVIO 2-channel interfaces, which require a POE source so you'd need a POE network switch to drive them, or a rackmount-style device. Probably not viable for this particular situation because most Dante equipment is on long backorder at the moment but hopefully in the next 3-6 months Audinate will have their chipset manufacturing cleared up.

I don't know how you're driving a long 3.5mm cable to the amps, but that's a suspect in the quest for a quality signal chain. Probably not the cause of your ground loop problem, but really should be driving the amps off balanced signals and it looks like the Gigaport only has unbalanced connections. Would be better off with a direct box, possibly a Whirlwind PCDI depending on your typical use case that allows you to convert an 1/8" TRS or dual RCA's into Left/Right balanced XLR's with a switch you can press to force into Stereo Left/Right on each cable to Mono Sum on both cables as-needed.
 
Get a Peavey USB-P direct box. It's a USB class-compliant device. It will show up as an audio device without the installation of drivers. It's fully transformer isolated (no ground to loop) and frankly will sound MUCH better than the 39 cent chip that drives the 3.5mm output on the computer. Oh, it's affordable at about US$80.

The issue with dimming noise - there is bad neutral connection and current is finding it's path back to origin via the equipment ground. It will be worst around 50% dimming.
 
Running the Dmx through an optical splitter might also fix the issue.
 
Get a Peavey USB-P direct box. It's a USB class-compliant device. It will show up as an audio device without the installation of drivers. It's fully transformer isolated (no ground to loop) and frankly will sound MUCH better than the 39 cent chip that drives the 3.5mm output on the computer. Oh, it's affordable at about US$80.

The issue with dimming noise - there is bad neutral connection and current is finding it's path back to origin via the equipment ground. It will be worst around 50% dimming.
I heartily endorse peavy usb-p. It’s been a great Swiss Army knife any time I needed to get sound from laptop to either power amps, pre-amps or a mixer
 

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