Ground Loop

I’ve been pulling my hair out, trying to piece together an even cleaner way to integrate various A/V components in the RETROvan.

My plan involves installing a second dual-gang outlet box on aft rear wall panel D6 using two USB-enabled 120V outlets and a pair of RCA audio jacks. This also involved carefully researching and purchasing three new cables and Leviton QuickPort modules. Those cables have to be the right lengths, they have to have the right connectors—and most importantly they have to be adequately shielded against RF interference.

The goal is to be able to connect my 5K iMac to the wall, and then to my AppRadio 4 receiver via the receiver’s AV input jacks, which are male RCA plugs. This direct connection would essentially eliminate any latency when playing music from my AKAI MIDI Controller keyboard, via Apple’s Logic Pro X DAW (Digital Audio Workstation). If connected merely by Bluetooth audio, you press a key and then hear the sound a half second later. And that’s not ideal. Normally this isn’t a problem because most of the time, you’re consuming A/V content not in realtime, but on a timeline that is kept in sync by your Mac and its operating system.

What I didn’t anticipate is something called “ground loop” interference. This manifests as a loud, low-pitched humming or buzzing noise playing through your speakers. This can happen when the components of your audio systems are connected to different power sources and/or at different voltages. There may be other causes, such as running an audio cable next to a power line, but that’s not a concern here.

In my case, my AppRadio is powered directly by a 12V circuit because it’s essentially a car radio. But my iMac is powered by a 120V wall outlet, fed from my ProMariner inverter. And apparently, the iMac’s headphone jack is not properly isolated or shielded. I know this because I can plug my new cable into the headphone jack of my TV or iPhone and the sound is crystal clear.


So next I tried a little gadget branded by Sabrent. It’s a USB Audio Adapter that plugs into your iMac and then you plug a 3.5mm headphone cable into the gadget. The assumption is that surely the iMac can produce digital audio that bypasses its own ground circuitry.

Same problem. Nothing but loud humming/buzzing noise. Apparently the iMac’s USB bus is still plagued by this design defect, and somehow that signal makes its way into any external analog jack no matter how far downstream it is. So next I tried isolating the problem with my favorite ThinkGeek external USB hub:

Same problem. I could try using a self-powered USB hub, but I’d still have to plug that into my 120V power so I don’t think it’s worth buying one.I eventually discovered this little device. It’s an inline Ground Loop Noise Isolator. I get it on Tuesday and can test things again then. My concern is that it’s basically a filter. So the question is, what other frequencies will it step on?

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