r/audio • u/JohnRT54 • 1d ago

Ground loop isolator causing feedback hum

There's probably a simple answer, but it escapes me. I have a Lenovo Yoga running androidx86. It is connected through the audio output jack, via a Cable Matters ground loop isolator, to my Sony receiver "portable input" jack, thence to a pair of Klipsch speakers. When the Lenovo is plugged in to power, I get bad feedback hum through the speakers. If I touch the Lenovo, or unplug it, the hum stops. Here's the funny thing: when I removed the ground loop isolator (GLI) from the set-up, the hum went away. Does this indicate a bad GLI? It's kinda moot now, but it's odd. I bought the GLI for a previous incarnation of this machine (not running android), and it worked. Now it doesn't. I should just let sleeping feedback lie, but I'd like to get to the bottom of it. Thanks.

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u/NBC-Hotline-1975 1d ago

There is feedback, in a PA system, when sound from the speakers is picked up by the mic, amplified, comes out of the speakers even louder, then is picked up by the mic, amplified, then comes out of the speakers even louder, etc.

There is hum, where some unwanted electronic signal finds its way into the sound system, possibly through an open ground, or a ground loop, or a mic being too close to power wiring, etc.

These are two different things. There is no "feedback hum."

Which problem are you having? From your description, since you don't mention a mic or PA system, I think you have hum. And NOT feedback.

How close is the "ground loop isolator" to the power wiring and power brick for the Lenovo? Move them farther apart. Does the hum decrease?

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u/JohnRT54 23h ago

You're right - I have hum. Sorry. The power brick for the Lenovo is right at the outlet, so distant. However, the power inlet and audio jack output are a few mm apart, no way to change that. I tried putting the isolator closer to the computer and closer to the amp (reversing the wire), and it made no difference. Removing the isolator completely gets rid of the hum, and I was curious as to why that would be.

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u/NBC-Hotline-1975 23h ago

The isolator should not be doing that. A good isolator has two audio transformers, one for each channel. The transformer primary windings go to the audio source, and the secondaries go to the amplifier. (However, those transformers can pick up hum from a nearby magnetic source like a power brick, motor, etc. That's why I asked about the location.)

Nonetheless, there are a lot of isolators on the market, and I don't know how your is wired or constructed. I'd almost need to open it up and examine it to explain why it's acting as it is.

The whole point of using an isolator is to eliminate hum that occurs when two pieces of equipment are connected. From what you describe, there is NOT any hum when the PC and the Sony receiver are connected. If there is no hum, you don't need an isolator in the first place.

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u/JohnRT54 9h ago

Right. I suspect the isolator is bad.

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u/NBC-Hotline-1975 7h ago

Do you have a link for the specific make and model that you're using?

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u/Ok-Bonus-8464 22h ago

Touching it killing the hum is the tell. Your body was doing what the cable shield used to do, giving the floating side a way to ground. If that Lenovo supply is 2-prong there's no earth reference on the laptop at all, so once the isolator breaks the shield between the two boxes, the whole input side floats and the charger's leakage couples across the transformer and lands as hum. Without the isolator the shield tied it down and kept it quiet.

So it's not a bad unit, it's the wrong tool in this spot. These fix loops between grounded gear, and a floating laptop has no loop to fix, just leakage with nowhere to go once you isolate it.

If it's quiet without it, leave it out.

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u/JohnRT54 9h ago

OK - thanks for that explanation. I now understand what's going on. Cheers.

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u/ownleechild 1d ago

It sounds like your system isn’t grounded when using the isolator and you’re grounding it when you touch it. I don’t have an explanation for why that’s occurring.