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Signal Converter

December 20th, 2013

Sub woofer analog to digital signal converter?

Right now my home theater sub is running across a analog signal and is getting a lot of EMI in the wall. i need to fix this as it cause a humm in my sub woofer. i was thinking about converting the signal to digital and then back to analog.
I know for a fact it is not a ground loop hum. the first sign is that the sub is only two prong. i know with about 99% certainty that it is emi.
also the emi is coming from running the wire next to electrical conduit.
And i have done testing i put an extension cable between the outlet it is in now and moved it next to my receiver, and used a different cable and the hum was eliminated.

It is completely possible for units with two prong plugs to exhibit ground loop hum. Three prong plugs are absolutely not required. (One possible route is coupling from the audio signal ground plane to the AC power line neutral... this can happen via a leaky grounding capacitor, capacitive coupling in the power transformer, many things.)

If it's low-frequency hum then it is almost certainly ground loop hum, as this simply does not radiate much from AC cables (because the current comes back just as much as it goes out, so the fields from the two conductors of the AC line tend to cancel each other)... nor is it picked up easily by nearby audio cables. Coupling, if it happened, would be inductive. Shielded cable will not help here. What will help is shielded twisted pair cable (the shielding is for RF-coupled interference, see below) fed by a balanced driver and connected to a balanced input. Since your equipment likely does not have balanced outputs or inputs, you can get the same effect by using a pair of hum eliminators (which are nothing more than transformers) at the ends of the cable - again with STP cable in between.

If it's actually ground loop hum you can get away with just one hum eliminator.

On the other hand, RF-coupled interference (this is also referred to as capacitive coupling) can radiate from AC power cables. The sound usually has a higher frequency, raspy, "buzzing" character rather than the typical sound of AC hum. Its usual sources are light dimmers, electric motors with brushes, and other loads that are switched on and off abruptly and rapidly. Shielding the audio cable does help with this; twisted pair cable and balanced ins and outs nomrmally not needed.
Ferrite cores on the cable will help against RF-coupled noise, but only if you separate the cable and run just the "hot" lead through the core. You can get XLR plugs that have these cores built in.

Ferrite cores do not do a thing for ground loop hum.

n.b.: Grounding just one end of a shield helps eliminate ground loop hum, but can hugely exacerbate RF-coupled interference, as the "hanging" shield may act like an antenna.

I notice you said "99% certainty". If it's not 100% then you haven't done any actual testing that would eliminate one or the other. Humor us, ok? Try plugging the sub into the same wall outlet as the rest of the system - how much trouble can that be? Try moving the line level cable to the sub around, etc., before making any more assumptions. Certainly before buying anything.

Heck, even if it is RFI, it may be being picked up directly by the sub's plate amp. (Most of them are completely unshielded.) And this could be entirely dependent on the sub's position in the room.

Regarding a digital cable... this would most likely work, contrary to the other poster, as long as the interference level was not too great. Reason: digital is completely forgiving of interference, UNTIL the interference gets too strong. Then of course digital goes from "perfect" to "very noisy" very quickly. But it usually takes a LOT of noise to do that. A workaround for that would be to use an optical SPDIF cable between the DAC and the ADC... it obviously will be completely impervious to both inductive and RF-coupled electrical noise. Accordingly I have linked what looks like a suitable and really inexpensive ADC and DAC pair below.

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