REL hum... from RCA connection

I know REL hum has been discussed, but seems like it’s usually from the High-level speakon connections…

I’m getting hum from both of my brand-new REL T/5i subs through the RCAs. Hum reduced when crossover frequency is lowered. it’s not bothersome when music is playing, but it’s noticeable when music is paused/silent.

When amplifier is off, hum is still present, but lessened.

if I disconnect the RCA cables, the hum goes away.

RCA cables are not cheapies, but not esoteric ones. Run is fairly long tho, like 20 feet.

I did NOT get hum with my Cambridge MINX subs. (Nor did I get hum when I demoed some SVS subs.)

This sounds like you may have a ground loop in your system. Essentially you have components receiving ground signals that are out of phase. There are two methods to resolve this one is to simply use a cheater plug that turns the 3 prong outlet into a 2 prong outlet. The other and more advisable method is to plug the device causing the hum into something like a HumX device which essentially does the same thing but in a safer way.

If your subs are not plugged into an outlet that is on the same run as your pre-amp, try to get them all plugged into the same outlet run.

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Looking at these HumX things, they’re around $80 each, and I’d need two. Not a fan of that, when my Cambridge subs didn’t hum.

This wouldn’t be possible, as the subs are in the living room, and the audio equipment is in a closet in another space.

It’s highly likely a ground loop. The ground current is now making its way from your preamp to the REL inputs which suggests that’s now a path of lesser resistance than what the other subwoofers offered – or the other sub didn’t have a proper ground.

One test is to lift the REL’s ground – that should make the hum go away, but now you have your RCA’s negative pin as the ground path from subwoofer to preamp – not good either.

The other high quality option you might have is REL’s wireless bridge.

Try the cheater plugs as a temporary solution and then at least you will know the hum is a ground loop. Also are the two subs also plugged into different circuits from each other?

I can test to see if they’re on different circuits, but I unplugged one from power, and the other still hummed. So I’m guessing that might render the circuit question irrelevant.

I guess the question is why these hum when my others, which were connected the same way, don’t? (While the RELs have a ground pin on the plug, the Cambridge Audio subs DO NOT… if that’s a clue of any kind as to the ground-loop issue.)

So adding the cheater plugs will put you back to two prong just like the Cambridge which is likely your only feasible solution that doesn’t cost since the gear is on a completely different circuit.


I’ll try a cheater plug to test our theories, but I don’t reckon I’d like to leave them in that fashion.

Not sure I know of another solution really that won’t cost.

So is it really just that the REL’s are grounded? Why are they grounded while the Cambridge Audio subs are not grounded? Is there a NEED to ground one vs the other?

Interestingly the cambridge subs don’t require polarization; plug can be flipped.

If the cheater plug clears it up then yes that is the issue combined with them being on a different circuit than the source of the IC’s.

yah i get that but I hate fixing a symptom instead of the actual problem!

Yes, you have a ground loop.

The REL probably has a grounding pin because it has exposed metallic parts that need to be grounded so you don’t electrocute yourself when there’s a fault within the unit.

It is recommended that all devices in an audio system be connected to the same power distributor to prevent ground differentials that cause hum.

The best solution is REL’s wireless transmitter. Next best is an XLR with the grounding disconnected via a lift box.

This Arrow unit for $199? plus I’d need one for each sub?

That’s it. Not sure if you need one or a pair of those. Give REL a call.

looks like I would need one for each sub.

I’m sure the audiophiles will scoff, but I think my decision is made: No RELs for me, at least not in this space.

If you have a REL why not use the speakon connector that it comes with. it’s about 25ft long and should eliminate the hum as it will ground the sub to your amplifier.

Can’t you go via a 1:1 transformer? - I think that’s what bands use as ground loop isolators. I’ve used that method satisfactorily.

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Everything I’ve read seems to indicate that the high-level speakon connection often results in hum, and requires an RCA to be run from the amp to the speaker to get rid of it.

Interesting. Do you have an example?

Again, makes me nuts that I have hum with this upgrade, when neither my Cambridge nor the SVS I demo’d hummed.