Hum in Sub when RCA is plugged in at only one end

[Sobhano]

Hello,

A friend of mine has a subwoofer that as soon as an RCA cable is plugged to it, starts to hum. The other end of the cable doesn't even have to be connected to anything at all and can be on the floor or in hand. As soon as the cable is pulled out from the back of the sub, the hum stops.

The cable in question is a 25' Acoustic Research MS253 Audio RCA Mono Subwoofer Cable 25'.


I'd like to help without going on a goose hunt. Any ideas?

I suspect they have a ground loop and I tried a cheapo surge protecor but that didn't help. Also, tried few of the cables mentioned above and all did the same. Tried another low quality RCA and the hum decreased but was still there and loud.

TIA.

 

[TLS Guy]

Hello,

A friend of mine has a subwoofer that as soon as an RCA cable is plugged to it, starts to hum. The other end of the cable doesn't even have to be connected to anything at all and can be on the floor or in hand. As soon as the cable is pulled out from the back of the sub, the hum stops.

The cable in question is a 25' Acoustic Research MS253 Audio RCA Mono Subwoofer Cable 25'.


I'd like to help without going on a goose hunt. Any ideas?

I suspect they have a ground loop and I tried a cheapo surge protecor but that didn't help. Also, tried few of the cables mentioned above and all did the same. Tried another low quality RCA and the hum decreased but was still there and loud.

TIA.

Is the sub three pin AC plug or two pin?

Most cables will hum open circuit like that. Short the live pin on the RCA not plugged in to the sub, to the shield with a jumper cable. The sub should then be silent, if not the sub is faulty.

 

[Sobhano]

I'll check on Sunday and do what you said. Decent Pinnacle system otherwise but too many devices and suspect loops – the sub is 3rd party though.

Thank you.

 

[highfigh]

Hello,

A friend of mine has a subwoofer that as soon as an RCA cable is plugged to it, starts to hum. The other end of the cable doesn't even have to be connected to anything at all and can be on the floor or in hand. As soon as the cable is pulled out from the back of the sub, the hum stops.

The cable in question is a 25' Acoustic Research MS253 Audio RCA Mono Subwoofer Cable 25'.


I'd like to help without going on a goose hunt. Any ideas?

I suspect they have a ground loop and I tried a cheapo surge protecor but that didn't help. Also, tried few of the cables mentioned above and all did the same. Tried another low quality RCA and the hum decreased but was still there and loud.

TIA.

An unterminated input cable on an amplifier indicates that it's picking up the hum from something near it. A ground loop can't occur when the sub isn't connected to another piece of equipment and no power strip, surge protector or power conditioner will do anything for this case because that's not where the hum is coming from. It's likely that the audio cable and/or the subwoofer amp is near a power line and it's picking it up through induction. If the hum disappears when the tip of the RCA plug on the far end is shorted to the ring, this means the noise is coming from something near the cable. I put the comment that applies to this in bold type, italics and underlined it.

What happens if you plug the cable into the receiver? Does the hum disappear? If it does, don't worry about it. Is the input level control of the sub turned up all the way? That's another problem that needs to be fixed- they're not made to be run that way with no input connection or signal. What's happening is, the tip of the RCA cable is acting as an antenna and the amplifier is increasing the level of the 60Hz magnetic field being produced by power cables or a motor that's nearby. If you were to plug a cable into a guitar amp without connecting it to the guitar and you turn the volume control up, it will do the same thing. Then, if the cable is unplugged, the input jack's shorting tab will contact the input tab and the noise disappears for the same reason it would if you short the tip to the ring of the RCA jack. This will cause no damage because it's at the input.

 

[zumbo]

I would like to add that all equipment needs to be OFF when making connections.

I go as far as unplugging my equipment.

 

[Sobhano]

Well, it makes the same hum even when it is plugged to the subwoofer connector on an Elite vsx-21 receiver. I took it to the new house that my friend just bought with hardly any electrical devices connected yet (just finished getting painted, flooring, etc.), placed it far from any outlet and it still made the same hum when connected it to a new Elite vsx-33 receiver. Might be the built-in amp finally giving up. The only thing I can think of that I did wrong was pulling the sub cable while the volume was ~2/3 on.

Already have few extra subs and another Emotiva Ultra 12 on the way for replacement so I'll find a good use of it.

Thanks for all of the great and informative help. I'm much more careful with pulling/connecting cables now.

 

[highfigh]

Well, it makes the same hum even when it is plugged to the subwoofer connector on an Elite vsx-21 receiver. I took it to the new house that my friend just bought with hardly any electrical devices connected yet (just finished getting painted, flooring, etc.), placed it far from any outlet and it still made the same hum when connected it to a new Elite vsx-33 receiver. Might be the built-in amp finally giving up. The only thing I can think of that I did wrong was pulling the sub cable while the volume was ~2/3 on.

Already have few extra subs and another Emotiva Ultra 12 on the way for replacement so I'll find a good use of it.

Thanks for all of the great and informative help. I'm much more careful with pulling/connecting cables now.

Pulling the cable shouldn't have damaged the input or amplifier but it can damage the woofer if it causes excessive excursion. That also shouldn't cause it to hum. Can you connect it with a different cable? A piece of coax with F-RCA adapters will work fine. If it doesn't hum and works fine, it could be a problem with the shield in the cable.

 

[TLS Guy]

Well, it makes the same hum even when it is plugged to the subwoofer connector on an Elite vsx-21 receiver. I took it to the new house that my friend just bought with hardly any electrical devices connected yet (just finished getting painted, flooring, etc.), placed it far from any outlet and it still made the same hum when connected it to a new Elite vsx-33 receiver. Might be the built-in amp finally giving up. The only thing I can think of that I did wrong was pulling the sub cable while the volume was ~2/3 on.

Already have few extra subs and another Emotiva Ultra 12 on the way for replacement so I'll find a good use of it.

Thanks for all of the great and informative help. I'm much more careful with pulling/connecting cables now.

I suspect the amp is shot, but you need to see what happens when you short the input.