Puzzling subwoofer

[Diaphone]

Hi,

I have a Q Acoustics S700 subwoofer which has developed a puzzling hum.

It is silent when switched on, but hums as soon as anything is plugged into either - or both - RCA inputs. By anything, I really mean anything. Plugging in a lead on its own causes it to hum, whether the lead is connected to the amp or not. The longer the lead, the louder the hum. As soon as the centre pin of the lead touches the inside of the socket, the hum starts. If I touch the pin on the other end of the lead, the hum becomes louder. It does the same if I touch it with another lead. It behaves identically whichever of the two inputs is used.

The hum is at the 50 Hz AC mains pitch (I checked it with the piano). The sub is powered by a two-pronged mains lead. I have tried different mains sockets, two different extension leads, and directly into different sockets. I have switched off and unplugged every thing else in the room, and I have tried the subwoofer in the middle of the room. But all to no avail. It's the same with everything I've tried. Nothing stops the hum, apart from removing the RCA leads.

Any thoughts or suggestions will be gratefully received. Thanks!

 

[Diaphone]

Another piece of the jigsaw which might help. I've now attached the speaker leads from the amp to the high level inputs of the subwoofer. In a couple of the amplifier's listening modes (stereo and stereo plus the centre speaker), the subwoofer works perfectly. In the other modes (Dolby pro logic etc) there is a noticeable (but less loud) hum. When the amplifier is switched off, the loud hum returns. I'm puzzled (admittedly, that doesn't take much!). Any ideas, please?

 

[Russ_64]

I would contact Q Acoustics customer support, it may be a fault on the amp.

 

[Diaphone]

Thanks, Russ_64. I've now wired it in parallel through the high level inputs to the front left and front right speakers, and it's presently working fine with that. Having both channels connected has totally eliminated the hum. I've also set things up with an extension lead so that the amp switches on (to standby) at the same time the subwoofer is switched on, which stops the subwoofer from producing any hum, and the subwoofer sounds great (the neighbours may disagree!). I don't need a 5.1 setup with this, so it's fine that way.

I'm keeping my fingers crossed that it will carry on working well with that setup; without the high level inputs I'd have been stuck. I'm still puzzled about how it manages to pick up a mains hum at 50 hz just by plugging a lead in, and without an earth connection being on the lead to the mains.

 

[BriD]

It's a ground loop (kinda).

Disconnect the high level stuff and have just the RCA so it's humming.

Then with a piece of wire (speaker wire is fine) touch a screw on the casing of your amp (or if it has a turntable grounding post use that) and then a screw on the amp plate of your sub.

Hum should vanish.

 

[Diaphone]

Thank you, BriD. That makes sense, and I'll give it a go. At the moment it's behaving well with the high level inputs, but it would be good to know that it will behave without needing those.

Just one more question, please. I presume I'm not putting too much of a strain on the amp by having the each front speaker wired to the amplifier with the subwoofer inputs wired in parallel with each speaker (i.e. front left speaker wired to the amp along with the left channel of the subwoofer in parallel, and the same with the right hand speaker and the right channel of the subwoofer). The amp is rated at 4 to 16 ohms, and each speaker is rated at 8 ohms. I presume that the impedance of the sub will be high which should be ok. I am getting more than sufficient power to the speakers when wired with the sub in parallel, but it dropped off when I played around with wiring them in series. Thanks.