Major hum on new Subwoofer

[Sutch71]

Hi, I recently upgraded my subwoofer from a Monitor Audio Silver to a Monitor Audio PLW15 - however I am now experiencing a pronounced hum on the subwoofer.

I think I have isolated the source to my plasma, as when it is powered off (not unplugged) the noise goes and when I change channels (even without a source selected) or if a brighter screen comes on the noise level is increased. I have unplugged everything and am convinced that the problem is my plasma.

The plasma is wired directly into the mains on one wall and the sub is plugged on the wall to the side. Having looked at the wiring - everything looks ok. I have plugged my old sub in and the noise is just not there at all, plug the new sub in and the noise is there - I am presuming its down the to lower frequency response of my new sub but any thoughts or ideas would be very helpful.

Thanks, Lee

 

[Sutch71]

Just to add, the sub woofer is secound hand, not new and my cable is QED performance. Thanks

 

[GSB]

You could always plug the sub or tv into a different socket,or into a surge protector or into an extension....just to see if it cures the problem.

 

[michael hoy]

Does the hum appear with or without the subwoofer cable attached.

If still appears without the subwoofer cable, try pluging the sub in upstairs, hopefully on a different part of the ring mains and see if it still appears.

 

[Sutch71]

Thanks chaps, the noise is there only with the cable attached. I have tried different sockets and plugging in a mains silencer near the source and the subwoofer but nothing shifts it.

The noise is also more pronounced when the Plasma has a picture on it (especially white)

 

[GSB]

Wish i knew of a fix for you.

Can't understand how the plasma is affecting the sub,which is connected to your avr,where can the interferance come from....has to be socket related :?

 

[Sutch71]

Thanks, I will try and earth the amp into my silencer block tomorrow to see what affect that has. My plasma runs via the amp - its almost like the sound that a tv would make if the tuning was off slightly and brighter pictures produce more noise. Its very strange as my old sub doesnt have the same issue.

 

[robjcooper]

If the sub sounds like a motorboat running, then that is probably an earth loop problem with the amp in your new sub (mains hum is 50hz so you would have heard it on your old one too). You could try this as it worked for me - make up a lead (any old bit of cable will do-just strip off the insulation on both ends) which will reach from the earthing point on your amp to the metal back plate of the sub. Connect one end of the cable to the earthing point on the amp then turn everything on. The sub should be humming as you've noticed. Now touch the other end of the cable onto the back plate of the sub (where all the controls are and onto whichthe amp is usually mounted) - you may have to try a couple of places - screws are usually a good place to start. If this does stop the hum, then find a way of permanently connecting this cable to a point on the sub where the hum disappears

My AE sub is connected from one of the screws on the backplate to the earthing socket on one of my Pure AV boxes and is now as silent as, well a very silent thing...!

Good hum hunting!

Rob

 

[Sutch71]

Thanks Rob, i'll give that a try. I have just noticed tonight that if I change the sub to stereo input the hum goes away - change it back to LFE and its back again so it must be noise through the cable as oppossed to mains - I will try the earth and see how that works.

My Amp is about the other side of the room though so it might be a stretch - I have an earthing point on my russ andrews power block, I wonder if its an idea to earth the amp casing to the powerblock?

It is like a motorboat engine, that unfortunatley gets noisier dependent on whats on the plasma screen

I hav re-plugged my old sub back in as a test and its absolutley silent as the holy night!

Will report back, thanks.

Lee

 

[abacus]

Double check to confirm that you have set the sub to LFE mode, rather than 2 channel mode. (Assuming your using the LFE output on your amp)

Hope this helps

Bill

 

[Sutch71]

Thanks Bill, all straight on that - my LFE is selected and the sub is recieving a signal on test and on bluray play (I have even swapped from sub 1 to sub 2 outputs and removed the 12v cable) - there is still just a loud audible hum accompanying the LFE.

Thanks for your comments

Lee

 

[ellisdj]

Have you tried a different subwoofer cable - I had this once before when the cable was creating an earth that shouldnt be there?

I would also give MA a call and ask them for some advice but I would puit money on that the sub cable is causing the earth as when you remove it its gone.

I bet if you wiggle it the hum would stop and start as the wires will make and break contact - it did on mine and my cable was brand new.

Audioquest sub woofer cables have a spade you can connect to the sub like what the earlier post suggested to isolate the earth.

 

[Sutch71]

Thanks Ellisdj, I have tried a couple of leads with the same results. Funnily enough the humm gets louder when the plasma has a brighter picture on and in fact when turning the plasma off it clears the humm somewhat.

I have put a couple of calls in to MA with no call back as yet.

Will report back

Thanks, Lee

 

[proffski]

Some friends had the same problem up the road recently with the same subwoofer. I attached a mains pluf with the Live and Neutral pins removed, just a wire from the top earth pin and grounded onto a handy screw on the matalwork, hum went instantly. Before doing that I'd be tempted to check for fauly issues first though in all your equipment including weird hum loop currents. Much equipment has class Y or class X capacitors accross switch mode power supplies (like in your TV) which could be causing the problem. Keep us informed.