Two subs causing hum

[jk330ci]

I have a monitor audio rsw12 subwoofer as my main sub and thought I would try a second sub to see what it sounds like. I have an old rel q150 knocking round so I connected it to the RCA outs of the rsw12. When I connected it there is a loud hum coming from both subs. If I then disconnect it the rsw is silent and the rel only has a faint hum which has been there for years. The gains on both subs are around 10 o'clock and I have tried swapping the phase of the rel.
my reason for trying this out is iv read various threads about multiple smaller driver size subs giving similar effect to a larger driver. Obviously neither subwoofer is going to reach down to subterrainium levels but the increase in surface area may help as iv always felt I could reach the limits of the ma sub pretty easily.
Anyone have any ideas why I'm getting such a loud hum, ground loop maybe.

 

[noiseboy72]

I would say you have a ground loop. This is often an issue with domestic, unbalanced equipment.

You may need to play around with earthing your other sub, but this may not cure it and you might end up fitting an isolator to the signal input.

Another method would be to use the high level speaker inputs, as these will be isolated by the amp anyway.

 

[jk330ci]

I'm tempted to change my pioneer vsx-921 for something like a denon avr x4000 or a higher model from the pioneer range with duel sub outputs with sub eq, I wonder if they would still hum connected to different outputs. My goal is to eventually change the sub to eithe something by svs or DIY

 

[noiseboy72]

The hum is more down to the sub than the amp. You might want to get rid of the sub first.

 

[jk330ci]

Just fancied trying two. I'll stick with the ma rsw12 as it's the superior sub out if the two. Maybe look at something by svs next then.

 

[spyder viewer]

You could try splitting the low level output from the rx and feeding both subs simultaneously instead of daisy-chaining them.

 

[jk330ci]

Right so I thought the hum completely went when I disconnected the RCA from the ma sub but it was still present just not quite as loud. It's only when I disconnect the rel from the mains that the ma sub stops humming. They are both plugged in to the same socket via a 6 way block, so the hum is coming through the mains supply. I'm in no doubt it's the rel humming as it's always had a hum where as the ma is silent when the rel is out of the equation. Funny thing is iv sent the sub back to rel who have fully tested it and everything is in accepted parameters.
Its bugging me now, as both subs should be working fine and yet the rel is still causing a hum. I know my mains supply is not great and have tried cheap mains conditioners with no help.

 

[spyder viewer]

Well it does appear to be an earth loop. When the Rel is on it's own, is the humming of an acceptable level? If it isn't, then that's where we need to start. Otherwise you could try decoupling both the line and earth feeds in the input lead with suitable capacitors. I don't know if there's any commercial product would do this? Also. you should try to connect the s/ws directly into their own wall outlets.