Depth subwoofer hum

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Bruce Weiland

Well-known member
Apr 22, 2006
Reaction score
Round Rock, Texas, USA
Hi guys,

I haven't posted here in an awfully long time. However I've been enjoying my Prodigy and pair of original Depth subs for quite a while now. :(

Of late I've been playing with passive bi-amplification, and room correction, which brings me to this topic.

Beginning at day one of ownership my Depths have always had a hum as soon as the amps click on. It really didn't matter whether I used a cheater plug, inverted the plug, ran multiple wires for grounding, shorted the inputs, or whatever. As soon as the amps click on there's a hum.

Since I generally listen relatively loud I don't hear the hum when playing music. And when playing at background levels at the opposite end of the room I don't hear the hum. But it's always there and in the back of my mind and bugging me just a bit.

Recently I read a thread that mentioned that some of the depths had a problem with their servo units and that cause the hum. In the conclusion of that thread the person had sent the subs back to Martin Logan, electronics were changed and the hum was gone.

I also read a thread about carvers true subwoofer Mark II where a couple of individuals had problems with grounding getting loose in the amplifier. The solution was to open the sub, tighten the ground, and the problem would be solved for several more months.

I thought I would investigate that. In the process I learned something very interesting. :rocker:

The hum is being induced by the magnets of the woofers.

With the sub powered up as I moved the plate away from the cabinet about an inch the hum went away. Moving the plate back against the cabinet cause the hum to return.

So if any of you have had an ongoing hum problem you might try removing the eighth Allen screws that hold the plate against the cabinet and simply moving it out about an inch. If that makes your hum go away then I think I have a solution that's pretty simple and does not require the unit going back to Martin Logan for repair.

My plan is to buy Allen screws 1-1/2 inch longer than the OEM screws. I will trim 1 x 2 fur strips to make a frame that goes along the outside edges of the amplifier plate. This should move the amplifier far enough away from the magnets to get rid of any hum. Although the additional volume of air in the cabinet may have some effect on frequency response, I'm not concerned. I will handle any aberrations with digital room correction.

Hope this info helps someone is the group.