Subwoofer Amplifier Hum Only — Help

Not to beat a dead topic here, but I have scoured the web in search of a solution or answer to a nagging subwoofer hum. I believe this problem is a bit unique, as I’m beginning to doubt that it is a ground loop-related issue (which, in fact, is covered web-wide), but do not have the technical prowess, nor the electrical knowledge to define the problem or fix its source.

First, some background: I had purchased a MartinLogan Subwoofer a few weeks ago to fill in the bottom-end of my Harbeth speakers; it’s a 2-channel rig only (no satellite/cable/HDTV) that includes an Integrated Tube Amp, a Benchmark DAC, Cambridge Audio 840C, Audio Magic Power Purifier and various PCs, Speaker Cables and Interconnects from Ridge Street Audio, TG Audio and Virtual Dynamics. I have never had noise, hum, clicks or anything of that nature pre-subwoofer; my system was satisfyingly dead-quiet. Once I integrated the sub into the system, I can hear a very faint hum coming from the subwoofer-only when I am two feet or closer; I don’t believe the usual hum volume from the speakers has changed any since this was added. I bring this up for context, as you can hear an ever-so-slight hum from the speakers, but you really have to press your ear up to the grille cloth…it’s not very audible now, nor was it before all of this. I’d say from a dB point-of-view, it is the same.

I have done all of the various things associated with trying to locate the source of a ground loop — cheater plugs on various PCs; unplugged all sources and plugged them back again; plugged them in the same outlet (a cryo-ed Oyaide variety); used audio ground isolation transformers; connected the amp to sub with Audioquest Sub-X sub interconnects (2), connecting the grounding lug wires on the Audioquests to both the sub and the amp; used about five varieties of shielded and unshielded interconnects (each RCA low-level connection efforts from amp out to sub in)…

Nothing has cured this ailment. I am beginning to think there is something the matter with the MartinLogan Sub’s amplifier. I would’ve thought one of these efforts would take care of the hum. As I say, it isn’t really very noticeable unless I’m changing out discs or placing my ear to the subwoofer — but it’s there nonetheless. The hum exists even when the ICs are disconnected; however, when it goes into standby mode or is powered-down, it is silent (no hum).

So with all of this, can anyone offer any suggestions, recommendations or sage advice to this problem? I am at my wits end and need help! Thanks so much all of you.
I'm guessing a couple of things are happening.You mention so-slight hum prior to the sub.That may or may not be amplified some by the sub,depending on the source of that slight hum.One thing to try if your very careful,try your cd player direct into the sub without any other component hooked to it.First off,VERY IMPORTANT,note the subs settings and turn the level/gain on it(sub) all the down before trying this.With the output of your cd player hooked up to the sub,level on sub turned down,hit play on the cd and slowly turn the level/volume of the sub up some so you could here the bass part of the music playing.Now with the music playing through the sub giving a low level bass,pause the cd player and listen for the hum,if it is present my guess would be the sub is the problem.Use extreme care with the subs level control doing this.This is the only at home test I could think of without swapping new components into the picture.Plug both into the same outlet while doing this trial.If you get carried away with the subs level/volume control and blow the sub,don't blame me.Don't forget the cd player is running at full volume,that is why you have to be careful with the subs level/volume control.It will be the only volume control with this cd direct hookup.With it in pause,you shouldn't hear that same level of hum.I hope this helps you find out if the sub is faulty.
Very interesting. I have never seen anyone offer this kind of test. How did you come by it? I wouldn't have thought to do that in a hundred years. At any rate, I'd never blame you (or anyone) if I screwed up (common sense must prevail, right?); but I see what you're saying about the gain/volume.
I'm messing around with it a bit right now. I will try this out and report my findings — good, bad or indifferent. Thanks for your insightful post.
I've been in this hobby with a friend I grew up with since our pre-teen years. We both like music and electronics.Years back I bought a used sub advertised in our local paper.I listened to it at the sellers house with loud rock.I heard deep bass and thought all was ok.When I hooked it up at home,I thought I herd a strange jingling/rattling noise.The only way I could think of was to hook it up to a source only so I wouldn't hear the main speakers.Trying this sure enough revealed a damaged voice coil.Since then I've used this method for listening to the sub only for defects.
Well, as per recommendation, it was indeed the amp in the sub after all. I ended-up taking it back and purchased an REL T-2, as I figured this might mate well with the Harbeth P3s I have in a secondary system (and its smallish size, like the Dynamo, might work in a 12' x 18' x 8' room). I also like how RELs utilize their high-level connection. The only problem I might initially have is setting the crossover for the REL, as it is reported by most that they need the crossovers set much lower and the gain a bit higher than one is used to. As the P3 measured pretty-much flat down to 63Hz in my room, I'm stumped as to what I should set it at. Anyone have suggestions, feel free to chime in.
At any rate, the Dynamo 700 was a much better performing musical sub than I ever would've guessed; I just couldn't get past the low-level hum in the room...