Belles amp hum ... rf problem?

Hi all. I have a noticeable hum problem which I have isolated as coming from my power amp, Belles 150a hotrod. The amp hums when I have it plugged into the wall with nothing else attached. The hum is not always present. I'll go a couple days without it and then it will re-appear ... comes and goes. I called David Belles and he told me the issue was likely a quality of electricity problem which was exciting the torroid transformer. (I live in a large apartment building in Manhattan.) He recommends a RF filter. Can anyone recommend a RF filter brand? Would something like a Blue Circle Noise Hound or PS audio ultimate outlet/power conditioner serve this function? If it is a electrical problem I don't think it's coming from within my apartment. I've unplugged everything that I can and the hum persists. Help it's driving me crazy. I may be imagining this but when it does hum the system doesn't sound as good. High frequencies are really grating. Thanks in advance for any suggestions.
open the unit and check for loose mounting of the transformer. try an ungrounded PC.

Hi Ryno. Thanks for the suggestions. Checking the transformer to see if it was mounted properly was the first thing I tried. Everything is nice and tight there. I also tried a cheater plug on the PC to lift the ground but that didn't do it either.

Do you think it's possible that an improperly grounded outlet would cause a hum? My Manhattan apartment building is pretty old (pre-war) and I have a feeling the wiring may not be ideal. I don't know much about the electrical end of things as you can tell ...
If anyone in your vicinity has a light dimmer or similiar device around(off of the same feed in your building) or in your apartment, this could be the culprit.
I looked for a week trying to find why my Belles amp had a humming transformer. Part of the time it was dead silent, then it would hum. I finally traced it to a Leviton dimmer in one room of the house. I replaced that switch and it hasn't hummed since.
The switch does not need to be on the same feed from your breaker. My amp was upstairs on a dedicated 30 amp service. The switch was downstairs on a 15 amp line.
I second Bigtee. I had same problem with a halogen light. On the light's low setting, my amp hummed like crazy. Switch it to the high setting, it basically went away. Toroidal's are very sensitive to this type of stuff.

Thanks for the responses guys. Actually I have 3 lights in my apartment that have dimmers but I always make a point of turning them off when I am listening to my system. Is it possible they are putting garbage into the line even when they're off? I think I will try replacing the dimmers with regular on/off switches.

If it turns out not to be something in my apartment causing the problem but say my neighbor. Would something like a power regenerator fix this? I've always been told not to use any sort of conditioning on my amp but I don't think I can live with this hum. If it's quiet, I can hear the hum across the room, 20 feet or so.
One more question ... Are all torroid transformers equally susceptible to this electrical hum? Does it have to do with the size of the torroid or is it more related to how the amp is built/designed? I guess what I'm trying to get at is, in the future if I decide to change amps am I going to have the same problem? Thanks.
just b/c the mounting of the transformer to the chassis is tight doesn't mean that its not the cause.

leave the lid off, next time it hums, go put your fingertips only (carefully! do not touch another thing!) on top the transformer and feel if its a mechanical hum or an electrical one. fingertips are sensitive enough to tell...if the transformer is moving, raise it off the chassis & put some rubber gromules inbetween the mounting plate of the transformer and the chassis.
Thanks for the tip Rhyno. I definitely would not have thought to try that. Hopefully I can get this figured out quickly!

Belles mounts his transformers very well and they are isolated. Unless the transformer is physically defective, I doubt that is where the problem is. It's something coming in on the line. Otherwise, it would hum all the time.
Good question about whether they all would do this. Most amps I've used in the past had an E-core.
I don't have a solution, but I do have a similar problem with the new Belles 150A Reference. The problem did not manifest itself until I attempted a speaker cable upgrade from the shielded LAT SS-800 to the unshielded Xindak FS-2 copper/silver foil. Result was a very loud hum, even with everything but the amp unplugged and the inputs from my 21A pre-amp disconnected. Problem is continuous with the unshielded cable, and is loud enough to make listening to music impossible (all with pre-amp completely removed from the sytem). Problem goes away when my old speaker cables are re-installed (yes, I checked multiple times to make sure I installed the new cables correctly). The hum changes pitch when I move the speaker cables around, but never goes away - I'm no EE genius, but to me, it would seem more logical that this is an EMI problem coming from the amp itself than a ground loop problem, since it goes away with switching only the speaker cables. I also tried lifting the ground from the amp PC with nothing else connected to the amp or plugged in to any outlets on that circuit - hum persisted. I also tried plugging the amp directly into the wall (my normal method) or into my Panamax line conditioner, with no change. I do, however, have dimmer switches installed throughout the house, and I'm not in the mood to replace every switch to see if that might help! Other than this recent challenge, I have been thrilled with the 150A Ref/21A combo. Any other ideas?
I had the same problem with the intermittent humming. I plugged my cord into a different outlet on the power strip and it stopped. Weird huh?
Hammer 223, as unappealing as it may seem, I would recommend checking/replacing your dimmer switches in your house. I had 3 dimmer switches in my house and after exhausting all other options I finally took these out and replaced them with regular on/off switches and my hum disappeared!

On another note, did you compare the 21A preamp to the 20A preamp before settling on the 21A? I'm interested in trying a tube preamp for my 150A hotrod but am tight on funds and am wondering how much of difference there is between the two.

Problem solved! I contacted David Belles, he understood my problem right away, and immediately sent me a pair of "RC snubbers" (his term) that easily installed over the amp binding posts. Slipped them on in 1 minute, turned on the power, hum was gone! I did not discuss the problem in detail with David, but I suspect the RFI from the toroidal transformer was the culprit. I would guess that these filters were not installed "stock" in order to maintain purity of design, since probably not all systems, and particularly speaker cables, would be as susceptible to the interference as my particular setup was.
interesting, my hotrod 150A just blew up this past monday out of now where. i was making dinner my music selection had ended and out of nowwhere a large hum and popping sound on the right channel. so much power went to the right channel that it caused my warning LED to come on in my 2ce sig II. i quickly turned the amp off waited turned it back on same thing. unhooked righ channel turned amp on left channel was fine. powered down then hooked right channel speaker to left amp channel and bam, no power at all the amp would not even power up. as we speak it is on its way to david belles himself under warrenty to fix the oddity. . hmmmm