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In my home studio I have several amplifiers:

  • Fender Blues Junior III (guitar combo, all tube)
  • Gallien Krueger MB150E (bass combo, solid state)
  • SWR Studio 220 (bass head, tube preamp)
  • Alesis RA 300 (monitor amplifier, solid state)

All of them hum audibly when simply switched on, with the volume and gain all the way down. That hum can be heard faintly up to 4 meters away (as far as I can get without going around a corner).

With at least the Alesis and SWR, the hum persists when the speakers and all inputs are disconnected.

The Fender, being a full-tube amp, is not meant to be switched on without a speaker load attached.

In contrast to the others, the GK does not hum when the speakers are disconnected; it does however continue to hum (with a bit of hiss) even with the volume turned all the way down and no inputs attached.

The Fender is new, and as such so are the tubes. The tube in the SWR is more than a decade old (but it's a pre-amp tube, so it should have a long lifetime).

All of the equipment is connected to the same power strip (which I believe should rule out ground loops?).

I assume the hum is coming from the transformers? Is this something that a line conditioner would have a shot at fixing?

I've searched a lot trying to figure out what the potential problem could be, but most of what I've found focuses on one of the amplifiers being faulty (which seems unlikely with 4 different amplifiers), or the hum being introduced in the input signal path, which doesn't apply here.

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I know that fluorescent light might wreak havoc on music equipment, but I was under the impression that it only should be noticeable with a speaker cabinet connected. Have you tried the amplifiers at other outlets in your home? –  Meaningful Username Mar 26 at 11:38
If the hum is NOT heard coming out of the speakers, then it's almost certainly mechanical. You should check the frequency: if it's 120Hz (or 60 ) it's almost certainly from the input line transformer, and may depend on output load(current). There's really not much you can do about transformer hum. –  Carl Witthoft Mar 26 at 11:49
@CarlWitthoft, On the Alesis there's a very slight amount of hum that's coming out of the speakers at around 50-60 Hz (I'm in Europe, so I assume it's 50), and the volume of the hum coming out of the speakers is independent of the gain on the amplifier. On the SWR I don't notice the hum through the speakers, though there is some present on the direct (balanced) out. (With the Fender it's hard to tell since they're in the same physical space.) I also suspected the transformers, but wasn't sure if transformer noise could be affected by a dirty power line in. –  scotchi Mar 26 at 12:20
@MeaningfulUsername, I have tried other outlets with the same problem and don't have florescent lights here. –  scotchi Mar 26 at 12:21
@CarlWitthoft, Also, to add to that, from the other stuff I'd managed to find online it seemed like most of what I ran across saw relatively loud transformer hum as something that you got from especially bad / faulty transformers, which seemed odd since at least two of the amps (GK and SWR) are basically top of the line stuff and since there's a sample set of 4. Is transformer hum audible at 4 meters away semi-common? –  scotchi Mar 26 at 12:40

2 Answers 2

Have you tried mechanically isolating the cabinets,as in resting them on carpet or rubber, rather than putting them on, I suspect, a hard surface which may even be a hollow floor - creating its own soundboard. You say that the hum is still there even when no speaker is connected. This will eliminate a suspect ground loop, which incidentally, shows up as very close to a G note - it's 60Hz.There's a possibility that an inputted instrument is very close to an amp., which can produce a hum, too, and I don't mean feedback.

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In general, the way you fix this is disconnect EVERYthing so you don't have hum. And one by one, plug stuff in and see when the hum starts. Even if a device isn't ON, it can still cause ground loop interference just due to the ground in the connecting cable.

Although disconnecting your ground on the main wall plug MAY get rid of your hum, do NOT do that. That can cause fires if your equipment breaks. Find out where the offending equipment is and kill IT. (Or leave it disconnected until you absolutely need it, etc.)

I'm not sure about your exact config. I don't have any guitars, just digital piano and digital drumset.

I've found that the usb cables that send midi to/from my pc both have "ground loop interference" that cause a digital whining.

I've read up on it recently, and they SAY that if you cut the netted ground connector in the usb cable, and possibly the black ground wire that'll kill the hum. Need to try it out this week. This is relatively safe as usb doesn't have high voltage, just +5 volts so no fires.

The procedure goes:

  • cut the netting in the cable that carries ground. If that cures it, you're done.
  • cut the back ground wire. If that cures it, you're done.
  • if it doesn't, buy another usb cable and you're just plain screwed. Try a midi interface and use that if your instrument has old school midi. Old school midi requires opto-isolators to get around just that problem.

Good luck to ya.

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I can reproduce the same hum by taking one of the amps to another room where none of the other stuff is around, plug in one single item and still get the same sound, so don't think it's a grounding issue. Also I'd already tried with a ground-lift just as an experiment and that didn't help. –  scotchi Mar 26 at 16:21

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