Tracking down hum in several amplifiers in a home studio

Question

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.

Answer 1

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.

Answer 2

In Europe where there is no fixed assignment for "neutral" and "live" in most outlets used, it might help to reverse the orientation of the plug by 180degree.

In many musical equipments audio signal ground is connected to the ground wire of a grounded outlet, which is further down in your house connected to the "neutral" wire of your outlet. The direct connection between the power ground and signal ground is a problem that mostly affects so called asymmetric inputs (only two contacts, no separate shield) while a symmetric/balanced input will not cause those problems if only proper balanced (3 pin/contact) cables are used and the instruments provide a balanced output as well. a so called audio-transformer/line-balancer can be used for instruments that do not provide balanced outputs.

However here comes the bad news: Basically any electric device in your house, sometimes even your neighbours house can influence your audio equipment, it could even be the wiring of your house itself, when ground and neutral are not kept separate or the "ground" of your house is not good enough (resistance to real ground). Sometimes finding and eliminating these influences is therefore frequently rather more a Art then a Science. You can buy expensive line conditioners and might find they fix your problem or they do not. Some equipment is even not designed that well to prevent a certain "ground" hum, which could in these cases be just an underrated capacitor or bad design.

A separate ground just for your audio equipment and a split transformer (letting power flow through but electrically separating the cables) to prevent direct loops via the "neutral" might fix your problem permanently but is quite costly. This is one of the parts that you will find in professional power conditioners. The other parts are usually for limiting higher frequency electric pollution.

If you want to try finding the noise source in your house, you can sometimes speed up the process by taking out all fuses beside the one your equipment is connected to and see if this eliminated the hum, only bad thing if you find out some fixed appliance like a water heater causes the noise.

BTW. As someone else already pointed out Fluorescent/Neon Lights, of any kind should be banned in the vicinity of any audio equipment. At least make sure they stay off while you record.

One more thing: Sure the transformer, as many other electric devices, can produce some hum of itself, but if properly mounted to a non-resonating surface if should not be audible from more than a yard/meter away.

Here a Link to another article that might help you in the location of the source of the sound as well.

Answer 3

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.