Ground hum problem - asking for a friend!

[moid]

Hello everyone

I had a conversation today with a friend of mine who has recently moved house and now discovers his new house comes with fun electrical issues, mainly whenever he plugs his guitar into his pedals / amps etc he gets a loud humming sound which he says is ground hum. He has experienced this a few times before at gigs, but never at home and it's driving him nuts. He has found a device that Harley Benton used to make called a Harley Benton Groundbox whicch appears to solve the issue he has (he found a video demonstrating it here):

However Harley Benton no longer make such a device and we can't even find one second hand in the UK. He asked me if I could build such a thing, but I can't find a schematic for it anywhere and I'm a little bit worried about making anything that plugs into 240V AC mains power. I have found that Behringer make something called a Behringer MICROHD HD400 Ultra-Compact 2-Channel Hum Destroyer

https://www.behringer.com/product.html?modelCode=P0387

which is cheap and available in the UK (and doesn't need a connection to mains power) - would any of you know if this box would achieve the same goal? Thanks for your thoughts!

[amptramp]

I would make sure the effects circuits and the amplifier are plugged into the same power bar.  This could avoid some (but not all) ground loop problems.

The hum isolator appears to be a 1:1 audio transformer which disconnects the effects ground from the amplifier ground.  The comment that the Behringer unit allows you to convert single-ended unbalanced signals into balanced confirms it.  There are other ways of doing the same thing - radio frequency isolators or optical isolators can do the same thing without the frequency limitations of an audio transformer but they would need power supplies; the audio transformer does not.

[moid]

Thanks amptramp, that's really helpful - I have told him to try the power strip first (his pedal board is massive and is actually two boards) so he might well be running each board off different wall sockets. I told him to get the Behringer device and try that too - it is very cheap so if that solves the issue then he'll be really happy!

[PRR]

Yes, get EVERYTHING on ONE power strip (or one plugged into the other so they are all-the-same).

While a dozen room-heaters on one outlet-strip would burn down your house, dozens of pedals is like one tiny desk-heater.

In the US we have a "3 light tester" which reveals some kinds of un-connected grounds. Does the UK sell such a thing to homeowners?

(Should be 6 to 12 bucks, this was an odd-lot deal.)

https://www.morganinspectionservices.com/three-light-tester-vs-suretest/

[Clint Eastwood]

Hello Martin from the UK,

It sound to me like your friend might have an earthing problem with the electrical outlet he is using, or possibly the whole home system. This could be dangerous, especially when using high voltage tube gear. Here is a link that explains how to check earthing using a bulb or a multimeter: https://www.wikihow.com/Check-Earthing-at-Home

[bluebunny]

In the US we have a "3 light tester" which reveals some kinds of un-connected grounds. Does the UK sell such a thing to homeowners?

Yep, I have one.  They should be readily available - if not from B&Q/Wickes/etc. then from Rapid/Farnell/CPC...

[Rob Strand]

The main point is there's different types of grounding issues.

That demo could be a little rigged rigged because AFIK those Boss amps do not have the chassis connected to mains ground and they are known to produce hum/buzz.    The solution is typically to connect the chassis back to mains earth (there was a recent post about this).   No isolation transformer needed in this case.

These days the power supplies for the pedals are *not* connected back to mains earth so when you use the Boss amp with pedal board you don't get a ground.  In fact the Boss amp will buzz with or without the pedals connected.

If however you have a normal amp which does have the chassis connected to main earth and then you connected second line from the chassis (or signal ground) back to mains earth then that could *cause* a hum loop.   A completely opposite conclusion.

You can get genuine hum loops with a ground amp when you have an old school pedal or rack units which *also* have a connection back to mains earth.   That's the old school classic hum loop (and the reason why some older amps had the ground lift switch.  This makes the amp like the Boss amp.  The switch would be illegal in UK, EU and AU.)

Another old school problem was hum from pedals when more than one pedal was powered from the same DC power supply.   This is a completely different type of hum loop and it's caused by the power supply ripply passing through the audio.   Nothing to do with mains earth.

What that box might do is isolate the signal with a signal isolate transformer then the the signal ground on each side of the transformer is connected back to mains via some sort of grounding network (like a cap or a cap in parallel with a resistor.)    That fends off most of the cases except old school pedal board hum from using the same DC power supply.

What the other guys have said are also possibilities.

A good starting point would be what Clint Eastwood said.   If there main earth has a problem then the amp won't have a proper connection back to earth and that could cause it to buzz like the Boss amp.   If there is a serious earthing problem in the house then connecting the chassis to a mains earth won't work because the house earthing is already stuffed.     (And yes earthing issues are a real hazard, especially for UK, EU and AU as there is an assumption that the earth is working in order to provide a back safety mechanism.)