What can I do to eliminate 60 cycle hum from my Marshall amp?

[peterwagner]

Must be a grounding problem, a ground loop. Are the THD outputs balanced? You can use this, http://www.radialeng.com/di-twiniso.htm

[Kiwiburger]

Buy an Orange.

AC hum is hard - I constantly struggle with this. You have to learn to identify the many ways in which AC hum can piss on you from all directions.

First up - does this amp hum with no guitar lead plugged in? If yes - is it an acoustic hum from the transformer, or an electrical hum from the speaker? Acoustic hum is easy - move the head out of the room. Or get a power inverter that supplies a pure sinewave.

AC power is basically like the output of your power amp if it was supplied with the loudest 60Hz sinewave you could provide. The actual tubes or transistors in your amp run on pure DC - and its the job of your power supply to deliver pure DC which is free from any hum. Unfortunately, to do that properly costs serious money. Have a look at the cost of a laboratory power supply.

The cheap power supplies in mass produced guitar amps just aren't that good. The tones that you heard from the classic guitar gods were frequently from highly moddified amps with superior power supplies. Forget the endorsement deals that have been made to promote the sale of mighty rock amps to the peasants - what you are shown and what actually was used are two different things. Not everything with Marshall on the front is gold. Gold is rare and not given for the peasants.

So if your amp hums before plugging anything in, you have some serious work ahead of you. You might be lucky, it might just be a bad tube. Or - you might be able to clean up the hum by adding some rectifiers and capacitors and running the tube heaters off DC instead of AC. (I can't believe any amp maker has the nerve to heat tubes with AC and expect the peasants to be happy with the hum - but it costs a few extra dollars to do it right, so they gotta keeps their account happy).

A good amp tech should be able to fix it so it doesn't hum.

If the hum only happens when you plug your guitar in - that's a whole different kettle of worms. Is the hum directional? So many power transformers radiate hum for miles.

I'm considering building a faraday cage, to kill the last traces of hum.

My worst source of hum was my AC ground. The best faraday cage in the world won't kill hum if it's right there on your ground, which all your audio "shields" connect to .... I recommend a dedicated earth spike and isolated ground receptacles.

There are multiple levels to this problem - and when you solve one, it exposes the next one.

Good luck.

[gurubuzz]

it might not be ac hum.

it could be rf interference from TV Transmitters...

Read this article.



here is a quote

"the noise that sounds like a power-related buzz is often the video-sync portion from a local TV station. It's 59.95 Hz, and if you listen closely (which is “easy” at metal levels), you'll notice a slow “phasing” sound as the vertical sync “comb filters” against the 60Hz power line hum. For guitar cables, place the clam just before the plug at the input (destination) end. This simple contraption creates a choke — essentially, an inductive high-frequency (RF/TVI) noise filter. The result? A joyous 80-percent reduction in that nasty RFI, followed by the question, “Now, how do I get rid of the rest?”



and the full article

The Tech's Files -- Summer DIY Fun --More Guitar Amp Tweaks

[Stephen Pruitt]

Thanks for the suggestions, guys. For the record, my THD Hotplate is NOT a balanced connection. 1/4" single cable in and 1/4" single cable out.

It appears that I have my work cut out for me.

:-(

Stephen

[gurubuzz]

don't feel so sad...

try the ferrite clams first ...
cheap ...easy...


try a DI on the output with a ground lift switch

also make an extention lead with the earth not conected...

try different combinations of gear in your set up with the earth removed, starting at the amp.


sometimes it could be one piece of gear causing it...

and the golden rule " make sure the gear U are using is all running on the same power circuit and make sure that the circuit
is only physically earthed at one point...

but before mucking around with earths try the ferrite...