Weird humming picked up by mic when trying to record just the amp

Question

I recently got a SM57 mic and wanted to record my guitar amp. I have a Fender Frontman 25R. The mic is plugged into a Scarlett 2i2. I'm recording via GarageBand. Unfortunately, in a barebones signal, there appears an obnoxious humming!

The humming is definitely caused by the amp because when I turn it off it disappears (there's still a hiss but that probably won't make it through the mix especially if I add a touch of EQ-ing). It's definitely a bad cable or pedal cuz nothing is plugged into the amp. It happens regardless of whether the amp is plugged into the wall outlet directly or through a power strip.

I've read that "ground loops" can present a problem but I'm not exactly what the loops in question are. The Scarlett 2i2 is plugged into a USB hub whose power cable is plugged into a surge protector into a different outlet than the amp... it's a lot of connections I know but this is part of my apartment desk setup rather than a dedicated studio.

What could be the issue here? Is it a small fix or am I screwed by virtue of having to record in my apartment? If the issue is insurmountable I might have to return the SM57 and stick with just a DI guitar signal and the amp sims in GarageBand :(

Edit: I forgot to mention but the humming is only heard at the end of the SM57 -> 2i2 -> GarageBand chain. I don't hear it directly from the amp with my ears. So either it's non-existent in real life and is induced into the signal chain, or it's present in real life but it's super faint and is amplified by the signal chain. The former seems more likely.

UPDATE: The issue occurs even when my laptop and the 2i2 are powered by the laptop's battery rather than a power adapter plugged into the wall.

Answer 1

Does your amp have a TRS or XLR input? If not, then that's probably your problem. Plug the mic directly into the Scarlett with an XLR cable. Don't use your amp to effect your mic signal, use a dedicated effects unit with XLR connections.

If you still insist on using the amp to effect your mic, it's definitely a 60-cycle AC hum try the following diagnostics.

Check the amp first. Do you have a headphone jack for the amp? Is there a hum in the headphones? Do you hear the hum in the headphones of the Scarlett?

Use the shortest cables possible for all of these tests. Cables are antennas. Make sure none of the cables lie on each other or any of the devices.

First, make sure that you have a clean power source. Turn off everything in the house except the amp and see if the hum continues. LED light, light dimmers and other electric or electronic devices can generate hum, even if they are on different circuits. If the hum does go away then try plugging the amp into other circuits in your house.

You say the hum goes away when the laptop and Scarlett are running from battery. Did you plug the Scarlett directly into the laptop or are you still using the USB hub? If you're using the powered hub that might be the source of the hum.

I assume that you don't have anything else plugged into the circuit except the amp, the Scarlett and the laptop. If you have anything else plugged in, even empty cables, remove them and try the diagnostics above again.

Does your amp have a ground loop lifter? If so, engage that and see if the problem goes away. Try plugging your setup into plugs somewhere else to see if the hum goes away.

Your laptop is also a source of RFI. Put the laptop on a long USB cable away from the Scarlett, mic and amp and see if the problem goes away.

If you still have the problem I would suggest getting a power conditioner for the amp and testing for the hum. Order from someplace that has a good unconditional return policy with free return shipping.

As a last resorts, you can put a noise gate in your signal chain but that takes away way too much dynamics.

Answer 2

The SM57 is a dynamic microphone that works by a small wire coil attached to the microphone membrane moving in the gap of a magnet and converting the resulting changes in the magnet field to a signal voltage.

This coil will also pick up external field changes. Like the field of the amp's mains transformer.

Try using monitoring (with headphones) and moving the microphone to a position and orientation where the hum is minimized. My guess is that you are picking up from a position too close to the equivalent of the amp head.

Answer 3

There's no electrical connection between computer and guitar amp, so it isn't a ground loop.

Am I correct in saying the SM57 is correctly connected to the XLR input of the Scarlett 2i2, and you can get a clean recording from other sources?

Does the buzz change as you move the mic and cable around? I'd EXPECT a buzz if the mic cable was draped over the amp, or the interface was sitting on top of it!

Can you try a different XLR cable on the mic?