From the course: Real Recording School Weekly

Tracking one guitar with two amps: Ground hum - Pro Tools Tutorial

From the course: Real Recording School Weekly

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Tracking one guitar with two amps: Ground hum

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- [Instructor] In the studio, sometimes we will use two guitar amps sharing the signal from a single guitar in order to get bigger tones, or to get a wider stereo sound. I have two other videos covering aspects of how to split the signal, plus how to deal with polarity and phase issues. But in this one, we'll cover ground hum issues when splitting the guitar signal between two amps, and how to solve those problems. For our examples, I recorded a guitar split via a Little Labs PCP-buffering splitting device, and via a simple jump ring between two guitar amp inputs. Both situations present different ground loop scenarios, where amps are sharing grounds and causing extra hum to occur. Our first example is the PCP splitter with no earth lift engaged. I have all the levels boosted to the maximum in this session, so we can hear the hum. (electrical humming) Note that you have different hum coming from the left and the right channels. Those are the two amps and the way that they are affected by the ground loop situations. Zooming in, you can see the audio on these tracks. It's there. (laughs) Listen closely, you can hear some definite low hum. (electrical humming) Next I lifted the shared ground from the amp on the right by depressing the earth switch. And here's what it sounds like lifted. (quieter electrical humming) Compare that to the previous, not lifted. (electrical humming) You can obviously see that the audio on the right side amp gets quieter. and the hum gets quieter. And there are some rather random background sounds as always. (laughs) So then I set the PCP switch back where it was and tried another trick. These little ground adapters can be used to lift the ground of a guitar amp. They can be dangerous if done incorrectly, so if you have any worries or concerns, never try this. It is best to use a volt meter to see if any of the cables, guitar, or amps have a voltage difference that could harm the user. In this first example, I have treated the guitar amp on the right with one of the ground adapters. (electrical humming) On the second, I treated the guitar amp on the left with the ground adapter. (electrical humming) If you're getting the feeling that this is a moving target, you're correct. Let's go back to the initial with no lift. (electrical humming) And using the PCP to lift on the right. (quieter electrical humming) That's again, guitar amp lifted on the right side, (electrical humming) guitar amp lifted on the left. (electrical humming) We're hearing a lot of different things there. You can also try the ground switches built into some guitar amplifiers. Not all have them, and many times I find them ineffective for some reason. Here I tried the amp on the left's ground switch when I had both amps jumpered together to split the guitar sound. And here it is jumpered. (electrical humming) And then here it is using the ground lift on the amp on the left, the switch on the back. (electrical humming) So it does change. (electrical humming) The frequencies change, the amount of noise changes. Once again, a moving target. If you look at these examples in spectral view, it's easier to see what is going on. The bottom line that we see down here is always the 60 hertz hum coming from the AC power from the walls. Hard to hear on most stereo systems, or headphones even. Very, very low. But all the other lines that we see are harmonics that are created above the 60 hertz, at 120 hertz, and above that, at 240, and those are more audible. But then above that are further harmonics and noise. Very quiet, very hard to hear. (laughs) I boosted this as much as I could. And the different examples, these are all taken from the Pro Tools session we were looking at earlier, and you can see that there is a constantly moving target of low hum and high noises. I'm just gonna skip through them here. Very quiet. In this example here, you get more on the left side. So you can see that eliminating hum when using two amps is important. And these ground loops and the different amps and the situations between them will cause audible problems. In my experience, like I've said before, it's always a moving target, and different amps, stomp boxes, and others will generate different hum situations. Use all these ideas as a starting point, and clean up your guitars so you can just hear the music.

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