Ground hum is one of those issues that comes up regularly, and the first thing worth saying is that it's commonly confused with rumble. Both are in the low end and can sound similar when training your ears, but they're completely different problems with completely different causes. Rumble is mechanical, it comes from platter bearings, suspension, motor vibration, or the vacuum system physically coupling into the lathe. Ground hum is electrical, and once you know what to listen for and how to look at it, it's actually one of the easier problems to diagnose.
The quickest way to find you're dealing with ground hum rather than rumble is to open a spectrum analyser and look at your noise floor. Ground hum shows up as a very specific spike, 50Hz in the UK and Europe, 60Hz in the US. It's not a broad smear of low frequency energy like rumble tends to be, it's a precise, narrow peak sitting right on that frequency. If you see that spike, you've got a ground loop. Rumble tends to show up as a broader, less defined lump of energy and often a little lower in frequency, sometimes as low as 20Hz or 30Hz.
It's also worth knowing that ground loops don't always just produce a single spike. They can produce harmonics too, so you might see a spike at 50Hz, another at 100Hz, another at 150Hz, and so on up the frequency range getting lower in volume. These harmonics can sometimes be more audible than the fundamental frequency itself, particularly the second harmonic at 100Hz which sits in a range where our hearing is quite sensitive. If you're seeing a series of evenly spaced spikes climbing up your spectrum analyser, that's a strong indicator of a ground loop with harmonics rather than any other kind of noise.
Ground loops happen when multiple pieces of equipment in your signal chain are connected to mains earth at different points, creating a ground loop. The practical way to find it is methodical, turn things off one at a time and watch your spectrum analyser each time.
Start with your full chain running and the hum present. Then after each cut, switch off individual pieces of equipment one at a time, starting furthest from the cutterhead and working towards it. Each time you switch something off, check whether the hum disappears, reduces, or stays the same. When switching off a particular piece of equipment makes the hum go away, you've found either the source or the piece of equipment creating the problematic earth connection.
Unbalanced connections, standard RCA or TS jack cables, are particularly prone to ground loop issues because the cable shield acts as both the signal return and the ground reference. When two connected pieces of equipment have different earth potentials, current flows through this shared shield and appears as noise in the audio signal. Balanced connections cancel this out, which is why they're preferred in line level audio. We should note though, balanced cables should only be used in the line level audio path, your cutteramp to cutterhead must be unbalanced speaker cable.
Once you've identified the source, there are a few ways to address it depending on what's causing it.
The simplest fix for a specific connection causing the issue is a ground lift adapter or an inline ground isolator on that connection. These break the earth loop at a specific point in the chain without affecting the audio signal. They're inexpensive and worth trying, although, we have found they don't always work.
For a better solution, a mains power conditioner with proper filtering and earth management is worth serious consideration. A customer of ours used an IsoTek Evo3 Cleanline and it eliminated their ground loop entirely. The Cleanline is specifically designed to filter mains noise and resolve earth loop issues, and it's a product that's worth the investment if you're running a professional setup going by the tests our customer did. Cheap power strips with surge protection do not do the same job, if you're going to address it at the mains level, get something properly designed for audio use.
Also worth checking is your general mains wiring if you're in an older building. Poor or inconsistent earthing in the building itself can cause persistent ground loop issues that no amount of cable rearrangement will fully resolve. If you've ruled everything else out and the hum persists, it's worth having an electrician check the earth quality at your sockets. We had this issue in our house years ago, and we used a UPS power supply, this helped us decouple from the mains fixing our issue.
The key takeaway is if you have a low end noise issue, use your spectrum analyser first. If you see 50Hz or 60Hz spikes, possibly with harmonics above them, you've got a ground loop and it's fixable. If you've got broad low frequency energy with no specific centre frequency, you're looking at a mechanical rumble problem and the fix is completely different. Diagnosing correctly before you start changing things saves a lot of time and frustration.