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I think grounding is too often overlooked when it comes to understanding how an electrical system operates. When it comes to guitars, improper implementation of grounding schemes often leads to what are called ground loops, which is a major source of hum and noise. Without understanding the purpose and proper use of grounds, it’s very hard to pinpoint problems whenever there’s unwanted noise suddenly coming from your amp.
The ground’s basic function is to provide a common path for currents to return to their sources after they have flowed* through circuitry. It is convenient to make this common path zero volts, that way there is a “reference” point for all the circuitry that’s sharing it. It should be pretty simple to understand: a circuit is basically a complicated circle; signals are made somewhere, and their currents need a path the return to their source.
In an amplifier, for example, the chassis is often used as the ground reference point for a few reasons. For one, since the ground’s voltage is desirably zero volts, it is unlikely that you will get shocked by touching the chassis and another (hopefully) grounded thing, like a microphone. Another good reason is that stray electrical fields interact with the chassis before interacting with the circuitry inside, and since the ground is ideally a single connection to a lot of higher-resistance circuits, any induced current has nowhere to go but back to its source within the chassis.
This is part of the reason why single coil pickups are so susceptible to noise; there is no easy path for these induced currents to flow when the coils are in these electrical fields, so they flow through your guitar cable and get amplified by the amp, instead of having a quick path to return to its source inside the guitar. If there were a big hunk of shielding surrounding the pickup, then noise would be significantly reduced, and in fact some guitars and basses have such a shield surrounding the pickup. But take note that if this shielding is not actually connected to the ground, then it has no effect. This sometimes happens with guitars that have sheets of metallic foil lining their inside cavities. Likely, the guitar builder was told that this shielding reduces noise, but never realized that it is only effective when it is actually grounded. Additionally, pots and knobs are typically shielded as well. This explains the large “solder blobs” found on the housings of pots.
Now, sometimes it seems like there’s an unacceptable amount of hum from an amp even when your guitar is turned down. This is especially the case when you start using complicated strings of guitar pedals. Additionally, you may have a guitar that seems to be wired with plenty of connected shielding and lots of ground points, yet there is still hum and noise. Or, when you’re at a gig and the soundguy makes you use a DI box, suddenly your signal becomes noisy and really makes you worried you’ve done something wrong. These problems usually have to do with a phenomenon called ground loops. A ground loop is simply when there are multiple paths in the ground network that a signal can return to its source. When this occurs, bad things happen. A ground loop may be simple to understand in theory, but hard to spot in practice. Let’s break it down.
Here we have a simplified circuit of a guitar and an amplifier. The signal flows through the guitar circuitry, then through the amp’s circuitry. We can represent the the return path by using the ground symbol, as seen on the right diagram above. The two diagrams are equivalent, and the right diagram better shows that these two elements may be a part of a larger picture, sharing a common ground with many different devices.
Now, even though a guitar cable looks like one wire, it actually contains two: the tip for the signal, and the sleeve for the return. The sleeve doubles as a shield from electrical fields for the signal’s path. So if we include the guitar cable in our ultra-simplified guitar/amp diagram, we must show that the guitar itself is not directly grounded, but it is grounded through the sleeve. To put a ground symbol in the guitar diagram would be redundant and imply that there is some other connection between the guitar and amp that is separate from the sleeve. This would, indeed, be a ground loop.
Let’s pretend for a moment that we did ground the guitar independently from the sleeve; we’d have a situation very similar to what we see above. Every electrical path has some sort of finite resistance. Ideally, our guitar cable has a very low resistance so that signal can pass through unimpeded. In reality, there is a very small resistance in both the tip and the sleeve. Normally, this isn’t a big deal, because in comparison the amplifier has a much larger resistance, and therefore the interaction of these small resistances doesn’t really matter. But, our redundant ground line also has a finite resistance. An example of this redundant ground could be a guitarist’s sweaty hands touching the strings (which are usually connected to the guitar’s ground) and a grounded chassis of an amp or microphone, for example.
Anyway, having this second ground creates a loop. In an ideal world, this would not affect the signal very much even if the guitar cable and the extra ground had significant resistance. But we must remember that there are many stray electric fields all around us, and this loop presents a problem when current is induced in these loops. Because of the two paths, current can now easily flow around the low-resistance loop. What used to protect our circuits from these stray fields is now leaking a small part of these unwanted signals into our signal-bearing circuits, which gets amplified. These stray signals may be very small, but so is our guitar’s signal, so it becomes a problem very quickly.
Usually, your hands actually have a large resistance and won’t likely cause a ground loop, but one notorious case of ground loops being a problem is a poorly-powered effect pedal scheme. Ideally, effect pedals are powered from an isolating power supply. What this means is that the ground in the guitar’s path is electrically isolated from the main’s neutral (the “cold” AC prong), which eventually connects to the main’s ground (the big third prong on an AC power cable). Because of this desire for electrical isolation, only two prongs are typically used for effects pedals. The third prong is there for safety reasons, and effect pedals generally are low-powered devices not drawing enough power to cause a hazard in case of a failure. The idea of this electrical isolation here is that the pedal is able to reference the zero volts from the main’s ground (not a shock hazard), but doesn’t create a redundant return path for the signal (a ground loop).
Unfortunately, many effect pedal power supplies are cheap and do not properly filter this isolation, effectively creating a “virtual” ground loop in the isolation circuitry. Currents are induced through the sleeve of your guitar cable, go to your guitar’s amp, go through the ground to the neutral, and then eventually pass through the AC neutral via this virtual ground loop and back to your guitar cable’s sleeve. A small amount of this induced current leaks into your guitar and mixes with your signal, causing hum.
There are two solutions to this problem, and both involve killing this ground loop while maintaining one return path. One way is to simply invest more money in a better isolating power supply for your pedals. These filter the ground isolation so that the pedals still reference the ground potential while isolating unwanted signal flow. The other solution is to actually disconnect the sleeve going to your pedal. It sounds crazy, but guitar signal is already returning via the AC ground, so the extra sleeve ground is unnecessary. It is, however, important to keep one end of the sleeve connected so that the benefits of its shielding are maintained, and the sleeve going to your guitar must always be connected on both ends for signal to actually have a path to return.
A similar situation happens with DI boxes. The mic cable coming from a DI box is grounded at the mixer board, and so is your amp. Improper ground isolation within the DI box can create a ground loop between the box, the amp, and the mixer board. In the schematic above, the sleeve of the mic cable is ideally isolated from the guitar cable’s sleeve, but poor isolation makes this a problem. This is why many DI boxes have a ground lift switch; cutting the ground at the box cuts the ground loop. The mixer doesn’t need your ground, because the signal already has a return path through the mixer board’s ground and through you amp.
(Just a quick side-note and reminder, these schematics are extremely simplified; I know a DI box has a balanced output. These are just for demonstrating the concepts.)
The key to maintaining good grounding practice is to recognize and implement singular ground paths. Often, this mentality is called the “star” grounding technique. In any electrical system using grounds, there is one point in the circuit that is decidedly the ground “source”, and the rest of the components reference this “source” only once. The same is true when wiring a guitar. You don’t need to connect every single pot housing to each other; one connection is sufficient. Connecting the shielding of your guitar’s inner cavities to the ground multiple times also is not a good idea. It is often overlooked that the bare shielding wire in pickup wire bundles goes to the housing of the pickup, so physically attaching the pickup to a ground point via a screw actually creates a small ground loop when the bare wire is connected to ground. A little attention to these details can salvage a noisy guitar rig and be a lifesaver in the studio.
A couple of side-notes to finish things up: First, musicians somehow seem to discover that hum problems are usually “solved” by removing the large third ground prong from one of the pieces of equipment that they own (you’ll find this with mixers, amps, active speakers, etc.), usually by physically removing it or by buying an adapter that does so. This is called “lifting the ground”. Yes, this often does solve the ground loop issue that we’ve been discussing because the mixer/amp/etc. has electrical isolation between the neutral and ground. BUT! This is ONLY ok when the device has been specifically designed to lift the ground (like with DI boxes for example). Remember, the third ground prong is there for safety reasons, and removing this safety feature could cause some serious problems in certain situations. Even some old equipment designs are missing this third prong (back before it was considered an issue), and these devices should be modified to be updated as soon as possible (see more info here).
Second is just a small note about ¼” guitar jacks: these are often redundantly grounded to both the chassis and another ground point. This is because the sleeve electrically connects to whatever the jack is tightened to, so if the chassis is bare metal and is grounded, the sleeve will electrically connect to the chassis, and a separate ground connection is just redundant. Often, though, it is desirable not to directly connect the sleeve to the chassis, and in these cases plastic washers prevent this connection, and then the sleeve may be wired to whatever specific ground point is desired. Just a heads up on that small ground loop issue; it seems to be overlooked far too often.
back to basic passive guitar electronics
*2023 footnote: I probably should have led somewhere that I’m not an experienced electrical engineer and that I was writing these articles for study and exploration. I realize I don’t really convey the concepts here totally correctly (for example, it’s not technically correct to say that current “flows” through circuitry), but I’ll leave this article as-is for posterity anyway. There are still some practical ideas that could be fun to explore.
Another 2023 footnote/tip: One common source of noise & buzz (especially 50/60Hz mains hum) is the simple fact that signal cables are too close to power cables or are running in parallel with them. A super easy fix to a noisy rig is to take some time to physically move all power cables, power plugs & receptacles, and power supply processing areas (like a PSU box) away from signal cables and signal-bearing parts of components. If you do need to run signal lines over power lines, run them at right-angles (perpendicularly) so that their EMFs have different phase angles and don’t induce as much current. I probably could have mentioned that in this article back in the day but I wasn’t really thinking about all the practical aspects! Experiment and have fun busting hum!