In order to build the guitar systems without the hum you have to know how the ground loop arises and when it will be the ‘hum’ cause. ‘Hum’ is the sound which occurs after amplification of the voltage induced in cables by the disturbing electromagnetic field. Disturbing EMF is generated near to supply transformers, and sometimes even near to supply cables with the flow of big current (e.g. 50A). The bigger the transformer power (and its size and weight) the bigger the disturbing field.
Fortunately intensity of this field drops considerably along with increasing the distance. For example the typical 100 Watt tube amp supply transformer mostly interferes in space up to 1 meter around itself but in 2 or 3 meter distance the field is almost completely omitted. It is easy to check it with the single coil pickups guitar.
But what is exactly the ground loop?
The ground loop is any fragment of our system in which the ground wires (signal or power cables) make the closed loop (in other words the coil).
If such a coil is enclosed by disturbing electromagnetic field there appears in it the current induced by that field. This current puts on the signal cable ground the voltage which generates audible hum. It is particularly heard when we use the high gain (overdrive channel). There is also another rule – the bigger ground loop size the bigger hum we hear.
How to avoid the ground loops?
Firstly, building the systems you have to do it avoiding appearance of the ground loops. Especially those which are related with the amp as an amp has always the supply transformer inside.
The best way is to draw your installation scheme to see at once where the ground loops are.
You have to remember that inside of some devices there are used the isolating elements in order to avoid the potential ground loop. For example every of MIDI connections is not the galvanic one because just after the MIDI IN connector there is an opto-isolator which provides the isolation. Similarly to the relay type outputs (SWITCH OUTPUT) in the controllers (e.g. G Lab GSC) where the isolation is provided by the electro mechanic relay. More complicated is situation with the multioutputs power supplies for guitar effects even though many of the producers indicate exactly where the isolation occurs (e.g. G LAB PB-1). In many devices there are the switches named GROUND SELECTOR or GROUND LIFT that serve to eliminate the ground loops and are also necessary because lack of the connection with the ground makes also the hum problem. You have to remember also that ground loops appear also due to the supply cables containing protective wire (Protective Earth named „grounding”). Guitar tube amps posses a connection between signal ground and protective wire which you have to remember about. That’s why connecting the guitar simultaneously to two amps by regular A/B/Y switcher creates the ground loop and hum.
Please notice that not every ground loop makes the ‘hum’ herable. If it is small size (e.g. between the devices in pedal board) and there are no bigger transformers (>10W) in pedal board and around it, you will not hear that a ground loop exists even on the high-gain.
If we use an amp and stompboxes the ground loop arise mostly from connecting to common supply the effects placed between the guitar and an amp input and the effects connected to the amp effect loop.
You have to pay attention with preamps and other devices (rack effectors), which features a power supply with ground (PE connector). If the PE wire is connected with signal ground (as it is in many preamps) and our amp or power amp also has a connection between PE wire and signal ground there will appear a ground loop (more or less hearable). Good solution in such case is to use low resistance (of shield ground) signal cable.
An example of a ground loop which appearance is not always hearable is connecting to the amp FX loop the effect. You will not hear the hum of the ground loop with high signal level on the loop (it is not always so) and which signal (SEND and RETURN) cables connected to the effects are placed as near as possible one to another (the best way is to stick them every 20 cm with plastic band clip on the final stretch near the amp).
Basic rules of building the guitar system:
||to the effects system situated between the guitar and the amp input from the system with the effects connected to the effect loop (there shouldn’t be connection between them e.g. by the power supply or looper),
||the connection of earth with the signal ground should be only in one device (usually in the amp). In some devices e.g. power supplies or “bigger” effects the supplying wire has the protective contact but mostly it isn’t connected or connected thru the resistor (from 10 to 50 ohm) with signal ground,
||to use the effect power supplies with isolated outputs to supply the particular effects,
||to supply from the same voltage source (if necessary) only the effects and other system elements that are in the same fragment of the system (e.g. on the effect loop),
||to keep the distance between or to isolate the metal cases of the JACK connectors to prevent from the accidental connection with other ground – especially that from footswitches and amps controlling signals.