Most passive guitar circuitry consists of the same ‘basic’ modules, i.e. volume and tone controls, which are connected in parallel mostly via a switch which lets you choose different combinations of your pick-ups. In the following diagrams, ‘signal in’ will either be a wire coming from your pick-up, or the ‘signal out’ from a previous module. The last ‘signal out’ will be connected to the output jack.
An important concept in electronic circuits (not only for guitars) is the ‘ground’. The ground in a circuit is a reference voltage that is used all over the circuit, and is indicated by the following symbol: . The name stems from early electronic circuits like telegraphs which actually used the earth, or ground, itself as one connection and a single wire as the other. For instance, many active circuits use the negative pole of the battery as the ‘ground’. In guitar circuits the ground is always the outer ring of the output jack (i.e. it is connected to the largest metal segment on the plug).
Using a common ground for everything allows to express the signals of every other point in the circuit with one single value, being the difference between that signal and the ground. It also simplifies diagrams because instead of drawing connections between all grounded points, they can simply be marked with the ground symbol. When shielding a circuit inside a cage (or metal box, like the foil method described on the main page), you should always connect this cage to the ground. The reason is obvious: the grounded cage (acting like a Faraday cage) will capture all noise signals and prevent them from reaching the actual signal wires. Because the ground is used as a reference it doesn't matter how much noise it receives.
This is the scheme for a standard volume control. The component used is a potentiometer, often abbreviated to “potmeter” or “pot”. It consists of a ‘track’ of graphite, which is connected to the two outer pins, and a taper which slides over this track as you turn the shaft. This way, the impedance between the middle pin and the outer pins varies. The potmeter in the image (at the left) is viewed from the bottom, i.e. with the shaft pointing away from you.
For guitars, mostly potmeters of around 470kOhm are used. This is rather high compared to other audio circuits, but as guitar pick-ups can have impedances of up to 16kOhm, it's a logical choice. However, potmeters with a lower value will do too, if they are approx. 10 times the impedance of your pick-up with the highest impedance. E.g. if you have a 4kOhm single-coil and an 8kOhm humbucker, you should use pots of at least 80kOhm. But if you're not sure, just use 470kOhm.
As you can see on the scheme, the metal casing of the pot must be connected to your circuit's ground. If your pots are mounted on a conducting plate, it suffices to ground one of your pots' casings, all other pots will then automatically be grounded if they make good contact with the plate. Otherwise you'll have to solder a ground wire to each of the pots' housings.
Important: you must always use logarithmic potmeters for audio circuitry like this. This is indicated by “LOG B” or “B” on the pot's housing. For this reason such potmeters are also sometimes referred to as “audio taper” pots. Do not use linear potmeters, with “LIN A” or “A” indicated. The reason for this is that the human ear has a logarithmic response (in both volume and frequency). There are also “LOG C” potmeters (so-called ‘antilog’), which are reversed compared to B pots. This is certainly not what you want. A log B potmeter will actually make the signal vary according to an exponential curve if you turn the knob clockwise, and this is exactly what we need.
A standard tone control consists of a capacitor connected in series with a variable resistor. The way this works is as follows: a capacitor conducts electricity in a better way as frequency increases. By connecting the capacitor between signal and ground, high frequencies will be “short-circuited” and low tones will pass. The amount of ‘grounded’ high tones can be controlled by the variable resistance.
The value of the capacitor determines its frequency response. The higher the capacitance the more high tones will be grounded. Capacitance is expressed in the µF unit, “micro-Farads”. For guitars, capacitors between 0.02µF and 0.05µF are used. This only counts if you use them together with 470~500kOhm pots. The value of the capacitor should be around 0.02µF if the pick-up it controls has a high impedance (like a humbucker), and around 0.05µF if the pick-up is low-impedance (e.g. a single-coil Stratocaster pick-up). Such capacitors are ridiculously cheap, so when you go to your electronics store it would be a good idea to buy a whole range so you can experiment to find the optimal value.
For the pot, the same comment on grounding and type (Logarithmic B) is to be taken into consideration. There are other ways of creating a variable resistor from a pot, e.g. by just grounding only the middle pin, or grounding the rightmost pin and connecting the signal at the middle pin and leaving the remaining unconnected. This is not recommended however, because the unconnected pin of the pot will be ‘dangling’, causing noise on your signal. The more grounded stuff in your circuit the less noise.
There are several ways to make a pick-up selector circuit. For instance you could provide a mini-switch to turn on/off each pick-up. But if you want to keep the amount of clutter on your guitar as low as possible, it may be a better idea to use a single selector switch. The most popular model is the Stratocaster switch, which has 5 positions. If you have three pick-ups A, B and C, this allows you to switch your pick-ups in the combinations A, A+B, B, B+C, and C. How this works is illustrated below. You can also use the switch with two pick-ups and additional tone control circuits.
As you can see, the switch consists of a rotating metal plate which is always connected to one contact “S” (the ‘signal out’ of the switch), and can be connected to one or two of the other contacts at the same time. It has a series of such contacts at each side. So you could switch two separate circuits at the same time. Whether you'll be using both sides of the switch depends on the rest of your configuration. If you only want to use the switch to select pick-ups you only need one side. It may then be a good idea to connect the corresponding pins of both sides together, resulting in “double” contacts. This will reduce the risk of crackling during switching as there is less chance of a bad contact.
An example of a circuit where both sides are used is the standard Stratocaster circuit, as shown below.
This circuit uses only one capacitor with two pots. The capacitor is connected between ground and taper pins of both potmeters. Depending on the position of the switch, none of the pots is used, only one, or both at the same time.
If you have volume controls for each pick-up separately a selector switch may be redundant, for you can turn off a pick-up by setting its volume to zero. But of course this is slower than a switch and will wear out the potmeters more quickly.
An active circuit contains some form of signal amplification, therefore it requires external power. For a guitar the only practical power source is a battery, although it would be possible to feed DC current through one channel of a stereo guitar cord. In its simplest form an active guitar circuit consists of a classic passive circuit, with an amplifier between it and the output jack. Besides providing a stronger output signal this may seem pretty pointless, but it isn't. A guitar with only a passive circuit will sound different depending on the impedance characteristics of the amplifier or even just the instrument cable it is connected to. Especially tone controls will behave differently. By ‘buffering’ the output with an amplifier the output sound will be much more consistent. And, active circuits can be more versatile, in theory you could put an entire effect unit inside your guitar. The only considerations are power usage (you don't want to drag along a whole arsenal of batteries), and noise. Active circuits will inevitably produce noise which will be worse with more shoddily constructed circuits. When incorporating an active circuit in your guitar you may still want to provide a switch that allows to bypass the active part, for in case you run out of power at the wrong moment or if you simply want to feed the raw output of your guitar directly to the amplifier. And, don't forget an on/off switch. You can create an automatic on/off switch with a stereo output jack and a mono guitar plug, by using the ground segment of the plug as the switch.
This circuit combines all of the above (except active circuits), plus some extra features. The switches S1 to S3 are part of the potmeters, which are “push-pull potmeters”. This means they have a built-in switch which can be controlled by pulling or pushing the pot's shaft. The switches are used here to use both coils of the humbuckers (normal mode), or only one coil, which gives a clearer sound (but more hum).
The switch S4 is a phase switch, which allows to reverse the middle pickup's phase. When the selector switch is in position 2 or 4, this results in a distinctive sound, often used for “surf”-type songs.
As you can see, this circuit uses the tone control method I previously advised against. This is because it was already soldered when I bought it (it actually is part of an old guitar), and I didn't yet have the time to change it. Besides, since the entire circuit is encapsulated in a cage of grounded foil already, it won't have much effect.
Only one side of the selector switch is used, but as mentioned above, both sides are connected. Despite this, the switch did start crackling annoyingly after a few years. This can be fixed by spraying the contacts with contact cleaner like Servisol Super 10. The same goes for potmeters. If using contact cleaner doesn't help on crackling potmeters, they may be worn out and need to be replaced.
Coming back to the topic of the ground in electronic circuits: mind that not just all parts in your guitar's circuit should be grounded to minimise noise: ideally all metal parts should be grounded. This may be impossible for some disconnected parts but the farther they are away from the circuit the less influence they have anyway. By far the most important large metal parts are the strings. If you don't connect the strings with your circuit's ground, your guitar will produce an annoying amount of noise and hum as soon as you touch the strings. So you should connect your circuit's ground to the bridge, which (being metal itself) will ground the strings too.
The problem with grounding, especially grounding the strings, is that there is an increased risk for electric shocks. Many an artist has received shocks while simultaneously playing the guitar and singing into a metal microphone because the microphone was on a different ground voltage than the guitar. When the difference is really large, a large current can flow with potentially lethal consequences. I do not recommend disconnecting your strings from the ground as this will make your guitar unusable. Instead, you should definitely make sure that all stage equipment is connected to a single grounding point or use isolation transformers where necessary.
There is a ‘trick’ which involves connecting the bridge with the circuit's ground through a capacitor instead of a wire. If the capacitance is large enough, this will effectively filter out hum and noise without allowing the flow of a large continuous current (capacitors block DC currents). However, this is not as safe as it may seem as most capacitors will short-circuit when subjected to too large a voltage, which is exactly the moment when things get dangerous.