home | tumblrblog | musics | electronics | ask | about | more
Tone circuits found in guitars are typically the simplest form of what are called filters. Mathematically, filters can be very complicated to express, but conceptually the filters found in guitar circuits are very simple to understand. Usually, the filter in a guitar is built using a capacitor and sometimes a resistor to create an RC circuit. Depending on how the capacitor and accompanying resistance are arranged, you can create either a hi-pass or a lo-pass filter, which do just what they sound like: lo-pass passes low frequencies by attenuating highs; hi-pass passes high frequencies by attenuating lows.
The configuration of whether a filter is hi-passing or lo-passing is easy to see if you simply think that high frequencies like to go through capacitors, whereas low-frequencies simply can’t go through them. A capacitor is actually made of two conductors separated by some insulating material. Current doesn’t actually flow through capacitors at all, but the effects of alternating signals makes their way through; the higher the frequency, the easier for the signal to pass through. In the lo-pass circuit—typically found in Tone controls—the high frequencies would rather go through the capacitor to get to the ground, whereas the low frequencies can’t get through, so they get sent out of the guitar. In the hi-pass, the opposite occurs: low frequencies can’t get through the capacitor, so they flow through the resistor to the ground instead, leaving only the high frequencies to leave the guitar.
One quick note about their schematic symbol: many capacitors are not polarized, meaning you can wire them in either direction. The major exception is when using higher-valued electrolytic capacitors. These are almost never used inside guitars, but you never know what you’ll run into. They will be clearly marked with an arrow flowing to their negative terminal. Because of these electrolytic caps, it is typical to see a polarization symbol even if it is not necessary. The curved part usually is connected to side that will have the lower voltage, like ground, for instance.
Now, what about this situation? Neither component is ground-referencing, so you might initially assume that it would not affect either highs or lows: highs go freely through the cap, and lows can go around the cap through the resistor. However, you must remember that every sub-circuit must be connected to a larger circuit, where there are more components to complete the circuit. In this situation, highs do indeed flow freely through the cap, but lows have a greater resistance since they must drop across the resistor, therefore later on they will appear to other circuitry as having a lower voltage. Therefore, it is a hi-passing circuit.
The value of a capacitor affects the crossover frequency where the capacitor starts affecting things, but so does the resistance that the capacitor sees. This frequency value is inversely proportional to the cap’s value and the resistance it sees. The easiest way to think of this is that high frequencies are more likely to flow through larger caps, and lower frequencies are less likely to flow through large caps. It’s a complicated concept to wrap your head around at first, but the more you see it in use, the more that will come together.
Also keep in mind that the resistance that a capacitor sees affects the crossover frequency too (the R part of the RC filter). Therefore, circuitry before and after a capacitor will affect the way the capacitor filters the signal. This is why different pot values matter, and why different pedals change the guitar’s tone, even some when they’re not on. There are too many variables to be able to easily generalize some assumptions; the best advice is to simply recognize this fact. If you are more interested, the best thing to do is to start studying RC filter topologies.
Rarely, inductors are used to filter a guitar’s signal. We already discussed the effects of inductors when talking about pickups. Inductors are like the opposite of capacitors; low frequencies can pass through no problem, but higher frequencies get converted to magnetic fluctuations, so they have a hard time getting through inductors. Keeping this relationship in mind makes understanding their use much easier.