500 kilo, 250 kilo. Ohm My…
A popular way to get rid of some highs in your tone is to solder a 500k ohm resistor on your 500k ohm pot to effectively turn it into a 250k pot. There are even some wiring schematics around that make a 500k pot a 250k pot depending on the position of your five way blade, so you have a 250k pot with your singlecoils and a 500k pot with your humbucker. It is also known that a 500k pot will give you more highs than a 250k pot. But the “why” remains a cloudy subject. So let’s get to the bottom of it: why does the value of the pot matter?
In order to get an answer to the question we have to take a closer look at what a pickup, tonepot and volume pot actually are in an electronical sense. The volume pot and tone pot share one major componant: the potentiometer. It’s normally thing more but a current divider: it divides the current into two parts relative to the resistance. One part goes to where you want it to go, and the rest bleeds away through the ground. Here’s a picture:
The law of Kirchhoff states that a voltage has to be equally distributed over all resistors in a circuit, relative to the resistors. That law is of great help to understand what a pot does. Suppose that a pickup produces 100 volts and the potmeter is 100 ohms. When the pot is fully open, the first resistor (R1) is 100 ohms and the second resistor (R2) is 0 ohms. The entire 100 volts goes over the first resistor, because the law of Ohm comes into play. That law gives the relation between voltage, current and resistance: U=IR. The U is voltage, I is current and R is the combined value of R1 and R2 (so, R1+R2=R). In other words, if R2 is 0, the whole signal has to go to R1, because you cannot devide by zero. That goes the other way around, too: If R1=0, R2 has to be 100, so the signal has to go all the way on R2, and since R2 is connected to earth, we cut out our signal.
To understand how pots work as a tone pot and why the higher valued pots bleed out less highs, we have to take a look at something called filters. Filters come in all sorts, shapes and sizes and can be very simple – like the tonepot in a guitar – or incredibly complex. They can filter out some frequencies, but also boost. Filters can be passive or active, meaning that the signal gets buffered and amplified to compensated for signal degradation (which will inevitably happen if you put a lot of components in a circuit).
A tone pot is a simple filter. Because it has just one filtering component, it can be called a first order filter. Because of its construction it allows only low frequencies to pass, so the full name is first order low pass filter. The ‘unusual’ part of the tone pot is that it allows you to change the resistor part of the entire filter. In other words, a tone pot with a 500k ohm pot will give you the same sound rolled half way down as a 250k pot in the same circuit. The amount of highs that can actually be rolled off depends on the capacitor and resistor. The resistor will cut off an initial amount of highs but the final outcome when rolled down depends on the capacitor too. So, using a higher value capacitor will roll off more highs than a lower value one.
The entire filter also has a resonance peak, though. This means that the way the filter filters differs with its components. In practice you can say that a 500k pot and a 470 PuF capacitor will roll off different high frequencies (and other frequencies, too!) than a 250k pot and a 220 PuF capacitor. Playing around with this will give you different results and might lead to a tone you like.
The volume pot is a whole different story. The volume pot works in conjunction with the guitar pickup itself. The pickups we regularly use are passive pickups; no battery required. These pickups can be seen as second order low pass filters with a resonance peak. A second order filter is a filter with two filtering components: a capacitor and a coil. The coil of the pickup is actually both at the same time. All the windings of the coil create a capacitance which bleeds out some highs, too, next to the coil bleeding out some highs. The resonance peak is the frequency at which the pickup resonates, which can be seen as the frequency the pickup puts out most easily (this is not entirely true, but it’s an easy way to look at the resonance frequency for the time being).
The term ‘load’ is being used more often and mostly in relation to tube amplifiers. It’s a known mantra amongst players who use tube amps: never leave your tube amp on without a load, or it will harm your amp! The load means nothing more than that it ‘takes away’ the voltage and/or current of whatever is producing this signal. There are two kinds of loads that are relevant: resistive load and capacitive load. A capacitive load will lower the frequency of the resonance frequency. A good example is a long guitar cables: the longer a cable is, the more highs you lose. A resistive load will lower the amplitude of the resonance frequency. Coincidently, because a volume pot is a resistive load, you lose some highs when you roll down your volume pot.
A second order low pass filter has the following, general schematic:
The resistance of the pot is a variable one, but its location does not change. The next schematic will just show the resistance that the pot is dialed in to at any given time. This can be anything between zero (making a short circuit so no signal passes through to the amp) or the maximum of your pot (e.g.: 250k or 500k ohms). I didn’t feel the need to draw in the entire potentiometer (as a voltage divider) because all we’re interested in is the value that works on the pickup. Hence, the schematic with the resistance of the volume pot is the following.
The resistance of the potentiometer that goes over the second order filter is actually the most important bit. It changes the amplitude of the cut off frequency. The lower it is, the harder it attenuates the higher frequencies because of the relative impact it has on the impedance of the second order filter (i.e.: the pickup). The higher it is, the less impact it has, the less it can shift the cut off frequency, and the less highs you lose.
The actual reason to why this is the case is very difficult to explain. Here’s a simplified version: the resistor acts as a load over the system (the pickup in this case) and if the load is not ‘matched’ with the system it will draw more current than the system can provide. As a result, the load will draw out more current where it can, and that’s in the higher frequencies, of which there is plenty of ‘power’. For this you have to imagine the entire sine wave of electricity being pumped out by the pickup as you play as having spikes in certain frequency bands, and the average of all those spikes together is what we perceive as the total output.
For the load to be a match, the design of the circuit has to be considered. The pickup maker designs a pickup with specific parameters, and the bigger the mismatch ‘above’ the value of the entire impedance of the pickup., the less current is being drawn and thus the less highs you loose. Why a lower value resistor draws more current is very easy to understand in terms of math, though. The law of Ohm becomes very useful again. The law of Ohm gives the relation between Voltage, current and resistance, U=I * R. The amount of voltage stays the same, so if R drops, I has to go up in order to keep U at the same level, and vice versa.
Interestingly, this also happens in your tube amplifier. If the impedance of the cab and amp don’t match, the speakers will draw more current if the mismatch goes down (I.E.: the speakers ‘need’ 8 ohms but are getting 16 ohms) or will draw less if the mismatch goes up (I.E.: vice versa, speakers are getting 16 when they should be getting 8 ohms). The problem with the last option is that your amp can’t properly release its power, so a ‘build up’ of energy will happen inside the amp. The result is that you might destroy your tubes or even worse, you might even destroy your output transformer. This won’t happen the instant you plug in the mismatch, but it’s not a wise idea nevertheless.
But back to pots! Some players absolutely love the small attenuation of highs by a lower value pot, others can’t live without the highest of the highs. Some players want so much clarity and definition and as little coloring of the pickups and pots as possible that they choose to use low impedance pickups. Some players find these pickups to sound cold and sterile, others love them. Different strokes, different blokes.
With special thanks to Chris Winsemius.
Further Reading: Getting The Most Out Of Your Guitar’s Controls While Playing