Posted Jan 23, 2012 08:08 AM
Another element of tone that is often overlooked are a guitar's electronics. How a guitar is wired and how the pickups are configured has a paramount effect on the final sound. In order to control the volume and tone on a guitar, a component called a potentiometer, or pot, is used. A potentiometer functions as a potential divider. This can be considered as two resistances in series which effectively divide the signal from the guitar's pickups in the same ratio as that of the resistances. Commonly found values are 250k (single coil), 500k (humbucker), and 1M.
A potentiometer, or pot, is a variable resistor. This means that as the knob shaft is rotated, the DC resistance will change. A pot is very simple by design, and once the components and their purpose are outlined they should become less mysterious.
Potentiometer Resistance Values
There are 2 main values of pots you can wire with passive pickups: 250K and 500K. The 250K pots produce a warmer sound and are usually used with single coil pickups to tame their brightness. 500K pots, which are brighter, are used in conjunction with humbucker pickups. A humbucker by design will cancel out some of the high frequencies in a signal, and the use of 500K pots is in order to make up for some of the brightness that is lost. A change of potentiometers is a great way to modify a guitar that is almost to sounding like the player demands.
Potentiometer Taper Values
In addition to the value of the pots, the taper of the potentiometer is also something to be considered. There are two types of taper, audio/log and linear. A linear taper provides a 1:1 ratio in adjustment, while an audio taper is logarithmic. Often times volume controls utilize an audio taper pot, in order to give a smoother transition. Tone pots are usually of the linear taper variety, as the fluctuation of this parameter is hardly noticeable to the human ear.
Both audio and linear pots have the same total resistance. The difference between them is when the position of rotation reaches a 50% value. Linear pots are usually marked with a B or Lin. A linear pot will reach 50% of its total resistance in the 50% rotation point. Audio taper pots are usually marked with an A or Aud and will decrease most of the resistance in the last 50% of the rotation. The audio taper is perceived as a more gradual reduction. Some manufactures use audio taper pots for both volume and tone controls. Some use linear taper pots for both volume and tone, and still others use linear taper pots for volume and audio taper pots for tone. Experimenting with different taper values can really help one customize the sound of their guitar even further.
Higher frequencies travel more readily to ground, and a guitar can sound muddy as the volume is rolled off. Many builders overcome this problem by using a "treble bypass" capacitor between the input and the output of the potentiometer. The higher the value of the cap, the more high frequencies are allowed to travel through it. A tone pot uses the same properties of a cap, but instead of letting the frequencies slip by to the amp, they are sent to ground. Most tone control caps are of a higher value than treble bleed caps, so the overall effect will be more noticeable and will provide more tonal flexibility.
The capacitor and tone pot are wired together to provide a variable low pass filter. This means when the filter is engaged only the low frequencies pass to the output jack and the high frequencies are grounded out (cut). In this application, the capacitor value determines the "cutoff frequency" of the filter and the position of the tone pot determines how much everything above the cutoff frequency will be reduced. Generally, larger capacitors will have lower cutoff frequency and sound darker in the bass setting because a wider range of frequencies is being reduced. Smaller capacitors will have a higher cutoff frequency and sound brighter in the bass setting because only the ultra high frequencies are cut.
For this reason, darker-sounding guitars like Les Pauls with humbuckers typically use larger capacitors to cut off less of the highs and guitars like Strats with single coils typically use smaller capacitors to allow more treble to be rolled off. Keep in mind that the capacitor value only affects the sound when the tone control is being utilized. The tone capacitor value will have little to no effect on the sound when the tone pot is in the treble setting.
Many guitarists do not realize how much of their tone is affected by the guitar's controls.
Audio taper pots are typically used for volume controls, as the change in volume due to the rotation of the pot is much smoother. The total resistance of the volume pot is connected directly across the pickups, and functions as a "load" on them. Because of the nature of the pickup's complex internal impedance, this will have the effect of attenuating the extreme ends of the tonal range (usually more noticeable with high frequencies) and reducing the sustain.
All passive tone controls are what's known as cut circuits, meaning they can only be reduced in volume. Some perceive turning a tone control down as boosting mid-range, and that is untrue. In fact, when you turn a passive tone control down you are only cutting high frequencies. It is simply a capacitor and a potentiometer wired as a variable resistance in series.
This type of circuit is called a "first order" filter, and has the effect of gently rolling off frequencies above a certain point determined by the combined AC reactance of the capacitor and the DC resistance of the potentiometer. usually the resistance of the pot is the same as that of the volume pot. Tone pots usually have a linear taper to give a smoother range of control, as the even ratio suits the application of the tone control better. There does exist one-size-fits-all tapers, which are pretty much the average of an audio and a linear taper pot.
By the nature of the circuit used, a tone control will always filter off some high frequencies even when turned fully clockwise, and many players who find that they never touch their tone controls will disconnect them to add a bit of extra "sparkle" to their sound. Whether there is a perceivable difference in totally disconnecting the tone control or not can be debated, the most famous proponent of disconnecting the tone from the circuit would be Eddie Van Halen.
Selector Switch and Jack Socket
Since the load on the pickups is determined by the total parallel resistance of all pots that are being used at a time, using fewer pots will reduce the overall load and give a slightly brighter sound. Also, connecting more pots is the same as using lower value pots, two 500K pots will loose or "bleed" the same amount of treble frequencies as one 250K pot.
To lessen the effect, switching should be designed to remove pots from the circuit when the related pickup is not selected. An example of this would be a Les Paul. The bridge controls are out of the circuit when the selector is in the neck position, and the neck controls are out of the circuit when the selector is in the bridge position.
Pickup selector switches and jack sockets should be kept clean at all times. These devices are very vulnerable to degradation like dirt, grease, tape residue, and corrosion. When the contact areas of these parts does become contaminated, it can cause loss of volume, reduced frequency content, increased noise, pickup hum, and intermittent failure.
How Cable Affects Tone
Cable is often disregarded in the search for better tone, but it is critical. Type of cable, type of connector, etc. all have a great deal of impact on the final tonal outcome.
Any cable will exhibit an inherent capacitance - if you check out any bulk cable manufacturer's spec sheets, you should see this quoted as so many pF per meter. This can vary from as little as 100pF/m or less for high quality cable up to some 400pF/m for cheap cables. This additional capacitance, when combined with the pickups own internal resistance, will form an additional low pass filter which will roll off a good proportion of the high frequencies that your instrument produces. The effect is exacerbated when the volume control is backed off, as the increased resistance will effectively lower the filter's cutoff frequency.
Many brands of cable are known as insulated, which means the cable has a thin, conductive plastic coating over the insulating sheath of the inner core, but underneath the outer copper braid. This coating is very effective in reducing ambient interference and handling noise. The advantage of this construction, however, is quite often negated by poor cabling and terminal connection. This is typified by a muddy, indistinct sound.
Another area that can make a big difference is the connectors that are used. Brass plugs are known to become covered in a very thin layer of copper oxide, which, in addition to not looking very good, also happens to function as an insulator! Gold-plated plugs are known to give superb performance in equipment that is cabled up and left undisturbed (home stereo, home studio, etc.), but when gold plated connectors are used for equipment that is continually being plugged in and out, the layer of gold (a rather soft metal) will gradually wear away, revealing the base metal underneath. Nickel connectors are pretty much the industry standard, and will certainly stand up to repeated insertion and removal.
In the effort to improve one's tone, often they need look no further than their guitar's electronics. Some simple experimentation with different elements of the guitar (pots, pickups, caps, etc.) can really lead to some great tonal discoveries.