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August 21, 2010

Pots for guitars

Pot Basics Potentiometers or Variable resistors as they are sometimes called, are commonly used as Volume, Tone or Pickup Balance Controls. A pot is basically an electrically resistive element or track with an extra connection known as a wiper. The wiper travels in contact with the track as the knob is turned. The pot can be connected in circuit to form a variable resistor between the wiper and either end of the pot track, or connected as a potential divider such that the wiper taps off a proportion of the voltage signal present between each end of the track depending upon its position along the track.
Guitar Volume Pot
An Electric Guitar Volume Control is normally wired as a Potential Divider. The clockwise end of the pot track is connected to the hot signal from the pickup and the anti-clockwise end, to the signal ground. With the knob turned fully clockwise in the 10 position, the wiper is positioned at the top end of the track, this is where the hot signal from the guitar pickup is connected. In this position 100% of the pickup output is transferred by the pot wiper to the guitars output. At the opposite end of the track, the volume pot is connected to the guitars signal ground. With the knob turned to the zero position, the wiper is then positioned at the ground end of the track and no output signal is transferred to the guitar output. For knob settings in between 0 & 10, a proportion of the signal is transferred to the guitar output. The proportion of the signal obviously depends upon the knob position, but also upon the taper.
Guitar Tone Pot
An Electric Guitar Tone Control is normally wired as a variable resistance. The anti-clockwise end of the pot track and the wiper form the variable resistance which is connected in series with a capacitor. The capacitor and pot are connected between the hot signal from the pickup and the signal ground. When the knob is turned to the 0 position, the pot is zero resistance and the capacitor is directly shorted across the guitar pickup signal. The Capacitor effectively filters the highs from the signal. With the knob turned to higher positions, the pot series resistance is higher limiting the current and therefore the effect that the capacitor has on filtering higher frequencies from the Guitar Signal.

Pot Taper
Taper refers to the relationship between the mechanical position of the pot and the actual electrical characteristic of the resistive track.
Linear Taper Pots
What is a linear taper pot? Linear refers to the law of the pot being a straight line. With the pot set at position 5 or 50%, the potentiometric output or the resistance between the end and the wiper will also be 50%. The electrical characteristic of the pot directly relates to the mechanical position of the wiper. This is appropriate in many circumstances of variable resistor use, but as it happens, not in Volume Control applications. Linear Taper Pots are more suitable as tone control pots, but rarely installed this way, due to manufacturing cost considerations.
Audio (Log) Taper Pots
What is an Audio Taper pot? Audio or Log Taper refers to the law of the pot being logarithmic. This comes about solely because of the way the human ear hears sound pressure levels. For the human ear to register sound as "twice as loud", about 10 times the sound pressure level has to be output! The higher the Voume knob is turned, the faster the output increases. For the most part, Audio Taper Guitar Pots are not a perfect log curve, but it's hard to tell. For a left handed player, turing the pot up is in the anti-clockwise direction instead of clockwise. With a logarithmic, audio taper pot, this also means that the curve has to work in the opposite direction. Left handed pots are available for this purpose.

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