Lecture 21: Electric Guitars
• The potentiometer is a resistor with a user-adjustable amount of resistance.
• The minimum and maximum resistance values are fixed during design and manufacturing
• It works by taking advantage of something called a voltage divider.
o As we know, in a circuit with series resistors, we have individual voltage drops across each
resistor. We can determine the drop across each using Ohm’s law.
o If we were able to change the values of resistors in real time, we could ‘control’ the amount of
• Easy method for calculating Req for parallel resistors. This is called product over sum. Vs=5V R1=4Ω R2=8Ω RL=12Ω
• So the simplest wiring diagram for an electric guitar would just have a pickup (voltage source), a
volume pot (voltage divider) and an output jack.
• Since we now have a voltage divider in our toolbox. We can apply it to other types of circuits.
Namely filters. We know that:
• Now the value of R is not fixed, we can adjust it. By doing so, we can alter the value of fc.
• Any complicated circuit can be modeled using a voltage source and a series resistor. • A slightly more complicated circuit, the ’66 telecaster.
• A different approach, the Gibson Les Paul
• In 1950, the fender telecaster was released and proved that there was a market for solid body
electric guitars. The Gibson guitar company, which was a well respected, high quality
manufacturer was taken by surprise. They brought in guitarist Les Paul as a consultant on their
own solid body guitar. Les Paul was an inventor in addition to being a musician. While the
fender designs were well received by the public, they were relatively simple, scrappy, durable and
easy to repair. Gibson on the other hand always built a more ornate and delicate instrument and
the Les Paul model, released in 1954 was an example of this.
• A potentiometer is a user-variable voltage divider.
• A guitar tone circuit uses a voltage divider as part of a low pass filter allowing the player to vary the
tone in real time.
• Product over sum is an easy way to determine the equivalent resistance in two AND ONLY two
• Looking at how components are connected in an actual circuit we can draw a schematic using
• Guitar pickups work by disturbing the magnetic field of the pickup by the vibrating string. Only
ferromagnetic strings will work on a typical electric guitar. • Changing magnet type, string type, number of turns of wire can all affect the character of the signal
the pickup outputs to the remainder of the circuit.
• Changing circuit components such as capacitor types and value, pot type and value can affect the
character of the signal that the GUITAR outputs to the a