# Circuit approximation of Induction in guitar pickups

I was trying to learn the way basic guitar circuits work, the other day, and I stumbled upon a thing that I couldn't fully understand.

According to this site, a simple guitar circuit can be made as shown on the image below.

As i understand the tone knob is represented by an LP filter with a variable cutoff frequency changed by the variable resistor, where higher the resistance is the brighter the tone is(higher frequencies in the signal). The volume knob on the other hand is a simple voltage divider represented by a potentiometer, regulating the voltage levels of the output signal.

Considering the way the tone RC filter is wired, it should only work if the pickup behaves as an AC current generator. I did AC analysis in LTSpice and the results came out positive.

The thing is that it is quite counter intuitive to me since I have always thought that EMF is induced in the coil according to Faraday's law: $$EMF = -N\frac{d\phi}{dt}$$ and then the current in the circuit depends on the load connected to the induction coil. According to this I should replace the pickup with an AC voltage generator but then the RC filter would not work.

Could anyone help me explain the way induction works in this process because clearly I have a mistake somewhere in my way of thinking.

• A pickup isn't an inductivity, but an inductivity in series with an AC voltage source. – Janka May 28 '18 at 0:21
• I have simulated the circuit with a coil and AC voltage source in series. The AC analysis gave me the well known frequency response of an LP filter. Turns out you are right. Thank you for the answer. Could you please elaborate the answer or provide me with some links on this matter? I would be very grateful! – Phill Donn May 28 '18 at 2:56

## 3 Answers

Could anyone help me explain the way induction works in this process because clearly I have a mistake somewhere in my way of thinking.

A guitar pick-up has inductance and resistance. Inductance may be several henries and resistance may be a few kohm. The emf induced in the pick-up is in series with the inductance and resistance so you should model your pick-up as a voltage source in series with R and L.

The pickup coil(s) are wound around powerful permanent magnets. Since the strings are steel, plucking them modulates this magnetic field at the same frequency as the string.

Some guitars have several pickups to boost the signal, especially for high notes. The coil picks up these changes in field strength and converts it to a weak signal for the Amplifiers or effects pedals to work with.

The magnets do have a slight pull on the steel strings, which is why several tuning steps is required to keep the strings tight, account for aging, humidity, temperature, etc, then tuned to the exact note they should be in.

A guitar pickup sounds bright when it has a high resistance load because its inductance resonates with the capacitance of its cable. The resonance creates a large peak in the upper midrange frequencies. That is why most guitar amplifiers have an input resistance of 1M or higher. A lower resistance damps the resonance and mutes the high frequencies like this:

• Hi - This image appears to come from another website. Please edit your answer and add a reference to comply with site rules and reduce the risk of plagiarism claims. Thanks :-) (If you ping me in a comment using "@SamGibson" after adding the reference, then I'll remove this comment to reduce clutter.) – SamGibson May 28 '18 at 2:39
• I believe my question was directed more towards how to simulate the induction in the coil of the pickup rather than why guitar amps need to have high input impedance. But Janka has helped me with his answer so thank you for the effort anyway. – Phill Donn May 28 '18 at 3:00