Hey folks, I'm a guitar player that's been fiddling with electronics recently for the purpose of making modifications on my guitar, and I'm a bit confused when it comes to a very simple principle of electronics. A lot of online instructional content on guitar modification says that resistance impedes the flow of current, and that it reduces the voltage, and therefore the volume of the guitar's signal (because voltage = volume). However, according to Ohm's law, U = R x I, which means that voltage is proportional to resistance.
In other words, shouldn't voltage grow with resistance? Am I not understanding this properly?
V = R * I means that voltage increases in proportion to resistance
if the current is constant. There are very few situations where the current is constant, and so there are rarely situations where the voltage increases in proportion to the resistance.
The other form of the equation is I = V/R. Mathematically, that is the same, but it's more relatable to many real-world situations, where the voltage is constant and the current goes down when the resistance goes up.
A guitar pick-up will produce a voltage. The amplitude of that voltage will be roughly in proportion to how much the string is moving. If the pick-up is connected an amplifier, then the amplifier's input will have some resistance, so some current will flow into the amplifier's input, which is amplified and eventually speakers are driven from that.
If some other resistance is put in series with the amplifier input, that resistance will decrease the current, so a smaller signal gets through.
Simple example:-
Pickup produces 0.5 V
Amplifier input resistance 1000 Ohms.
Current flowing is 0.5/1000 = 0.0005 A or 0.5 mA
If a 2000 Ohm resistor is put in series with the amplifier input,
Pickup produces 0.5 V
Total resistance is 2000 Ohms + 1000 Ohms = 3000 Ohms
Current flowing is 0.5/3000 = 0.000167 A or 0.167 mA
This is smaller, so the sound is quieter from the same voltage from the pickup.
It is a similar situation with headphones and speakers. Speakers might be 4 Ohms, and lots of power. You plug in headphones, and you don't want anything like that much power to the headphones as they would burn out and deafen you. So extra resistance is put in series with the headphones, either in the headphones or in the socket or both, so that much less power goes to the headphones.