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How does a pickup work?


Crusader
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Hi there, I was wondering how a pickup actually works. I know a few basics like Faraday's Law but after Googling I can't find a clear answer to a few simple questions. I have a few but here are the first two

First of all, does the amp send voltage to the guitar?

Or does the vibrating string create all the potential?

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Guitar pickups rely on electromagnetism in order to generate a signal. At its simplest a pickup is a magnet with a coil of wire wrapped around it. Placing a moving ferrous metal object (ie, the string) near the assembly will cause a small current to be produced in the coil of wire which can then be amplified. Increasing the number of turns of wire increases the repreduced signal. Moving the string more (plucking harder), or placing the string closer to the pickup also increases signal strength. Using a magnet made of more powerful material also increases the signal.

You may remember experimenting with making an electromagnet at school by wrapping a wire around a nail, hooking the wire up to a battery and picking up steel wool with it. Electromagnetism is somewhat "reversible". The DIY nail electromagnet and a guitar pickup rely on the same principles, only in this case they are being used in a kind of back-to-front order - by introducing an external electric current (the guitar signal) you can create the magnetic field (the pickup) in order to move the steel wool (the guitar string).

First of all, does the amp send voltage to the guitar?

Very rarely. In 99.999% of situations the guitar is the only thing that will generate a voltage that goes down the cable. Volts coming back the other way either indicates a very specific usage of a particular circuit design which you're extremely unlikely to come across while bashing away at your favorite rendition of Smoke on the Water, or there's something wrong with the amp.

Or does the vibrating string create all the potential?

The vibrating string is part of the mechanism required to create all the noise. The pickup converts all the movement in the string into an electrical signal which we can then amplify/distort/filter/whatever.

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curtis pretty much nailed it if you want to learn a little more take a look at how a basic generator works. thats all a pickup is a generator.

when you pull a magnetic field across a wire it generates a electric current in a pickup the string pulls the magnetic field causing it to cross the coils. its really such a simple process that it amazes me how there are so many pickup designs

.

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if you think about it ie the pickup design it makes an interesting generator design. well its already been done but it is interesting. instead of moving coil, or moving magnet, a moving field...... just saying.

as far as the pickup goes try playing with mu metal to shape the field.

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Thanks for your replies

Looking at my question now I think I could have chosen better words. I did High School Physics where we made a simple electric motor and we learnt about Faradays Law and all that. I know the string vibrating through the magnetic field creates a signal in the coil. I just wasn't sure if there was already a small current going through the coil

I got my Multimeter working tonight and the amp seems to read 1.5 millivolts. (I don't know if I'm getting an accurate reading btw) But Curtisa you say "In 99.999% of situations the guitar is the only thing that will generate a voltage that goes down the cable" It just seems amazing that the vibrations of the string can create enough current. I thought the system might work like a Condenser mike but with less voltage

Am I completely wrong there?

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A guitar pickup is more similar to a dynamic microphone as a condenser (read: capacitor) mic requires a power source to bias the capacitor. The element of a dynamic mic is analogous to the strings of the guitar; their movement induces a current in the pickup/mic's coil. At least, that's my understanding of it. Even active pickups are still passive in how the current is (in/trans)duced by the strings.

I think I just made the word "transduced" up.

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I got my Multimeter working tonight and the amp seems to read 1.5 millivolts. (I don't know if I'm getting an accurate reading btw)

DC or AC? Depending on your circumstances you may see a handfull of mV "floating" at the input, but 1.5mV is likely to be the error in your multimeter rather than the amp trying to push some juice back up the cable.

But Curtisa you say "In 99.999% of situations the guitar is the only thing that will generate a voltage that goes down the cable" It just seems amazing that the vibrations of the string can create enough current. I thought the system might work like a Condenser mike but with less voltage

In general magnetic guitar pickups are low (ish) current/high impedance devices, meaning that they don't have much guts to push several volts of signal over difficult terrain (long cables, low-impedance "heavy" loads etc). A typical humbucker may generate a few hundred mV if you hammer away at a power chord, a signal which probably won't make it alive over a 100 foot cable plugged into a hi-fi stereo line input. If you crack open the top of just about any guitar amp and start tracing the first few components of input circuitry you will find fairly standard and specific building blocks that have been in use for decades, which allow such flimsy pickup signals to be brought up in level (volts) and grunt (current) in order to start moving a speaker.

A condenser mike utilises a completely different principle to generate a signal - a flexible diaphragm moving in sympathy with the soundwave hitting it, suspended over a fixed plate. The distance between the diaphragm and plate at any moment causes a change in capacitance between the two which can then be amplified. Electromagnetism plays no part here.

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Thats interesting, all I knew is that Condenser mics need Phantom power

But getting back to my first question "does the amp send voltage to the guitar?"

The answer is no, but how come when you touch the input jack it makes the speakers pop?

(i.e. when plugging in) the pickup isn't even connected yet

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But getting back to my first question "does the amp send voltage to the guitar?"

The answer is no, but how come when you touch the input jack it makes the speakers pop?

(i.e. when plugging in) the pickup isn't even connected yet

There are two required conductors in a guitar in order for rock 'n roll to be made. One is obviously the signal conductor from the pickup(s) carrying all the good juju that we want to amplify. The second conductor is ground, which serves several functions in a guitar - it's used to shield against external noises, earth metal components to minimise hum and (more importantly) provide a return path for the signal we want to amplify. In the same way a battery has two terminals, one to "push" the volts out and one to "return" the volts from a circuit, the guitar needs both conductors to complete the signal path.

The construction of a normal guitar jack and socket does not allow the user to insert or remove the plug without inadvertently crossing over the two conductors or having a partial circuit (next time you have a Strat-style jack socket on the bench have a close look at it while inserting a guitar lead to see what happens to the various contact points rubbing on the plug). Guitar plugs and sockets are relatively loose fitting compared to some other plug styles, so the action of inserting or removing the plug may cause intermittent contact during movement. These factors generally result in the circuit being briefly incomplete during plugging in/out. For a fraction of a second the guitar stops being an instrument and acts as an antenna, picking up all sorts of garbage (radio signals, flourescent lights, refrigerator motors, aircon motors, even electrical noise picked up by your own body) which is faithfully reproduced by the amp as a familiar "blaaaaaat", "buzzzzzzz", "pop" or "hummmmm". The (yucky) voltage signal is still being generated by the guitar at this point, not by the amp.

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Wow I'm finding out more than I thought I would

Particularly this;

The speed of the "Alternating" in AC is what we hear as pitch. The magnitude of the AC swing is "volume" to us.

So;

If you play the first string at the 5th fret (A=440) The string vibrates 440 times a second, creating a current that alternates 440 times a second in the coil, and The distance the string travels from side to side determines the magnitude, (i.e. volume)

Have I got it right?

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On behalf of all the non-electronically educated such as myself, being on the outside looking in so to speak, I'd like to bellow a resounding

Three Cheers! to curtisa for his uncanny ablility to answer these questions in a manner we can relate to and understand. He's done so numerous times in numerous threads and I for one want to say thanks. I'm learning a lot.

Scott

.....please do carry on with your discussion......

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On behalf of all the non-electronically educated such as myself, being on the outside looking in so to speak, I'd like to bellow a resounding

Three Cheers! to curtisa for his uncanny ablility to answer these questions in a manner we can relate to and understand. He's done so numerous times in numerous threads and I for one want to say thanks. I'm learning a lot.

Scott

.....please do carry on with your discussion......

Thanks Scott, and you're most welcome.

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Three Cheers! to curtisa

I'll second that motion, actually I didn't realise anyone else was reading this

So I will continue

If the distance the string moves across the pickup determines volume. That would explain why the neck pickup is louder than the bridge (Given the same pickup) because you can clearly see the string moves more there

But why does the bridge pickup sound brighter?

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A plucked guitar string is not a simple single frequency sine wave but a complex waveform with a fundamental and harmonics. The bridge is in effect a mechanical filter and damps various components of the waveform. A string also has a different tone depending on where it is plucked as well (Johnny Cash at times used to strum half way up the fretboard to get a different sound)

Keith

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If the distance the string moves across the pickup determines volume. That would explain why the neck pickup is louder than the bridge (Given the same pickup) because you can clearly see the string moves more there

But why does the bridge pickup sound brighter?

Generally it doesn't. The higher frequencies have more or less the same amplitude as those further towards the centre of the string. The difference is that the lower frequencies have a lower amplitude at the sensing position. The ends of the vibrating string length have zero amplitude (string displacement from rest) the centre of the string (in theory) has the maximum possible amplitude, with a corresponding logarithmic dropoff in maximum amplitude from the centre to the ends.

Longer wavelengths (lower frequencies) require more string length for the shape of the wave. A fundamental frequency (half a sine wave, think of a skipping rope) has maximum amplitude at the middle of the string. It also adequately demonstrates falloff in a way. Shorter wavelengths have more of the string to play with, so many "complete wave lengths" exist along the string at any one point. These easily "pack up" towards the ends of the string meaning their displacement is more readily read by pickups away from the centre of the string length. Does this make sense?

A bridge pickup only sounds brighter because of the low apparent presence of lower frequencies. For this reason, bridge pickups are either hotter to compensate for the lower perceived output compared to the richly-fed neck pickup, or raised closer to the strings to increase output. I prefer to back off neck pickups instead.

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I think I've got this

Higher frequencies have less amplitude than lower frequencies, all along the string. While the lower frequencies decrease in amplitude closer to the bridge

So a bridge pickup sounds brighter because the higher frequencies still achieve full amplitude, while the lower frequencies have less amplitude

And a neck pickup sounds warmer (and louder) because the amplitude of higher frequencies remains the same, while the amplitude of lower frequencies is greater

Given the same pickup

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Umm, not really. The full amplitude of lower frequencies can't "reach" the ends of the vibrating string. They have more room to vibrate in the middle area but are prevented from a wider amplitude nearer the nut/fret and saddles. Does this make sense? Higher frequencies with their shorter wavelengths can "fit" closer to the nut/fret/bridge. I was going to make a drawing but got sidetracked. I am butchering the idea somewhat, but the concept is more or less valid.

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