Pickup Inductance

The voice of an electric guitar is its pickup. Yes, there are lots of other components like wood, bridges, nut etc., but the sound comes out of the pickup. The effect the rest of the bits have will be a rant for a different day. Frankly there’s more than enough materiel just in the pickup to keep me going for now.

So how does a pickup work? Well, the best place to look for information is Helmuth Lemme’s website. This man is seriously knowledgeable. Buy the book. I did.

I must also mention here Bill Lawrence. A bit of a hero of mine, he really knew his stuff and produced some genuinely innovative pickups which deserved to be far better known. His pickups are some of the most expressive and distinctive I have used.

In case you don’t want to check out the link, I’ll summarise. A pickup is actually a very simple device which makes use of some complicated physics. It’s a length of very thin wire wound around a magnet. When a metal string vibrates in the magnetic field of the pickup an alternating current is created in the wire coil. We send this current into an amplifier to increase it until it will move a loudspeaker. Then we hear sound.

There are design variations – sometimes the magnet is underneath and the wire is wound around a former – but as far as physics is concerned it’s the same end result. Whether you are making a single coil like a Strat, or a P90, or a mega death metal overwound humbucker you still have a magnet (or magnets), a coil of wire and some bits to hold it together. The way these components are assembled does influence the way the pickup senses the movement of the string, which is part of the reason why a Les Paul sounds different from a Strat. But it’s only a fairly small part of the reason.

A length of wire wound around a piece of metal (or a magnet) is called an inductor. It has a property called inductance, defined as, “the property of an electric conductor or circuit that causes an electromotive force to be generated by a change in the current flowing.” This property also works the other way around, so a change in electromotive force will generate a current – this is used in our pickup. We can measure inductance, although you need a special (but not expensive) meter to do it. It’s measured in Henries. Odd name, but true.

The pickup also has other properties we can measure. We can put a simple meter across the ends of the wire and measure its resistance. The wire is very thin so it does have a fairly high resistance. Usually the meter is measuring resistance to Direct Current (DC). Our musical note is Alternating Current. When you put AC through a coil the inductance effect we just mentioned does weird things to the resistance of the coil. For this and many other reasons the DC resistance doesn’t tell us much, although it’s the one figure manufacturers quote. More on this later when I engage rant mode.

The magnet is quite important. A weak magnet will produce a weak signal. We’ll talk about magnet swapping in another section.

There’s one more thing we can measure on our pickup – the capacitance. It’s very small – around 80 – 200 pico Farads. In fact it’s so small that we can ignore it for now.  Just don’t forget about it completely because it does have an effect on the sound of the pickup and if we change it we can change how the pickup sounds. We just have to change it quite a lot for anything to happen.

There is an equation which links the variables together to give us something new.

ω0 = 1/LC

L is the inductance in Henries. C is the capacitance in Farads. ω0 is the resonant frequency of the coil. The what? Well, read Hr Lemme’s page for all the detail, but in a nutshell it’s the frequency where the coil is most sensitive.

That’s right. A pickup does not detect all frequencies equally. Each coil has a different resonant frequency dependant primarily on its inductance.

To cut to the chase, a low inductance will favour high frequencies and a higher inductance will favour lower frequencies. The majority of pickups in common use have inductances which range between 1.8 Henries (a Gretsch Blacktop Filtertron) and 10 Henries (an original Bill Lawrence L500). There are also a few specialised pickups which are higher inductance than that – up to 15H for a Telecaster Hot Rail. I’ve measured over 100 pickups and tested them to see if this theory holds true. It does. Low inductance pickups are bright and twangy, high inductance pickups darker, fatter and more midrangey.

Here’s a graph showing the difference between the Filtertron (blue line) and the L500XL (red line). The Filtertron shows a distinct peak at 4500Hz, whereas the L500XL has a slight hump at 2000Hz and then tapers off. What this graph doesn’t show is the comparative output levels. The L500XL has a much higher output than the filtertron.

L500XL vs Filtertron

Conveniently Bill Lawrence made many of his pickups in different inductance values. The L500 comes in C, R, L & XL versions. The construction of all four is identical, only the amount of wire on the bobbins varies, and thus the inductance. My L500-XL is 9.2H, the L is 6.99 and the R is 4.78. I don’t have a C but it’s supposed to be 2.8H. The XL sounds huge – one of my favourite heavy rock pickups. Fat midrange and subdued treble. The L sounds more like a vintage PAF but with a slightly ‘honky’ lift in the upper mids and the R is clear and bright but not icepicky. The R is one of the nicest neck pickups I’ve used.

The ‘sweet spot’ is between 3 and 6H – the lower end is where Strat and Telecaster pickups live and the upper is a PAF. A P90 is also around 5.5-6, which we would expect as the PAF was designed to replace the P90 in a hum cancelling format.

It is no coincidence that this ‘sweet spot’ coincides with a frequency range between 2000Hz and 8000Hz. These are the frequencies where the human ear is most sensitive to small changes and where we perceive things as ‘musical’.

***RANT MODE ON***

Leo Fender, Seth Lover, Ray Butts and all the other people designing guitars and pickups back in the 1950s were not blind idiots bumbling about. They did not accidentally stumble across the exact right ‘magical tone’. They knew exactly what they were doing and what effect they wanted to achieve. The PAF installed in the Les Paul sounds awesome because IT WAS SUPPOSED TO.

***RANT MODE OFF***

Knowing the inductance of a pickup won’t tell you everything about how it will sound. The construction makes a difference because of (for example) the magnetic field and coil shape, which isn’t in our equation and affects the position and length of string being sensed, or eddy currents caused by covers, steel pole screws and the like. In particular the dual-coil design of a humbucker cancels hum but also some of the signal, resulting in a mellower tone than, say a P90 with the same inductance. A Telecaster pickup and a PAF both wound to 6 Henries will not sound the same, but they will have a similar tonal signature with a distinct presence around 3000Hz. The inductance will allow you to make a fairly accurate assessment of the overall sound you will get, particularly whether it will be dark and bassy, honky, nasal or bright, perhaps even piercing.

Notice that the equation does not include variables for things like the tension of the wire or the scatter pattern of the coils. There’s a reason for that. It’s because it’s irrelevant. It makes no measurable or discernible difference to the resonant frequency, which is what gives the pickup its voice. You might get a bit of random ‘something’ as an immeasurable artifact caused by a stray capacitance or a winding vibrating sympathetically, but it’s not going to be repeatable and certainly shouldn’t be used as a selling point.

I recently asked a pickup winder to clarify some questions I had about his pickups. I wasn’t trying to ‘catch him out’, I just asked what the inductance of the pickup was so I could gauge it against some better-known references. I was sent this statement:

“The output varies quite significantly, being that they are handwound…

As for inductance in general, I have yet to measure it as an indicator of sound (I find the wire gauge, wind tension, number of turns, and magnet type are the only reliable sources of information in that regard, really, but output does have it’s use in indicating what ‘type’ of pickup you’re dealing with), so I unfortunately can’t really say which pickup in the range would be nearest.”

***RANT MODE ON***

So basically, this individual, who is taking money off people for ‘custom winding’ pickups, has NO IDEA how a pickup works or what factors influence the sound it will produce. He can’t produce two pickups the same yet he names them and sells them as specific models in his range. He has specifically disregarded the main property of the pickup that is scientifically proven to influence the ‘voice’ of the pickup – inductance – and instead cited a load of largely irrelevant factors. The number of turns and wire gauge together will have a direct effect on the inductance, but he clearly doesn’t know why. The magnet type (he means the specific alloy of Alnico) has NO bearing on the frequency response at all. The wind tension shouldn’t have any effect, but the inconsistency in his DC readings (what he refers to as ‘output’) suggest that either the counter on his winding machine doesn’t work, or he’s pulling the wire so tight it’s actually stretching and thinning. This would show as an increase in DC resistance. To be fair, the tension of the wire and thus the ‘compactness’ of the coil will make a very slight difference to the capacitance of the pickup, but it would be so small as to be insignificant – 50 pF or so. I challenge anyone to tell the difference between two otherwise identical pickups, one wound with a loose coil and one with a tight one in a blind test. You can hardly measure the difference with laboratory equipment.

Bill Lawrence would be spinning in his grave.

Sadly this sort of clueless meddling is common. In fact it seems to be encouraged by the ‘cork sniffers’ on certain forums. You’ll see me writing about ‘Mojo’ a lot. I’m taking the mickey out of these charlatans who are trying to convince people to part with large sums of money for pointless stuff. It’s basically fraud. In the old West they used to talk of ‘Snake Oil Salesmen’.

Snake oil, originally a fraudulent liniment without snake extract, has come to refer to any product with questionable or unverifiable quality or benefit. By extension, a snake oil salesman is someone who knowingly sells fraudulent goods or who is a fraud, quack, or charlatan.”

Yeah. That.

While rant mode is still engaged I’ll state once and for all that DC resistance alone gives NO indication whatsoever of the ‘tone’ of the pickup, or its potential output voltage.

***RANT MODE OFF***

The reason for this is simple. There are several different wire gauges in common use. Thinner wire will have a higher resistance per metre than thicker wire. If you wind a pickup with thin 44 gauge wire to 10Kohms it will have LESS wire on it than one wound to 10Kohms with thicker 42 gauge wire. Less wire = lower inductance. The 44 gauge pickup will be a lot brighter than the 42 gauge one even though the DC resistance is the same. Why manufacturers insist on giving us the DC resistance alone is beyond me.

Don’t believe me? OK. My Seymour Duncan Custom Custom has a DC resistance of 14 Kohms and an inductance of 9.35 Henries. My JB has a higher DC resistance of 16.2Kohms and yet a lower inductance of 8.54 Henries. Why? Different wire gauge. The JB has thinner wire so even though the DC resistance is higher than the Custom, there’s less mass of wire on the coil. Technically the JB should be brighter, but both pickups have very high inductances so both of them will be pretty dark and the audible difference will be minimal.

There’s something else besides the amount of wire which can affect the inductance – any ferrous materiel attached to the pickup. A proper Telecaster pickup has a steel baseplate, usually copper plated so it’s easier to solder the ground wire to. This noticeably increases the inductance. My experiments show a difference of up to 1 Henry between a steel baseplate and a plain fibre one. That much difference is clearly audible.

Using huge pole screws has the same effect. Swapping the undersized hex-drive pole screws on a cheap 70’s-inspired overwound pickup for longer ones increased the inductance by 0.8 Henry. Again, an audible difference.

Metal covers also play with the eddy currents in the coil, which is why some Les Pauls had their pickup covers removed. German Silver is the ‘correct’ materiel for humbucker covers as it is relatively transparent. The baseplate should also be made of this materiel. Brass can kill some of the treble and is used on the Telecaster neck pickup for that very reason.