Innovative Synergies

Engineering Dimensionally Alternative Business Analysis 

Electro-Magnetic Guitar Pickups

Starting to Analyse Pickups!

Jan 2003, 2006, July 2007

Background

Some decades ago (about 1963) as a schoolkid, and loving the sounds made by electric guitars, I decided to make my own solid body guitar, for in those days there was no way that I would be able to afford one myself and I am sure that it was a long way from the top of my parents “must buy” wish list. 

After pouring over several electric guitar advertisements / photos, and having tried a few electric guitars in a couple of music and second-hand stores, some physical truths came out – firstly that I was a natural left-hander, so all those guitars were back to front for me; there was an importance about the twelfth fret, and the ‘action’ – movement required to get the string down behind a fret.  So the course was now rather clear that if I wanted to have an electric guitar, then I would definitely have to make it myself and convert everything to be ‘left-handed’!

With my mind now set, I saved my pocket money for several months and then purchased a machine head, (the metal winding mechanism that tensions the strings at the end of the neck), a bridge (the metal anchor for the strings) with tremolo arm, a pack of flat wound strings, a rosewood fretboard – complete with frets, a strip of plastic called a ‘nut’ to guide the strings at the end of the fretboard near the machine head, and most importantly a couple of magnetic pickups. 

In the ensuing weeks, I chose some not too dense hardwood – think it was Cypress Pine by the aroma, and carved out a neck to my own (left-handed) design – without any tensioning rod to de-bend the neck as the strings can warp the neck and play havoc with the ‘action’ – and glued the fretboard to the neck.  The guitar body was much the same story, and I carved it out of a block of Pacific Maple.  I used four wood screws to join the body to the neck, and then set about working where the bridge would be located, then the pickups, volume and tone controls, switches and output jack.  This electric guitar was a unique left-handed design that somewhat followed the Fender Stratocaster shape – but ‘chunky’! 

As electronics was my other childhood hobby, the wiring was all too easy, so in a matter of a month or so I had my own ‘electric guitar’ – it worked, had reasonable action, and to me it sounded much the same as many others, and I hadn’t broken the bank; so I pondered – why all the dollars on these things?  (At that time I didn’t have a business mind, and I had not realised that I had worked part time for some months to come up with this, and if I was being paid for the unique construction then yes, this guitar would have been very expensive, and no I didn’t have an advertising campaign or sponsorships going, or management overheads!) 

Some years later (about 1968) I reconstructed all these pieces and made a new neck and body for a 12-String Solid Body electric guitar.  To my surprise this guitar also worked very well for many years and the neck did not warp having had tensioned strings left on it for about 10 years, so I questioned myself about the pickups and the neck structure.  

In about 1980 I had the opportunity to purchase a right-handed bass Fender copy and me being left-handed, I made a left-handed body and fitted the right handed neck to the new body - and presto a left handed bass electric guitar - and it worked perfectly too!  You guessed it - a million more questions! 

As it happened, I had since then spent several years (1967 to 1984) in a telecommunications electronics laboratory that specialised in the manufacture and testing of analogue transmission equipment - including amplifiers, equalisers, filters, transformers, power supplies; all using a wide range of components including valves, transistors, FETs, Integrated Circuits, SCRs, ferrite and iron cored inductors and transformers! And if that was not enough there was engineering of digital computers, digital transmission systems, switched mode power supplies, and computer programming at all levels. This tremendous experience really bolstered my knowledge about electronic components, (especially precision inductors and a very wide range of transformers) electronic circuitry and advanced testing procedures which came into the fore a few decades later. 

By 1984 as an Electronics Engineer, the questioning went right to the bone, but I never had time on my hands to find out – or the people to ask.  This was a quaint subject, and everybody spoke passionately about the sound, using words like ‘hot’ – which I later found out meant ‘comparatively big output level’; ‘edge’ – meaning ‘resonant frequency peak above the human centre of the audio spectrum’; ‘muddy’ – meaning ‘upper frequencies filtered out’; ‘fat or thick’ – meaning broad resonant peak below the centre (low mid-range) of the audio spectrum’.  There are some other real ‘doozies’ that you will be sure to hear especially in music shops – and it really amazes me in how much sales staff really do not know – but create a mythical language to sell!  If you hear one of these words or phrases – stop and ask for a clear definition of the meaning, that is often more fun!  

More recently (2001) I have had some time and a little patience to get a much clearer picture of actually how electrical pickups work, what their limitations are and why. What I have put in this Website will seriously fly in the face of many sales people, advertisers, musicians and pickup manufacturers - but what I have put here is clearly supported by highly repeatable testing procedures with highly repeatable data - and this is the area that is very seriously lacking in this whole industry!   

Most of what has been written here comes from decades of thought and the results of practical data analysis that has occurred over the last 40 years (1967 to 2007).  Most of the BLACK ART myths associated with electric guitars are explained and DEMYSTIFIED - so Read On!    

Solid Body / Neck Thoughts

To me, there is absolutely no doubt that body/neck resonance plays a major part of colouring the sound from a stringed musical instrument, if the body has, or is, a resonator, but with a solid body guitar the sound is barely resonant and there is minimum inter-string coupling compared to an hollow/resonator body, and further the fact is that simply holding the instrument considerably damps any resonances in both the body and the neck. 

Most solid body electric guitars are manufactured in two main pieces – the body and the neck, and the joint is in my opinion the weakest link – further killing the ability for the notes to be sustained.  After hearing so much talk about various guitars and the sounds from these, and knowing that the sound can be coloured by ‘tone’ controls – it started to dawn on me that the body has much less effect on the sound than the pickup itself, and further; the bridge (body anchor point) may introduce just as much variation as the construction of the pickup and body construction itself. 

(2007) Every time a string oscillates, it pulls on the neck at twice the note frequency. There are two extremities in a single string cycle, so the neck gets compressed twice for every one strings' amplitude cycle - and the sting gets stretched twice for every one amplitude cycle.  Think of it like a kiddies swing. For each cycle of swinging, the swing seat passes through two quasi stable points where the amplitude is fully extended (and the swing frame stability is under maximum lateral stress). 

Other documents have clearly shown that a guitar neck has a natural resonance like holding a knife handle firmly to a table top and then flicking the blade - the blade oscillates in much the same way that a guitar neck would oscillate.  Most guitar neck manufacturers put an iron rod through the guitar neck and this rod is bowed to 'tension' the neck, and this way the neck can be adjusted to counteract warp in the neck over time (of which some is caused by the string tension). 

For analysis purposes, the neck would be of infinite weight and totally rigid and without any resonances, but in practice the neck must be light and reasonably rigid - and it will have resonances - but these will only affect notes created up to about the 7th fret and beyond that, the neck is virtually rigid.  So we have to look elsewhere for so called 'flat spots'.

The only other non-considered point is the bridge - where the strings lengths are adjusted at the body end, and sometimes this is the terminating point for the strings - all rolled up in one.  If there is going to be inter-string coupling, then the bridge is the obvious point - especially if the bridge connects across the strings and not directly to the body.  Flat spots are the result of strings inter-coupling and giving a somewhat cancelled response for a particular note - at a particular fret - with a pickup in a particular place. 

This argument is a long way from blaming the body or neck for flat spots and it points to the bridge as probably the most variable component in an electric guitar after the pickups and their electronic loadings and settings.  

Magnetic Pickup Thoughts

Over decades of winding and measuring several thousand coils and transformers to tight specifications, and used a wide range of magnetic materials, it came as no surprise to me that a large number/proportion of well meaning sales and musically based people would swear that the difference between similar solid body guitars is the physical construction of the neck, fret-board, body etc and a whole lot of things; but rarely have I heard of anybody to get inside and analyse pickups for what they are and then extend that learned knowledge and predict how and why a pickup will sound like – or the background knowledge to directly compare any two virtually identical guitars and in that compare the near identical pickups with factual reasoning and no a load of sales waffle. 

I am well aware that variations in guitar construction can make differences to the sound, but I am far more acutely aware that subtle differences in sound also start with the construction standards of a pickup – not whether it has been dipped in wax or whatever, but by subtle differences in the coil construction or magnetic circuit can make a profound difference to the sensitivity and spectral response of the pickup.  

Having engineered and constructed special pre-amplifiers that sit in the guitar body, it was more than obvious to me that the resistive and capacitive values of the volume and tone controls and cable, and amplifier input impedance all play major factors to the sound – far beyond that given credit by those selling solid body guitars. 

For decades, having considered the wide variations of sounds that can be sucked out of pickups, simply by changing the loading and /or position – it finally got the better of me and I decided to find out as best I could what makes some pickups work ‘better’ than others – and why.  There is also the golden fleece – to come up with the perfect magnetic pickup, and meaningful date about pickups is extremely scarce!  There is so much data lacking in Patent records that these documents are almost fasical - but they are well meaning.

Method and Approach

There are several different approaches that could be taken and each will give a little bit of data that can be analysed into information.  The most common approach is ‘just listen’ and make an ‘informed’ decision on how it sounds.  Having been there like everybody else, I decided that a little more in-depth analysis would tell me a wealth of meaningful information about these pickups and then by analysing this array of information some meaningful knowledge would come forth. 

To start with, there are several different types of pickups, single and multiple coils, Alnico and Ferrite magnets, some with pole pieces – so these pickups need to be categorised into types so that comparisons can be draws with reasoning and not just sales waffle. 

My first break-up of pickups is based on the number of coil structures – so there is ‘single coil’ as a stand-out physical entity, and there are a group of multiple coil pickups.  The multiple coil pickups are in most cases ‘engineered’ to minimise hum from not too distant electro-magnetic fields, and these are usually called “Humbuckers”.  There are nominally two types of Humbuckers: lateral coil and vertical coil. 

(In the early days of radio 1898, loudspeakers did not have permanent magnets, so Oliver Lodge invented a loudspeaker that had a 'voice coil' set in an annular magnetic field that was created by an electro-magnet.  The coil for this electro-magnet doubled as a power filter choke that connected to the high tension voltage feed to the valves, and as the current was full wave rectified from the power transformer secondary winding it had a ripple in it at twice the mains frequency.  This coil had a thick iron rod as the centre and a virtual ‘cup’ to return the magnetic field to the front where it had a thick iron front plate with a round hole a few mm larger in diameter than the central iron rod, and this formed an strong magnetic annular ring for the voice coil to move through.  As this coil formed part of the mains filter, the magnetic field had ‘hum’ in it from the rectifiers – so the hum was heard from the speaker cone!  By including a small second coil – connected in reverse – and wiring this coil before the first filter capacitor, the current variations were much larger – but in reverse, and by having the right ratio of turns, the high tension current was smoothed and the audible hum was ‘dinged’ or cancelled – hence the word “Humdinger”.) 

In guitar pickups the Hum Bucker pickup works in a very similar principle.  A distant interfering source (for example the sharp edged electromagnetic field from fluorescent lighting) passes evenly through both coils and apparently forms a cancelling output and that ‘bucks’ the interference.  How the two coils do not ‘buck’ local interference (string movement) is another story that is covered later! 

Another approach is to look at the coils themselves and make some sense of the resistive and inductive values that exist, and then put these values into a generator model that can replicate the expected frequency response.  This approach wipes an amazing number of beliefs and shows that there is a rather simple unified approach to quantifying pickups. 

The third approach is to look at the magnetic fields produced by the magnetic structures, and this gives a real insight into the reasons why some pickup structures are low output, while others are so susceptible to external interference.  This really exposes many myths, and opens up a Pandora’s box of issues. 

By putting all these approaches together, most of the mysteries abut pickups are explained and it then becomes rather straightforward to understand what is really going on! But first – how does the magnetic pickup work?

How Pickups Work

The Four Components

There are basically four components in the structure of the magnetic pickup and these four all work together to cause the transduction from movement into voltage for amplification.

The first and not so obvious component is the permanent magnet.  A permanent magnet is an object that holds (retains) its energy charge as a magnetic field and this field is concentrated through the magnet forming two ‘poles’ of concentration usually termed North and South.  The permanent magnet has a magnetic circuit that forms a complete loop with itself, and this field extends beyond one pole of the magnet, through the air (in this case), strings, through the plastic and through the body of the guitar and then back to the other pole in the magnet. 

The ability for any object to carry a magnetic field through itself is called ‘permeability’ and permeability is measured in relation to a vacuum.  For iron the relative permeability varies considerably with magnetic field intensity but the relative permeability is in the order of 200 to 1000 times that in air, vacuum, wood, or plastic.  Some special magnetic alloys such as Mumetal have much higher relative permeabilities in the order of 20,000 to 40,000. 

Just like an electrical circuit where there is Resistance to limit the Current flow due to Electrical Potential Difference, in a magnetic circuit there is Reluctance to limit the Flux flow due to Magnetic Potential Difference.  Unlike electrical circuits where the current density can be sharply defined with conductors and non-conductors (insulated wires), magnetic circuits are very leaky making analysis far more difficult.  In electrical circuits, wires have well defined current densities as the cross sectional areas of wires are tightly defined, but in magnetic circuits, Flux Densities (or field strength) are highly dependent on the position of the measurement and the shape of the magnetic field.

This magnetic field is the unseen second component in a pickup and this field can extend for several metres before becoming unnoticeable. 

The third component in a pickup is the coil winding and this coil is engineered to sit around the magnet so that it captures changes in the flux density of the magnetic field.  These coils follow both electrical and magnetic laws in that the resistance of the coils is entirely calculable through length measurements and the resistivity of copper.  In the other hand, inductance measurements are somewhat from first principles, but including a very leaky magnetic structure makes associating these calculations to reality rather difficult. 

Moreover it is equally important to position the coil so that it surrounds the highest magnetic flux density, as the voltage generated from the pickup comes from variations in the flux density caused by the relative changing position of the strings to the magnetic structure.  I have no doubt that there has been an inordinate amount of work done on coils, and to most people that have done this work it seems to be a black art – that is, the reasons for change have not been accurately quantified – so it is largely guess work! 

The fourth area is the strings themselves – be they iron or nickel, or other magnetic alloys.  The strings actually form part of the magnetic circuit and the magnetic flux actually passes through the strings.  I believe that this phenomenon is very poorly understood for several reasons – mostly aesthetic.  While it is obvious that the pickup must not be on both sides of a string – as it would interfere with playing the guitar – the size of most pickups magnetic coupling to the strings is at best miniscule with tiny rod magnets in many cases as the near point for the magnetic loop, and it is no wonder that many pickups are highly susceptible to external electromagnetic interference! 

What Really Happens

The above is a picture of an electric guitar (lefty) looking across the string plane (x) across the screen, and (y) from near to far - the brick wall!  The (z) axis extends from the floor to the sky through the body of the guitar.  Keep these Cartesian (x, y, z) coordinates  in mind as they are used extensively throughout these documents.  In doing this research work I re-discovered Polar coordinates (Radius, angle, angle, angle) and these are of immense value, and I have swapped around as necessary, but remember, this research is more applied (practical) than theoretical, making it fairly easy reading - because pictures tell more than words and pictures tell much faster.

We are now going to move from reality into the world of approximation with the use of several tools, and to get a little taste of it the picture below represents what could be seen if you could visualise the magnetic field looking end-on to one of the pickups!  (Just like the picture above where we are looking end-on to the pickups.)

The above picture is a Finite Element Modelling (FEM) representation of one of the pickups in the guitar above and the magnetic lines of force and field strength given in a two dimensional picture (‘y’ – along the string axis (left to right), ‘z’ vertically through the pickup (bottom to top) of a magnetic guitar string in the vicinity of a small bar magnet.  A magnet in this case is sitting vertically at the bottom with the string positioned horizontally across the top.  It is usual that the pickup coil sits over the middle of the magnet. Note in this case the string is not shown in its entire length, merely about 60 mm to get a good enough approximation of what is going on.  Because the string has a rather small cross section and is in the vicinity of the magnetic field, this magnetic field is concentrated in the string so internally, the string itself is fairly highly magnetised.  The FEM picture above assumes that the two dimensional slice is right in the middle of the pickup.

The next level of understanding is that the string is in an almost constant magnetic field and moving the strings' position relative to the magnet only minutely alters the high and consistent concentration of magnetising flux in that string, so the magnetic flux in the string does not move all the way along the B-H (magnetic) curve for that magnetic string, but simply hovers in a strongly biased position in sympathy with that part of the string moving in the magnets' field.   

The magnetic field around the pickup is concentrated through the winding by the pole pieces, and as the total strength of the magnetic field is changed in time - relative to the previous total field strength, it will cause a voltage to be generated in the coil.  When the steel string vibrates, it causes the magnetic field to fluctuate in strength - relative to the position of the string and the magnetic field, and an oscillatory voltage signal is generated from the changing magnetic field.  That’s how it works, but it is not that simple!  This is a very loosely coupled magnetic circuit, and there are a lot of stray fields that are highly susceptible to interference - which will come out as hum or buzz!  

Assume that the string moves in a sinusoidal fashion – towards and from the magnetic field.  When the string reaches its maximum and minimum positions, it is temporarily still – just like being on a child's swing.  At those instances, there is no changing in the magnetic field intensity – because the string is temporarily still – so there is no induced voltage from the pickup coil at that instant.  When the string is moving through its rest or zero position, the string is moving at maximum velocity (again just like on a swing) and at these points, the magnetic field is going through maximum change with time and this produces maximum voltages in the pickup coil – either positive or negative – depending on the direction of the string movement relative to the magnetic field, the polarity of the magnet, and which way the coil winding is connected. If the string moves in a sinusoidal fashion parallel to the pickup face, then the magnetic field will not be varied - and no voltage will be produced! 

So the instantaneous voltage produced is related to the rate of change of the string position relative to the magnetic field strength, and not the instantaneous position of the string.  Most people find this a tough bullet to swallow!  And it takes time! The voltage produced is not proportional to the relative position of the string near the magnet, but the rate of change in position of the string relative to the magnetic field.  It is worth reading these two paragraphs several times until this concept is clearly understood.  

Spectrum Requirements

The spectrum needs are wide. When we listen we perceive frequency as fundamentals and harmonics (logarithmically related), and the second harmonic is twice the frequency of the fundamental – and it sounds ‘smooth’, as does the fourth harmonic, sixth harmonic and the eighth harmonic etc.  Conversely, a fundamental with the third harmonic (three times the fundamental) sounds very harsh, as does the fifth harmonic, seventh harmonic etc. with the fundamental. 

The lower open E string has a fundamental of about 128 Hz, and significant harmonics are the second, fourth, sixth and eighth meaning the spectrum here extends to 1024 Hz.  On the twelfth fret on the upper E string the fundamental is about 1,024 Hz, and the spectrum extends to about 8.192 kHz.  On the 21st fret the fundamental at C#’’ is about 1,722 Hz, and the spectrum extends to almost 14 kHz!  The transduced (string movement to electrical) frequency response needs to be virtually flat from less than 100 Hz to greater than 14 kHz to faithfully reproduce the vibrations from the guitar strings - before anybody starts to play with the audio spectrum, develop distortion, or introduce echo and / or reverberation. 

A Few Good Pickups

Getting started on this journey was not that easy as it took some years to better understand coils, magnetics, and decoding sales talk from reality!  Having had a few pickups for several years, I decided to get some more pickups and then start measuring against known references and surely something would come out of the figures that would have some correlation. 

But first the pickups.  To simplify the documentation procedure, each pickup has been given a name and a very brief description so that with later document recording, the chance of assigning incorrect data is minimised.  (The names that are chosen may be incorrect – but they are just names to correlate data in later pages.)

I would like to acknowledge the assistance provided by Venue Music in Sydney,  Turramurra Music in Sydney and to Chris Kinman from Kinman Guitar Electrix in Brisbane for providing some of these pickups for testing. 

 

Strat 01

Single coil design with 6 Alnico 5 bar magnets of differing lengths. 

 

 

 

Strat 02

Single coil design with 6 Alnico 5 bar magnets of differing lengths. 

 

 

 


White Strat

Single coil design with 6 common length soft iron rods as pole pieces and a common Ferrite magnet underneath. 

 

 

Black Strat

Single coil design with 6 common length soft iron rods as pole pieces and a common Ferrite magnet underneath. 

 

 

Kinman Tele

Vertical Hum Bucker coils – with what ‘appears’ like 6 Alnico 5 pole pieces. 

 

 

 

Kinman Strat

Vertical Hum Bucker coils – with what ‘appears’ like 6 Alnico 5 pole pieces. 

 

 

 

Hum Bucker 01

Horizontal Hum Bucker coils with 6 Alnico 5 rod magnets in the main coil and 6 soft iron (Allen Key) rod pole pieces in the bucker coil.  A soft iron plate about 2.5 mm thick magnetically joins the coil assemblies.  

Hum Bucker 02

Horizontal Hum Bucker coils with 6 Alnico 5 rod magnets in the main coil and 6 soft iron (Chrome Plated Raised Head) metal thread screws as rod pole pieces in the bucker coil. A soft iron plate about 2.5 mm thick magnetically joins the coil assemblies.  

Now that we have a few good pickups – let the testing and analysis begin!

Copyright © Malcolm Moore, 2003.   Comments and Corrections are welcome