Category Archives: Uncategorized

Guitar Pickup Measurement Revisited

It has been 5 years since my first experiments with guitar pickup measurements began. I discovered several people that had performed experiments and published them on the internet. But why would I want to measure pickups? It began with a guitar. I had always been a Gibson player, but tried a Fender Telecaster one day and liked it so much I bought it. However, I was queasy about the non-humbucking pickups that it had. So I set about to replace them.

I expected to find some concrete wisdom online, but found nothing that satisfied my technical appetite. So I just asked the guy at the counter in the music store. He suggested a Seymour Duncan SH-2 Jazz pickup for the neck. I had already been to the Seymour Duncan website and found the “sound system” ratings that they publish – these are bar graphs showing bass, midrange and treble levels. I couldn’t make much sense of those so I just followed the guy’s advice. It did sound good, but it left me wondering why.

Later, I found myself in China with a bit of spare time on my hands. I wondered, “could I build a pickup measurement system using simple everyday parts instead of the lab instruments that many of the published researchers were using?”. So on the magnificent window facing wooden desk set of my study at the school, I built this prototype:

circuit layout
This is the whole test circuit, looking ugly now because it is built in air! At the top is the battery powered integrator. Magnetic probe is placed on the pickup.

It did not perform very well, but it did work. In the meanwhile, I found and began to collaborate with other people that had similar interests. That led to improvement of the same circuit. I made the first PCB, the V5.7, and built some systems which shipped to a few interested people all over the globe. This previous article explains more: http://kenwillmott.com/blog/archives/152

Gradually, the experience with the V5.7 produced a list of needed improvements. That kick started the development of the same circuit as a more manufacturable and versatile product. Fast forward about 2 years to 2019, and it is here:

V5.8 integrator top view
V5.8 integrator PCB and internal wiring
V5.8 PCB with standard components

So, what does it measure, and how could you use those measurements? The pickup sits between the strings and the amplifier, so it is a kind of gateway through which the sound has to pass. It’s also an electrical component, so it has a thing called an “impedance” – more on that later. As a gateway, it is not transparent, but has different degrees of response at different audio frequencies – together referred to as a “response”. This device measures the response, which can be recorded, studied, and compared with the responses of other pickups.

If you feed a constant, unvarying test signal to a pickup (instead of the vibration of a string), and plot the output signal at all the relevant audio frequencies, you get a “Bode plot”, which is just a graph of amplitude vs. frequency. A small test coil can provide a test signal, and an appropriate PC application can easily generate a test chart, such as this one:

Sample humbucker Bode plot – “raw” unloaded plot shown in white, load applied to simulate actual guitar circuit performance shown in green

It’s worth discussing a few common response features at this point – nearly all pickups share them because they all effectively operate the same way. Below 1 kHz, you can see that the response is “flat” – the amplitude does not vary with frequency (one implication of that, is that the term “bass response” of pickups has no useful technical meaning). Then, at some frequency which is about 2.4 kHz in this example, there is usually a peak (in this case +1 dB louder than the reference level below 1 kHz). Above that frequency, the signal diminishes rapidly and never returns.

How would this impact the perceived sound? Well, in the flat portion, all pickups sound the same. That is because the pickups are passing the string vibration identically and uniformly. The intensity (here, the height) of the peak imparts a bell-like resonance which may or may not be audible depending on the amplitude of the peak. The “cutoff” or drop in signal always follows the peak, so the frequency of the peak also determines the frequency at which the fall-off occurs. The fall-off of higher frequencies mainly influences the perception of treble – the “brightness” or “dullness” of the sound. Note that such perceptions are value independent – personal preference can translate “bright” into “shrill” and “dull” into “smooth” or some other adjective.

The overall similarity of the response of different pickups is due to the identical electrical model that they all share. Thus, disregarding small differences, nearly every pickup can be distinguished electrically by its resonant frequency, height of resonant peak or “Q”, and sensitivity to string vibration or “output”. There are other differences due to magnetic field geometry (“aperture”), non-linear magnetic field strength imparting harmonic content, and harmonic effects on the string from the constant force of the applied magnetic field. I am only listing the proven ones. However since these 3 factors will be similar between any two of the same general type of pickup, the response itself is a predominant and reliable indication of the sound.

There are many applications for the measurements. You can predict tone approximately, compare altogether different designs and evaluate the effects of different internal components such as magnets, compare different pickups of the same production run, detect defects like internal shorts that are not evident in a simple resistance measurement. With access to an online database, you can select pickups that are similar to ones that you already like, disprove specious claims about pickups, test new designs and so on. I have seen photos of similar devices taken on factory tours of some major pickup manufacturers, so they use them even if they don’t talk about it – they like to project an image more tailored to “connoisseurs” of tone, more in tune with instrumentalists.

V5.8 integrator on test bench measuring a single coil guitar pickup

The system works in the following way – a test signal is applied to a small coil through a resistor. This creates a magnetic test field that substitutes for a vibrating string. The coil is placed adjacent to the pickup, and a plot is created. There is a small problem – an alternating magnetic field generates a voltage signal that is the mathematical derivative of the field. This is predicted by Faraday’s Law. It is not part of the response of the pickup “as a filter”. So it is necessary to apply the inverse of differentiation, which is mathematical integration. The V5.x devices contain an “integrator” circuit to perform this function, as well as a highly sensitive preamplifier that prevents the circuit from itself affecting the pickup that it is connected to.

There is another way to measure pickups, by treating it as a device with no magnetic input and simply measuring the complex impedance at all frequencies. This is a perfectly acceptable method, but does not yield response curves directly, thus requires specialized software. It has the advantage that response curves for arbitrary loads can be simulated correctly, and that the test hardware can potentially be very simple. However, at this point in time it is under development by a few people who have not yet made it publicly available as a working package. I certainly hope that they do, but my hardware integrator solution works well now with generic PC software available for/with PC audio hardware or hardware oscilloscopes. Also, measurements with a magnetic exciter coil such as in my system, are more accurate when eddy current losses are present between the string and the coil (often true in metal pickup covers).

The V5.8 brings mostly practical improvements to the V5.7 that I developed and distributed PCB’s for several years ago. The assembly has been simplified by locating all the switches on the PCB. A variable capacitor makes calibration easy. Bulk procurement has enabled me to reduce the parts cost and so I can offer assembled PCB’s at a low cost, and the improved assembly also allows me to construct complete systems at a very reasonable price. As the market for it is not big, I have not constructed an e-commerce site for them. But if you want one, or you want to build one, just contact me and I will work something out with you.

It is an open source project, I have published all the PCB designs, so that anyone can make them. I’ve also provided schematics and parts lists. It can be found here: https://github.com/KenWillmott/integrator

Price List:

  • PCB only: US$5
  • PCB assembled with parts (including IC, switches not soldered): US$35
  • Complete unit in enclosure, assembled and tested: US$95
  • Magnetic test coil: US$15

Another in a Continuing Line of LED clocks

The clock collection grows with this double 8×32 LED display version. I actually promised myself I wouldn’t use the LED matrices again, for aesthetic reasons, but I already had two of them extra, and it was a chance to improve my housing construction techniques. Also, it is a leap away from the AVR based hardware platform to the ST Micro STM32F103C8 processor in the form of a Maple Mini. The advantage is that the Maple is smaller, cheaper and faster than a larger Arduino board like the Mega2560, and also has multiple serial ports which is important for interfacing an internet device like the ESP-01, a Bluetooth or a GPS module. The system is made with 2 MAX7219 based matrix displays, and a proto board housing the Maple processor, an RTC module for timekeeping, and an ESP-01 for WiFi access. Functionally, the system goes to an NTP server via WiFi, to get the exact time when power is applied, and every 10 hours after that. It uses that time to set the RTC in case internet is lost. WiFi and other settings are configured from a computer through the USB port that powers it. It understands daylight savings time, and has a database of about 30 global time zones. So once it is set up, it never needs any kind of attention unless WiFi setting have to change.

The housing uses high grade 1×4 pine, which is easy to work with and finish with polyurethane. There are three main aspects that make the box fit the electronics and display effectively. There is a saw cut all around the inside of the front that forms an escutcheon (holder, if you like) for the filter. In the middle of the interior, there is a post that mounts the display on one side and the processor on the other. A sheet of thin plywood covers the back.

No doubt, someone who is experienced in woodwork would suggest improvements in my method. But I don’t have a real wood shop, just a table saw and hand drill. So my method is evolving around those limitations. First I make the shallow saw cut about 5mm from the front for the filter. Then I cut the side, top and bottom pieces from that. It helps to cut slowly, which makes a cleaner cut across the wood grain. I measure and cut the center post and back cover. This leaves gaps where the saw cut meets the edge. I fill these by gluing in wood spacers, which i trim carefully after the glue is dry. The housing pieces are assembled by laying them square on the saw table and drilling for the screws. After assembly, I apply at least two coats of polyurethane from a spray can, with light sanding in between coats. It helps to let the poly dry overnight even though they talk about 2 hour drying time. When I’m happy with the outside, I paint the inside with black craft latex, to eliminate reflections and to minimize what can be seen through the filter. A future upgrade will be to hide the top screws with plug dowels.

I have been asked whether I can develop projects using more off the shelf components, so that other people would have an easier time duplicating them. There are problems in that. I always find weaknesses in the inexpensive hardware that I get, which not everyone has the patience to fix. For example, most low cost MAX7219 displays have an excessive LED current due to a poor bias resistor choice. I think it is because the designers think that people are too dumb to turn up the brightness in software. I chose to do SMD surgery to fix that on these display boards because I loath to run components outside their specified limits. The popular library to drive these displays views each 8×8 module rotated by 90 degrees, so I had to modify the code to change that. I also have found that the ESP-01 modules can be powered down when not in use, yet most hobby circuits don’t bother to do it. So in many cases, hardware and software modifications are required to make a project really useful.

In fact, I have two other clock projects in the works, where I am trying to create something that the average experimenter could duplicate. I am trying to avoid the special mods and things and put together a code base that isn’t also a hodge podge of modified libraries like I often have to use. Mainly here, I’m showing you how to take your clock hardware, and put it in a presentable and pleasing box.

Phantom Powered Guitar Preamp

The idea of putting a preamplifier in a guitar is hardly new. There are many advantages, but a big disadvantage is the requirement for power, which usually means that a battery has to go into the guitar. That means that a space must be found for the battery, it must be replaced when it expires, and has the potential to leak corrosive liquids inside the guitar if left for too long.This circuit avoids those problems by using a common idea from microphone technology, phantom powering. Many microphones obtain power for internal circuits from an ingenious circuit that allows power to be transmitted over audio cables. This system has become a standard, and is provided by most microphone preamplifiers and mixing consoles.

Originally, the power was used to bias condenser microphones, and so the voltage is quite high, 48 volts. Originally, a transformer was used to separate the audio from the supply voltage, but in recent years transistors became available that could do that, and so the “Schoeps circuit” was developed by the engineers at Schoeps Mikrofone to replace it.

The Schoeps circuit has been adapted and modified endlessly since its introduction, and does a fine job of supplying the power provided on a phantom cable, to on board preamplifiers of various kinds. It is a differential amplifier that provides a very low impedance output to drive the output cable, combined with a balanced power extraction circuit. It works well in different configurations, some having Zener diodes for regulation and some not. The driver stage can be configured as an additional regulation stage (which I have done), however this is optional and some designers omit it. It is the kind of circuit that will almost always work, but needs some thought and understanding to achieve the best results.

In my case, I adapted a JFET input stage from some microphones to work as a guitar preamp. The JFET is configured in differential output mode, in order to easily drive the output transistors and audio cable, which are also differential. It has an extremely high input impedance which means that it presents almost no load on the guitar circuit. Thus the guitar tone and volume controls are the only electrical load that the pickup “sees”. This avoids tonal variances that normally happen when a cable is plugged into a guitar. The lower impedance and differential (balanced) signals in the XLR cable are more immune to hum and interference, and can run up to 100 meters (several hundred feet) with absolutely no loss of tone.

The wiring in my guitar is completely conventional, with a three way switch and tone and volume control. The output goes to the preamp, which is connected to an XLR connector that replaces the usual output jack. In fact, it is possible to keep the original jack and have both powered and unpowered outputs from the guitar, but I didn’t want to drill a new hole. In future, I would because the XLR is not a perfect fit in the existing hole (to be honest, a bit of an understatement!).

Here is the installation. The prototype board fits nicely inside with a safety wrapping of insulating Kapton tape.


I wrestled with the design for a long time. There are a lot of variations on it online but no complete explanations that would allow me to optimize the components.

I thought about Zener regulation. The entire circuit is differential except for the JFET current. So I reasoned that I could omit it and depend on a pair of filter capacitors to do the job. What is strange about the Schoeps circuit, is that the pair of driver transistors function as a first stage voltage regulator as well as signal amplifiers. The collectors are common, and go to the first stage filter capacitor. Then a dropping resistor feeds the second stage supply, which depends on the second stage filter capacitor for audio frequency suppression. In my Spice simulations and also in testing, I found that this was more than adequate. There is no noise or instability from omitting the Zener, in fact it is probably quieter because Zeners are notorious noise sources. The only disadvantage that I can find in this arrangement is that all the component values are interdependent, and so you can’t really change one value or component without changing them all. The voltages have to be checked after construction to verify that they are sane. You should measure approximately 10 volts on C6. However, this circuit has a good chance of working the first time if it is built exactly as shown.

Note: R1 in the schematic above is just a simplification for the sake of Spice testing. It really represents the entire usual volume and tone control circuit of the guitar.

I’ve been playing my Godin Redline 2 with the circuit installed and I like it. I have it plugged into an Art Accessories Phantom II Pro, which then goes into my guitar amp. I can just as easily plug it directly into any phantom powered microphone preamp or mixing board, as I mentioned before.

Undercover Pickups

For quite a long time, I have been wondering about a phenomena called “eddy current losses” in guitar pickups. Essentially, these are signal losses due to various electromagnetic aspects of the pickup’s construction, that cause the tone to become dulled to some degree. All pickups have them, and some depend on a calculated amount of losses to produce a certain tonal balance. However, because the high audio frequencies are conducive to a sensation of “clarity” or “brilliance” in the sound, it is generally good to reduce the losses as much as possible.

While analyzing pickups and examining the analyses of other testers, I began to realize that the metallic covers that contain the coils and internal parts of a pickup, are a prime source of loss. This was actually known to the early designers of the 1950’s era. They responded by finding metals that have low losses, and used those as the base material for pickup covers. These would then be electroplated to any desired appearance.

J.R. Butts, a designer for the Gretch guitar company, chose a different way. He considered the electromagnetic problem more carefully, and designed a metal cover shape that was almost completely immune to the losses – the “Filtertron”. Subsequently, the Fender guitar company adopted the design for a specialty guitar – the “Cabronita”.

After 1960, nobody thought much of the whole thing. High quality pickups always used an alloy called nickel-silver, while the Filtertrons and some other covers remained the more inexpensive brass. But when brass is used in a non-Filtertron design, the sound is usually very dull due to the eddy current losses.

I wondered, why does the J.R. Butts design work so well? The patent mentions it, but offers little explanation. So I began some experiments to determine the exact nature of the eddy currents in a guitar pickup cover. These were extremely revealing. Soon, I realized that the Butts design barely scratches the surface of the techniques that could be leveraged to improve a pickup cover.

I designed and built several prototype alternative designs made from brass to test my theories. These were wildly successful. However, as I considered what I would do with my invention, I realized that I lack the funds and resources to obtain patents, trademarks, set up inventory, place manufacturing orders and such things that are necessary to make and sell a product.

guitar pickup

Prototype Humbucker Cover

So after almost a year of development, I feel that the best course of action is to simply release the information into the public domain. I hope that if it has some small success as a product, that I can at least boast that it was my idea. After all, it probably won’t be the last one coming from me.

The technical article is long, so I should gPrototype Telecaster Neck Pickupive you a summary. The idea is that by cutting small slots in strategic locations on the cover, the tone-sucking eddy currents can be mostly eliminated. This has two applications. One is that cheap brass covers can be used where a nickel-silver one would normally be used. That is a cost advantage. Another is that when a nickel-silver cover is slotted, the losses are so small as to be both non-measurable and inaudible. This means that a protective cover can be added to pickups that have previously shunned covers for reasons of tonal purity (this habit began with the heavy metal players of the late 1970’s).

There are a few different possibilities for placement of the slots, however not all are mechanically sound or aesthetically pleasing. Here is a practical alternative for the Tele neck design, fully tested and found to eliminate losses equally as well as the version shown above.

Here is the full story: pickup_cover_geometry

Rendering Again!

nine_lives_1 These are historical batteries that I have subjected to a cylindrical scan and then rebuilt, using computer rendering techniques. The floor is the actual floor outside my apartment in Hubei, photographed and used as an surface colouring. There are few of these batteries in existence now, because they leak and corrode with age.

Introduction

I am a person of many interests, and I plan to show some of my ideas and projects on this site. I hope that others will find some interest or novelty in them. Welcome to my world.