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 my wooden desk in China, 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.

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 (essentially, 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.

Preliminary Price List:

  • PCB only: US$5
  • PCB assembled and tested: US$25
  • Complete unit assembled and tested: US$85
  • Magnetic test coil: US$20