The Physics of Hum-Cancelling
Any hum-cancelling pickup comprises at least two coils consisting of both coil windings and magnetic fields oriented in opposite directions. Why does this work and why do we need to do it this way? Well, what is the noise that we refer to as “hum”? Hum results from the magnetic component associated with the AC power running through our walls and that powers our equipment. Electric fields are always accompanied by a coupled and associated magnetic field and vice versa. The fact of AC power demands that there will be an associated magnetic field. You can think of this magnetic field as a standing wave, the intensity and structure of which is a function of the particular electrical and wiring environment that surrounds us at any given point. The pickups in a guitar detect this magnetic field, fluctuating with a primary period of 60 cycles per second (at least in the US), the same way they detect the signal from the vibrating string and it manifests in the signal chain as “60 cycle hum”. The magnitude of the hum will depend on a number of things and can range from barely noticeable (with low gain in a clean electrical environment) to intolerable (with high gain in a noisy environment – such as the web of knob & tube wiring I was subjected to in my basement in Phoenix). The polarity (often mistakenly referred to as “phase” in the common guitar player’s parlance) of the hum signal depends on two things, the polarity of the magnetic field and the direction of the coil winding with respect to the magnetic field. Guitar pickups cancel hum by utilizing coils with opposite winding directions. Since the polarity of the 60 cycle magnetic field is fixed, two coils with the same number of oppositely configured windings will generate hum signals of equal magnitude, but opposite polarity, effectively cancel each other out when combined. So then, why do hum-cancelling guitar pickups incorporate coils that are both opposite in winding direction (“RW”) and opposite in magnetic polarity (“RP”)? The answer: to keep the string signal in phase. If the magnets internal to the pickups were not reversed, then the string signal would also cancel. By changing the winding direction, we flip the polarity of the string signal and by changing the internal magnetic field of the pickup (or more precisely the magnetic polarity at which the string is magnetized) we flip the polarity of the string signal back making it once again additive between the two coils. Note that the internal magnetic field of the pickup “does not care” about the magnetic polarity of the hum field. These fields are acting on the windings of the pickup independently.
So, if we have an equal number of oppositely opposed windings we can cancel external magnetic interference (hum), and if we have a cleverly configured system of an equal number of reverse wound windings and reverse polarity magnets magnetizing the strings, we can make a working pickup with hum-canceling capabilities. But let’s get back to the one-coil-per string configuration. How do these things manifest themselves in this kind of linear hum cancelling format? One thing you figure out pretty fast is that if you have round pole pieces, you set up magnetic field cancellation at the center line between every pair of magnetically opposed poles. This translates to output drop if the string moves in to this region. This isn’t an issue in the typical bridge pickup position as the string never really translates that far from the rest position immediately above the pole piece, even with wild bends. Moving away from the bridge however, in the middle and especially the neck pickup positions as in a typical Strat® configuration, this becomes a show stopper. My experience with these round pole, round coil one-coil-per-string pickups was that bending strings, or even hitting a string hard, at the neck position would cause the strings to spend a significant amount of their time in this magnetic “dead zone” and result in significant output loss. So, now I had a problem to fix, the resolution of which I felt might lead to some proprietary pickup design ideas and some potentially protectable intellectual property.
Addressing the Problem
The basic problem, as I saw it, was to get the pickup patterns of adjacent poles to overlap. My first idea was around the concept of overlapping coil windings. Another of the initial boundary conditions was that the pickups should look outwardly like conventional single coil pickups. We all know how conservative guitar players are about certain things, one of them being the look of their instruments (especially the more vintage designs), and boy have I learned this the hard way since then – but I digress. I was thus enamored of the idea of coils with overlapping windings around round poles. I did a lot of development around this idea, both in terms of building prototypes and intellectual property. The overlapping winding design definitely works, and we have some IP around the idea of trapezoidally shaped coils (or other overlapping geometries) both on round poles, or poles with a geometry that mimics the coil. In fact, we went so far as to send out prototypes on a limited basis. But ultimately it doesn’t work well enough, or at least not well enough that we could make what we felt like was a viable pickup for the neck position. You can make a great bridge pickup this way, and we tried a bunch of things to make it work in the other positions. One of them (also patented) was the idea of moving the magnetic seam to a position where it would be less significant to performance. (Another point that should be mentioned is the design constraint of limiting the number of magnetic seams in the pickup. It’s easy to envision that a configuration where every other pole is opposed would exacerbate this magnetic dead zone problem, so that the ideal number of magnetic seams in a pickup of this type is one.) In order to balance drop-out issues and hum cancelling performance, a good place to put the seam is between the poles associated with the A and D strings. This results in the seam being in a less catastrophic position, while also maintaining 2/3 hum cancelling (4 coils worth of hum opposing two coils worth of hum). This 2 versus 4 configurations works, but the viability of a less than perfect hum cancelling platform (combined with a necessarily boutique-level price point due to the custom nature of the product) was always in question and ultimately we decided it was not viable.
There were a few different design currents flowing alongside the overlapping coil/round pole platform ideas. I had started out winding my own coils, but when I migrated to the odd coil cross sections I elected to enlist outside winders better equipped to manufacture these odd shapes. I also went to a self-supporting coil format in order to get rid of the bobbin (and the volume it consumed) to free up more space to pack more windings between adjacent coils. Another constant struggle in this type of design format is to pack enough windings in there to get the output you need, as you can’t just keep adding turns – you’re limited by the available space between adjacent coils. Couple this with the low permeability of the AlNiCo alloys, which I was still using exclusively at this point, and getting the output to the point that the pickup responds as expected in a typical guitar rig becomes a real challenge. About when I was feeling like I was up against it with the overlapping coils on round poles design, the coil winder I was working with decided to bail on me. In retrospect, this was the best thing that could happen but at the time it didn’t feel like it, in fact just the opposite! (But, everything happens for a reason, right Mom?) I started looking for someone else to help me, and I re-contacted one of the first winders I had identified and who had initially rejected me. He had apparently got side-tracked by crazy guitar player-inventors before and he wasn’t about to do it again. Well, not to be deterred, I put on my Corporate Engineer hat, put together a butt-kicking PowerPoint presentation and went and sold him on the idea. Newly bolstered by a fresh ally, I went back to the drawing board. At this point I was starting to work on some ideas for a P90 format, and this is where the idea of overlapping “blades on an angle” started creeping into the picture. I was also still working with self-supporting coils and my new partner was doing his best to convince me that this was a bad idea. And he was right. The self-supporting coil format would have proven to be too expensive to be commercially viable. Thus, the idea of squeezing both the overlapping blades on an angle, and coils wound on a bobbin, into a Strat-sized format coalesced at about the same time.
So, I started going full speed ahead with this new design platform. Up to now, I had been working exclusively with AlNiCo alloys as pole pieces for all of the Strat® style pickups. I continued along these lines with the “blade on an angle” (which we will now begin referring to as “Zexcoils”) format and had some custom AlNiCo 2 and 5 mini-blades fabricated. I did quite a bit of prototyping with these materials, but ultimately I was never able to get an AlNiCo based Zexcoil to perform to my satisfaction, to the point that I felt like it was a viable neck pickup. While the magnetic seam problem was rectified to the point that signal drop-out was not an issue, there was always some “weirdness” in that region, things that sound like phasing when bending the G string, or even hitting the D string hard. The other issue was output. Both of these problems stem from the low magnetic permeability of the AlNiCo alloys. In terms of the magnetic seam, the relatively low permeability of the AlNiCo alloys doesn’t sufficiently concentrate the string signal in the pole piece resulting in lower relative output in the region of the magnetic seam, even in the presence of a significant pole piece overlap. In terms of overall output, the low permeability of the AlNiCo alloys, coupled with the limited space available to pack windings between adjacent coils (especially now with a bobbin based design) resulted in a pickup with insufficient inductance.
In the meantime, some of the initial work with the P90 format, although now on hold, had piqued my interest in exploring other pole piece materials. Of course, the obvious pole piece choice for a P90 pickup would be low carbon steel. This was where I started, but due to the elongated blade format, simple screws at the center of the individual coils would not suffice. I was playing around with “wings” to fill in the empty core space on either side of a central screw, and also starting to look at using thin laminations in this space. I started to see some interesting responses that caused me to dive in to the study of pole piece material response in excruciating detail and lay the ground work for our Tone Tuning Technology™.
But that’s where we’ll pick up in the next installment…