About those Fishmans

Over the last two years, I equipped two of my guitars with Fishman Fluence pickups, and while they do a great job of sounding how a guitar pickup should sound, I primarily chose to switch to them based on the design principle. I’d consider them the only real innovation in pickup technology since EMG released the first active pickups. Once I saw the first pictures of the PCB coils they use, I quickly figured out how they might work, and my hands-on experience with them confirmed what I was able to piece together. So this isn’t gonna be an article on how ‘good’ the pickups sound – which is subjective, of course – but on how they achieve things like being able to switch pickup characteristics on the fly. Keep in mind that I never tore down one of the Fluence pickups, so it’s only my best guess, but I hope an educated one.

First of all, let us describe a guitar pickup like a system for signal transmission, that can be described by frequency response, meaning that the transmission characteristics are dependent on the frequency of the signal. The input signal would then be the speed of the moving string, and the output signal would be the electrical signal generated by the pickup. Describing the pickup like this, we can assume that every pickup coil behaves like what is called a second order low-pass filter. This means that low-frequency signals are transmitted unchanged (the can pass, hence the name) while higher-frequency signals are attenuated starting at a certain point called the cut-off frequency, with the attenuation increasing with frequency. With filters of second or higher order, there is also a phenomenon called resonance, which occurs around the cut-off frequency, that is more or less pronounced depending on the pickup and leads to a significant amplification of these frequencies in the signal. I won’t go into details here, have a look at BuildYourGuitar.com’s article on that topic for thorough description. If you have a look at figure four of their article, you’ll see the frequency response I described.

Now this cut-off or resonance frequency is determined, among other effects, by the inductance of the pickup coil. You don’t need to look up that word, just know that the inductance is proportional to the number of turns a coil has… and passive pickup have a lot of turns, like several thousands. In case you didn’t already know: A passive pickup electrically only consists of a coil (or two for humbuckers), nothing else. Now the cut-off frequency is one of two (or three) defining factors for the sound of a pickup. So if your manufacturing process of the pickup isn’t stable, and you end up with one pickup having a few hundred turns more or less then the next, then it will sound differently. So you what all your pickups to be wound exactly the same, which isn’t a simple feat. Further, the cut-off frequency depends inversely on the inductance and hence on the number of turn your coils have: The more turns you have, the lower the cut-off frequency. So what did Fishman do?

They made coils out of multi-layered printed circuit boards (PCBs), like the ones the chips on your PC or phone are mounted on. These can be manufacture with high precision and highly reproducible. However, you can only have so many layers in a PCB and you can’t make the ‘wires’ on the board arbitrarily small. So Fishmans coils end up having significantly less turns than a passive pickup (I’d guess not more than a few hundred). This has advantages as well as disadvantages: First you might have guessed that the cut-off frequency of these coils I quite high, so high I suppose that is isn’t in the audible range any more. Meaning a pickup based on the coil alone no longer has a relevant cut-off frequency but transmits all frequencies equally. However this does mean, that the pickup would not sound like a guitar pickup anymore. I’d again guess it would sound somewhat like a piezo pickup, but it’s hard to say. However, the electrical signal strength (the voltage if you want) generated by a pickup is proportional to the number of turns. In fact, passive pickups only need so many turn because they are used to drive the electrical signal over several meters of instrument cable (which is just plain bad engineering nowadays, but let’s not go into that). So Fishman’s coils require amplification (that why we call them active), because of their low number of turns, but because of their coil’s low inductance they have a nice, linear frequency response. But how do you make them sound like traditional guitar pickups? And multiple, switchable ones?

Its really quite simple: You design electrical circuits (filters), that have the same frequency response as those old, passive pickups, and run the signal of the amplified, linear-response coil through it. And that’s a thing thats really easy to do. Make the filters switchable for different pickup characteristics and you are done. That the reason why some of their signature artists tell you that designing pickups with Fishman is super fast and straight forward. I presume that have a ‘clean’ pickup they install on the guitar, then use an adjustable, external filter (think: equalizer or filter in a synth) to dial in the frequency response the artist likes. Save the frequency response. Synthesize a small analogue circuit with the same frequency response to put in the actual production model pickup. Done. That’s at least how I’d do it. If you compare it to the hassle you have to go through when designing a passive pickup, where you can only guess the frequency response before you actually wound each pickup prototype… You can see how it not only saves time: Using Fishman’s approach you can respond to the request of you artist far better and in greater detail than with classical passive pickups. You could even make them sound not like guitar pickups at all, if someone wished for it. Actually, most of the Fluence pickups have solder pads that you could use to contact the coils directly. Maybe someday I’ll try to make my own signature set… All it takes is a filter.