Other parts of the website mention my home designed and built pickup tester(s), I have also used a coil and frequency generator to measure various things in a pickup. For some time, I have thought about building a tester that operated at higher frequency, but this time was based on a real guitar string, just like the before mentioned testers. Using the exciter coil is one thing, but using a guitar is the real thing! So here we go…
I have built one that is using a guitar string and can produce a “vibration” of a frequency above 1kHz, which is a frequency about the same value as the high E at the 24th fret. It is not so much the string diameter that was the focus for this, rather the frequency so it is just one diameter string that is used, an A string one that can produce a good output signal from a pickup. So in order to try out this new tester that I built, I grabbed a standard HB pickup, slugs on one side and screws on the other and stuck this PU in the tester. As you may know, my typical measurements are with the string placed above the 3rd pole of the pickup, so I did that and this is what came out of my oscilloscope, Fig 1.
As you can see this is a highly irregular voltage output, you would expect a pretty uniform amplitude but it is far from that. The waveform is close to sinusoidal, but not perfect. Due to the variation in amplitude, my first thought was that there was something wrong with my tester, but I realized that because I have multiple strings “in play” at the same time, that could account for the differences in volume, since it was not just the 3rd pole that was contributing to the output, it was also the neighbor poles. Since the individual poles contribute some to the total signal that would come into play because total symmetry could not be achieved with a standard PU with individual poles. So I thought if I use a dual rails PU, I should get a more uniform signal, so I grabbed one of those, but before I did I started to move the the Std PU from end to end and this is what came out of it, Fig 2.
As you can tell, the 3 different positions of the pickup produce 3 different waveforms, well, vastly different waveforms. The blue curve is the same as in Fig 1, I did not put them in phase which would have made it easier to compare, but that should be the least of our concerns. Please note the orange waveform, it is highly irregular, if you look at it, some of the peaks coincide with the red waveform, bit the in-between peaks are almost missing, so we get a totally different frequency spectrum for the orange than we do for the red (or blue, for that matter). I it was a video, you could see how the waveforms were morphing from one to the other when you were sliding the pickup back and forth. Now let us see what we get with a rails pickup, Fig 3.
What we see here is that the output voltage is still a bit irregular, but there are some differences, the rails PU will display the same style waveform no matter the location of the PU with respect to the string(s). I should say at this point the Fig 3 is a different time scale that Fig 1 and Fig 2, the frequency is the same. In this case, we do not have to deal with the individual poles because a rails PU gives us a uniform signal if we move the pickup from one end to the other with respect to the strings, but the fact that there is movement of several strings, there will be some difference in amplitude and it looks like this is repetitive. How about trying a PU with just one pole, Fig 4.
Here we see what happens if we reduce the pole piece count from 6 to just one, that should eliminate some of the drawbacks mentioned above and Fig 4 shows exactly that. In this case, I took a single coil pickup and removed 5 of the AlNiCo 5 magnets and left 1 in position 3 (not that it matters much) because it centers the PU in my measurement rig. We see (red curve) that the single pole PU gives us a waveform that it uniform in amplitude and it is shown how it looks if compared to the rails pickup from Fig 3. If the single pole is moved across the setup, only the amplitude changes slightly. Interesting observations showing how different types of pickups react to the same exact string setup, food for thought!
What did we learn from this? First of all, it looks like I have to make some improvements to my measurement setup to make it capable to run at high speed in a reliable manner and that need to only use a single pole if I want pure results. If I can improve the setup in such a way that the PU will stay in one place, I can get reliable frequency results both rail and standard HB pickups even if I see a variable amplitude because the purpose was to develop a frequency characteristic. We know from theory that the output amplitude will vary with speed so that can be worked into the test same way as it was done using the exciter coil. To round this off, here is higher frequency curve recorded with the single coil/single pole PU, Fig 5. Note that the frequency is about 1 kHz!
I hope that the figures were not too confusing because I used a mixture of time and measurement point number on the X-axis of the figures. Except for Fig 5, it is more the shape of the waveforms than the timing. In all, the conclusion can be that I with improvements to the mechanical system can achieve high frequency string movement and the quality of the waveforms can be improved by improving the stability of the tester.