Extended response pickups for bass

[rjb]

...but I have to do the recording.

BTW, when do we get to see pics of your prototypes?

[Mike Sulzer]

Here is the recording for the 8,000 turn pickup (about 2.14H inductance), same magnets, larger bobbin than above, loaded with 50K and the standard cable (no preamp). Unless you have lost all your high frequency hearing, you should notice the missing higher string harmonics. The 3 db point is about 3.7KHz.

The bobbin is deeper as well as wider and has room for additional wire.

C2.mp3

[Mike Sulzer]

Here is the recording of the 8000 turn pickup, loaded with 100K, using with a preamp, 1/4 of a LMV654, used as a unity gain follower. The high frequencies are back.

[Mike Sulzer]

Here I will summarize what I think the results so far mean. First, the sound of a bass pickup can be nearly the same over about a 700 to one impedance range. (That is the ratio of the inductances from A to D.) And this is not some sound that is all that easy to achieve, but rather nearly flat response over the necessary range. This is achieved by manipulating the load that the pickup looks into, from the low impedance pickup, for which the load is pretty much irrelevant in the real world, to the 2.14 H pickup where the pickup is connected to a high impedance preamp with a short leads.

[By the way I did not give all the details of that preamp. It order to get the sound in the clip it was necessary to bypass the power supplies with large polypropylene caps and smaller ones, and use a resistor in series with the output. The latter is a result of the sensitivity of that op amp to a capacitive load. I used a 10K resistor in series, which put the output pole at about 20KHz, which is high enough compared to the pickup circuit response that it does not matter.]

It seems pretty clear that the sound from a pickup is not determined by its impedance, but rather by its frequency response, a quantity that can be manipulated by controlling the shunt element of the voltage divider formed by the pickup and load. Frequency response is the fundamental parameter in audio. Of course, other things matter as well, but it is useful to determine important factors that affect the frequency response.

In most guitar pickup design, you start with the load, using either 250K or 500K pots, and then alter various parameters, especially the winding, to get the desired sound. That is, the impedance of the pickup is adjusted to get the desired frequency response given the shunt element. (Although the pickup capacitance is part of that shunt element, and so you really are adjusting the load somewhat, too.)

The interesting remaining question is this: how much below 10KHz? That is, at what bandwidth does the bass pickup lose the sound characterized by having all the string harmonics? It is likely that there is no exact frequency for this, but that the hearing ability of the listener, a variable, matters.

[CRU JONES]

Those samples sound good Mike! Very interesting demonstration, thank you I have a P-bass here. The pickups have a loaded resonant frequency of 2.7kHz and exhibit a low damping factor. Q is over 1 at resonance. Very much unlike your pickups.