Exciter/Test Coil: Coil Size + AWG

[guitarmd]

Looking at Ken's page here:

github.com/KenWillmott/integrator/blob/master/Test%20Coil%20Assembly%202021-03-27.pdf

I've been using a 49.2mm PAF style bobbin exciter coil for everything, which I ordered from him directly. And it works great. However, I've been thinking about making custom exciter coils, e.g. 53mm for Tele, 52mm for Strat, and so on, so the exciter coil will match the coil dimensions of the pickup being tested.

Thoughts? Necessary or unnecessary? Just occurred to me today and wondered if the exciter coil not matching the dimensions of the coil being tested will have an impact on the bode plot.

Also, I frequently get staggered pole pickups, and in order to get the test coil to sit flush, I think it would be easier to use a 52mm humbucker bobbin with oversized holes on the bottom, so the test coil bobbin could sit flush on the top of *any* pickup easily. The staggers can really throw it off, though to be fair, I've compared side-by-side with a cockeyed test coil and a perfectly flush coil, and the bode plot results were nearly identical, so I'm not sure how much this actually matters. 

On the topic of making my own test coil: I don't have any 30AWG in stock. I can easily get 30AWG magnet wire, but am wondering if 38AWG, which I already have, would also work. 

[ms]

I have been comparing two very different exciter coils, and will post results soon.  My conclusion is that there is very little difference in the measured frequency response even from coils that are very different.  It does not seem to be necessary to illuminate the whole coil, or just one pole piece.

KW recommends 100 ohms in series with the coil, I believe.  30 or 38 AWG does not make a lot of difference to the total resistance.

With some coils you might not get the same current at high and low frequencies; it is important to so, or compensate somehow.  I prefer to sample the current with a series resistor and then do cross spectral analysis so that the db and degrees at each frequency are independent of the amplitude and phase of the source.

An alternate approach would be to drive the coil with a very good active current source so that the amplitude and phase are very close to the same at all frequencies.  I have not tried that.

[aquin43]

Monitoring the current is ideal. Another way is to get the response to the generated field from a coreless coil with few turns. That will include the 90degree lead and any needed correction. A decent current drive can be made by putting the coil between supply and the collector of a transistor. The dc field created seems to be unimportant.

A simple series resistance (R) drive is difficult without calibration and compensation, if you are interested in the phase. Say you want an error of one degree at 10kHz. That will mean that the reactance of the coil will have to be below R*tan(1) = R*17m46. If R = 100, that leads to ω*L = 1.75 Ohms. At 10kHz, ω is 62k83 which means L cannot be greater than 28μH.

Of course, most people seem to want only the amplitude frequency response. That is much easier.

I have found at least one pickup that has a different frequency response at each pole when measure with a small coil.

[ms]

Feb 24, 2023 12:17:56 GMT -5 aquin43 said:

I have found at least one pickup that has a different frequency response at each pole when measure with a small coil.

I would be interested in seeing more about that.  I have been using small coils for some time, believing that it should matter in some cases, but I have not proven that.

Mike

[aquin43]

The pickup is a mini humbucker intended for jazz so it is not intended to exhibit a strong resonant peak. It has 12 adjustable poles and a metal case which gives it quite a high shielding loss. When properly balanced for loudness some of the pole screws are below the edge of the casing. Measured individually, these have more high frequency loss than the others. Another uncertainty is caused by it needing a large adjustment of the pole screws to balance it properly so it is probable that some of the signal from the string would come via the rails that carry the pole screws as well as via the screws

This is may be a special case but it does make an argument in favour of the tripole arrangement that you described in another thread.

[ms]

Feb 28, 2023 10:43:30 GMT -5 aquin43 said:

The pickup is a mini humbucker intended for jazz so it is not intended to exhibit a strong resonant peak. It has 12 adjustable poles and a metal case which gives it quite a high shielding loss. When properly balanced for loudness some of the pole screws are below the edge of the casing. Measured individually, these have more high frequency loss than the others. Another uncertainty is caused by it needing a large adjustment of the pole screws to balance it properly so it is probable that some of the signal from the string would come via the rails that carry the pole screws as well as via the screws

This is may be a special case but it does make an argument in favour of the tripole arrangement that you described in another thread.

Thank you, that is a great example of a case I have not looked at.  The simplest explanation I can think of (certainly not necessarily correct) is that we have two fields inducing changing flux in the pole piece:
1. directly from the vibrating string;
2. from currents induced in the cover by the vibrating string.

I would expect them to vary by different relative amounts as the height of the pole is varied; that is, a greater percent change in the field directly from the string because it is a small source further away.  Source 2 increases in strength with respect to source 1 in frequency and therefore alters the frequency response of the sum.