Pickups - Help And Advice Needed

[SwedishLuthier]

Please do not think that I’m picking a fight with you. I just want to understand your point of view (and hopefully learns a thing or two on the way)

I have to disagree about the relatiionship between magnetic field strength and high frequencies. I think that there is no relationship at all unless the field strength is quite strong. This at the onset of "string pull" or "stratitis", and I think the sound is better described as harsh rather than just just more high frequencies. It is, after, a non-linear affect.

I’m curious of what you base your opinion upon. Based on my limited pickup winding experience (about 25 done by now) the relationship between magnetic field and treble is rock solid. On the other hand I have a bunch of A2, A3 A4 and Ceramic HB bars together with some A2, A3 and A4 SC rods magnets on its way in for further experimentation...

Having that said I have exchanged the A5 magnet in a HB with ceramics (unfortunately I do not have any measurements recorded regarding magnet strength) and I noted a definite increase of the total output. I also noted that the treble was the part of the sound spectrum that was boosted the most. That might be part of the “loudness” effect. But I do not think so. Next time I might have to find a way to measure the effect of magnet swapping with an oscilloscope. That will give some hard facts. But nevertheless the percept effect of stronger magnets was definitely pronounced treble. If you would like to refer to that as harsh I’m assuming it is your way of describing that treble boost.

When you chopped up that Gibson alnico magnet, you observed exactly what the measured properties and theory of magnets predict: alnico magnets get weak quickly as you make them short in the direction of the field (from pole to pole). Ceramic magnets do no lose strength as fast. That is, while a long Alnico magnet might be stronger than a ceramic of the same shape, a short one will not be. This has to do with the B-H, or hysteresis loops, but we probably do not want to get into that here.

I’m very aware of all this. Nevertheless I had a P90 style pickup with alnico magnets and ceramic magnets in the same pickup. The result was very similar to the result of swapping an A5-magnet for a ceramic in a HB. In this particular case the chopped up A5 bar magnet had magnetic field strength of around 10 Gauss. The ceramic bar was slightly above 25. In the finished pickup the A5 magnets produced a resulting magnetic field strength at the pole screws of 7 Gauss and the ceramic magnets were slightly below 25 Gauss.

Compare this to a standard P90 pickup (a genuine Gibson P90) and you will se that it is exactly 25 Gauss. If we were to compare the pickups they would of cause sound different (one has a Tele shaped bobbin and 12 000 turns of wire. (a lot) and the P90 is, well p90-shaped with about 10 000 turns). Different, but with a strong resemblance of each other. When comparing the treble the T90s for sure doesn’t sound harsh at all. They both have a pronounced treble but the T90 actually slightly less top end than the P90! There are a lot of factors that this can be accounted for (higher turn count on the T90, different coil shape, different distance from magnets to strings, absence of eddy currents in the ceramic magnets and so on). But to me this is the proof for that a ceramic magnets does not equals harsh trebles. In this case we have a similar magnetic field strength and a similar treble.

ok thats fair enough - but i have never seen a pickup with an alnico magnet rated stronger than 5. Not saying they dont exist though

A6 and A8 bar magnets sized for HB use have been used but not very extensive. Mainly because they are rare to find and very expensive. The A6 is very similar to ceramics in sound. It takes a very distinguished listener to hear the difference. So because of the cost people just use ceramics. The A8 are VERY strong and seldom used because the treble is VERY pronounced. And if people rate the ceramic HBs as harsh…you figure it out.

This guy too explains that the material of the magnet is of absolutely no importance for the sound of the pickup: http://www.billlawrence.com/Pages/Pickupology/magnets.htm

Bill Lawrence is one of the most respected guys in the industry. He has worked for Gibson and Don Lace (Lace pickups) have learned a lot when working with Bill.

Edited July 14, 2007 by SwedishLuthier

[Mike Sulzer]

My experiments have been different from yours. I use a small Neodymium magnet on each pole piece. It is easy to vary the field strength by varying the size of the magnet (, although I have never measured the field as you have done*). I do not hear any significant difference in the sound of the pickup when the magnet size is changed until the onset of string pull is reached. You always need to listen at the same volume since a louder sound is clearer and more present even if otherwise no different. It is possible that I have missed differences because of not really listening carefully enough. That is, perhaps through too much reliance on the theory: increasing the degree of magetization of the string should change the volume, but nothing else as long as the vibration of the string is not affected by the magnets and the pole pieces are not saturated by strong er magnets.

*How do you measure the field?

[SwedishLuthier]

OK, I see. And the increase in treble that I am experience might as well be part of the "loudness effect" that the higher output is causing. When you are adjusting the listening volume of the altered pickup so that they have an even sound level, how do you do that without altering the sound? At the amp or with the volume pot?

For measuring the magnetic strength I use a Gauss meter from R.B.Annis, http://www.rbannis.com/products/magnets.html I have their model 25 50-0-50 pocket magnetometer. That covers the necessary range for use with guitar pickups. It is not a cheap tool but very valuable when trying to understand the physics of a pickup.

For future experimentation I will try to find some (hopefully free) oscilloscope and tone generator software. That way I will be able to use a speaker (the magnet/coil part should be enough) to create a varying magnetic field that induce a current into the pickup and measure the output. That way I can eliminate factors like string gauge, picking technique and such and only see how the response changes from changing magnets or adding windings or whatever. If anyone know of such a software, please let me know.

[Mike Sulzer]

Thanks, I will check out the meter. For analysis, I use Signalscope and Signalsuite on a mac. Not free, but inexpensive.

The guitar volume pot can cause a big change in the frequency response. The cable capcitance is part of the pickup resonant circuit, so as you turn the volume control down, the resonant frequency changes and the level of the high frequencies reaching the other end of the cable changes due to the series resistance. The gain should be changed with the first gain pot on the amplifier; the gain stages before this control need to be linear, that is, not overloaded.

You can use a speaker, or just wrap some wire around a steel core. I prefer to use white noise as a signal source and use the scope software in its spectrometer mode, averaging enough times to get a good response.

[SwedishLuthier]

Thanks Mike. I will have to check if Signalscope is availible for Windows.