Pickup Impedance and other musings.....

Discussion in 'Pickups' started by CharlieB, Jul 20, 2005.

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  1. CharlieB

    CharlieB Active Member

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    A lot of questions get asked about pickups, and TNT had an excellent bit of information in a recent post. Its stickied here for all you wanting to learn more about pickups.

    Let's start with TNT's post:

    Can you please go over the way you are measuring your pickups' impedances?

    To see if we're playing on the same playing field, and if your pickups are still connected to the guitar you should:

    1) Set your Multi-meter on 20k Ohms.

    2) Have a patch cable coming out of the guitar in order to test.

    3) Make sure all volume knobs on the guitar are turned up tp "10"

    4) Set tour selector switch to the pup you are testing.

    5) Put one lead from the multimeter on the side of the chrome end of the patch cable and one lead at the tip.

    6) Read the value


    I'd to add a few things, mostly technical gibberish to TNT's comments, as a way of clarifying some often mistaken ideas.

    When you do the test above, you are reading the DC resistance, not impedance. This is very important to consider, as I'll explain down further. The other thing to consider is that you are reading the combined resistance of the pickup, and (at least) the volume pot. The inclusion of the volume pot in the circuit is usually insignificant if the pot is turned fully "up" to max volume. You will get slightly different DC resistance measurements with a 1m, 500k and 250k pot.

    It should be noted, that DC resitance is not a good indicator of pickup output, unless, as a comparison of two otherwise similarly constructed pickups. Let me explain, briefly, some of the various factors to be considered.

    1. Windings. Here's where the DC resistance comes into play. Keep in mind that smaller gauge (hgher number value) wire will have a higher resistance, and may or may not have higher ouput than a similarly constructed pickup. It could be your pickup has more windings of the same gauge wire and shows a greater DC resistance, or it may have the same amount of windings of a smaller gauge wire, also showing a higher DC resitance.

    Resistance is a DC value, and is only the same as impedance when its measured at a frequency of zero.

    impedance
    (Z) Measured in ohms it is the total opposition to the flow of current offered by a circuit. Impedance consists of the vector sum of resistance and reactance.

    Coils, by their nature, tend to allow DC to flow while restricting the flow of AC. This is at first thought of as strange, since we're using a coil to produce an AC signal for the amp. Impedance expresses the variable nature of the resistance as the frequency goes up. The higher the frequency, the higher the impedance. Industry standards for speakers, have used a frequency 1000cycles as the point that all speakers are measured, but keep in mind that you cannot state an impedance, unless you also state "at such and such frequency" along with it. There is no frequency or test standard for guitar pickups. Imedance, as opposed to resistance is vitally important! As the actual impedance changes, so does the flavor and character or the pickup. But read on, inductance plays an even greater role.....

    There is another electrical value called inductance. Inductance effets the nature of a coil to be resonant in a certain way, when combined with other factors (like capacitance). Inductance is measured in "Henries".

    inductance
    (L) Property of a circuit to oppose a change in current. The moving magnetic field produced by a change in current causes an induced voltage to oppose the original change.

    Inductance is a better describer of a pickups nature than impedance. All other things being fairly equal, a low inductance pickup will have more highs, while a higher inductance pickup will have fewer highs. It should be noted that a by its nature, an inductor is an electro-magnetic device, and as such... we can also generally say, that for similarly constructed pickups, the one with the higher inductance will have greater output. Like DC resistance, using inductance as a way to compare pickups only works when the pickups are constructed very similarly.

    The shape of a coil determines its inductance (and other factors like interwinding capacitance, impedance, and resistance). Tall coils of small diameter and the same inductance as short coils of greater diameter will have different capacitance, and impedance (and therefore also DC resistance). Noted pickup guru Bill Lawence says coils dont care how the wire is wound on the coil, its strictly a matter of inductance. Taking this a step further, "scatter" winding to a certain DC resistance will produce coils of different capacitance and inductance than carefully laid windings of the same DC resistance. Winding for a certain inductance seems to be a more sure way of determining the pickups flavor, according to Lawrence.

    Magnetics. Using magnets of varying strength and in different layouts greatly effects the tone and output of the pickup. Strong magnets vs weaker ones, when used in conjunction with the same coil, will produce greater output. Seth Lover, inventor of the humbucking pickup, as well as others, have stated that the magnet type matters not. Its the strength in conjunction with the windings that matters for output and tone. That is, for example, an alnico-II magnet is just less powerful than an alnico-V magnet, and pickups using one or the other will be wound in various ways to take advantage of the different magnetic strength. Some have said alnico-2 is the musical magnetic material, but this is false. It is simply that coils, made into pickups with alnico-II seem to have a good musical quality when that particular magnetic strength is best utilized by the type and shape of the winding. The magnetic "circuit" will also vary the tone and output. Such is the case with the P90 vs Stratocaster pickup vs the Telecaster pickup. All three are single coil designs, but all three have different magnetic circuits. And all three sound hugely different. Then again, there are other pickups with blade magnets, multi row magnets, magnets laid on their sides, magnets used in pairs (like the Super Humbucking pickups)... you name it, its been tried.

    Ok whats all this mean?

    It means that DC resistance is only one of many types of indicator values that we can use to compare pickups. You can't just look at DC resistance, but instead, need to look at all the factors in combination. This is so devilishly difficult to do on paper, that we're bound to just try lots of pickups rather than figure it out!

    Anyone wanting to add to this - please send me a PM and I'll make inclusions (or revisions or deletions) on a case by case basis. Many thanks TNT for opening up this can of worms!
     
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