The Steel Guitar Forum

What device is used to measure pickup impedance? How do you do it? Where do you buy the device? Anything tricky about the process?

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E9 lessons
Mullen D-12/MSA D-12/Sho-Bud Pro 1 S-12/Melobar/Webb/Profex II

An impedance bridge, at a minimum. A sine, square and triangle wave generator and a spectrum analyzer come in handy, too.

If you're talking "ohms," an ohmmeter.

Nobody (except maybe Bill Lawrence) actually measures pickup impedence. The numbers people use are DC resistance measurements which can be made with an inexpensive multi-meter.

HELLO: IMPEDANCE IS A TERM THATIS GIVEN TO INCLUDE " ALL OF THE ELETRICAL PROPERTIES OF A DEVICE OR UNIT " THIS TERM INCLUDES [ FHE FOLLOWING ] RESISTANCE, MEASURED IN OHMS, CAPICASTANCE,, MEASURED IN MICROFARIADS,,,INDUCTANCE MEASURED IN HERORIES OR MICROHENRIES,,, MUTIAL GAIN OF COIL,, COMMONLY CALLED OUTPUT. RESONANCE MEASURED IN FREQUENCY REPONSE OR LACK THERE OF. HARMONICS, A FUNCTION OF FREQUENCY,, HOW BROAD OR NARROW THE SPRECRUM IS. AND AN EDUCATION ON HOW TO USE THE MEASURING DEVICES OR COMMONLY CALLED TEST EQUIPMENT...
MEASURING D.C. [ DIRECT CURRENT] WITH AN
"OHM METER ] " RESISRTANCE " IS ONLY ONE SMALL FUNCTION OF IMPEDANCE.. DO NOT BE MISS LED BY THE TERM IMPEDANCE AND D.C. RESISTANCE. THIS IS NOT AN EASY [ IMPEDANCE ] TERM TO FULLY UNDERSTAND....AN OHM METER WILL ONLY TELL YOU IF THE COIL IS OPEN OR NOT..IF IT IS OPEN IT WILL NOT WORK.
AN OHM METER WILL TELL YOU THE D.C.RESTANCE OF A COIL, BUT IS MEANINGLESS UNLESS YOU KNOW WHAT YOU ARE LOOKING FOR...DONT BE MISS LED.....HOPE THIS HELPS.......JENNINGS

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EMMONS D10 10-10 profex 2 deltafex ne1000 pv1000, pv 31 bd eq, +

The idea of impedance is opposition to current flow when a load is presented with an AC voltage. There is a big distinction between complex impedance and resistance (or resistive impedance). These impedances are all measured in ohms, and there are resistive, capacitive, and inductive components. This explanation is somewhat simplified, but gets to the nuts and bolts.

In the usual model, resistive impedance occurs at all frequencies equally, and is due to thermal losses from current flow. Since it's the same at all frequencies, it's usually measured at DC, which can be measured using a simple ohmmeter, as mentioned. In a pickup, resistive impedance is primarily due to the length of the pickup winding wire. Capacitance is the ability to hold charge between two conductors separated by a non-conducting dielectric, and is measured in Farads. Capacitive impedance happens when the frequency gets small enough that this charge accumulation saturates during a cycle, thus the current flow stops. Therefore, capacitive impedance decreases as the frequency of the applied AC voltage increases. It's possible to hear capacitive impedance in a pickup - if you've ever noticed that a pickup with a broken winding still produces sound, but it's high-pitched and shrill. That's the current flow due to inter-winding capacitance. Inductive impedance occurs if an alternating electric current produces an alternating magnetic field which, in turn, produces an opposition to current flow. Induction is measured in Henries. The magnitude of this magnetic field increases as AC voltage frequency increases, so inductive impedance increases with increasing frequency. Of course, pickups have significant inductance due to the magnets, and is a key variable in the sound. High-inductance pickups tend to choke off the high frequencies more, since inductive impedance increases with frequency.

To measure complex impedance with an impedance bridge (I used a General Radio 650-A bridge for years), you need a sine-wave generator and at least a set of headphones, but preferably, an oscilloscope. One attaches the load, signal generator (which charges the circuit at some frequency), and headphones/oscilloscope as directed. If you're using headphones, it better be an audio frequency that you can hear. Most often, people use 1KHz. Then, there are several dials that are used to 'balance' the bridge to that load. You adjust the dials until you get a minimum, or "null", on the headphones or scope. The resistance, capacitance, and inductance are calculated from these dial readings, using formulae in the manual. It's not a very complex process. If you can get a bridge, there are lots of manuals online - for example, scanned GenRad bridge manuals are here:

http://www.jamminpower.com/main/GR%20Bridge.jsp

These days, sophisticated network analyzers are available (e.g., the Agilent 8510) which automate this process. However, these are expensive and have a significant learning curve. The old GenRad bridges work great for pickup measurements at audio frequencies if they're calibrated properly. I wish I still had mine, I think my dad still has one.

b0b is right, pickup manufacturers tend not to talk about complex impedance much (Lawrence is an exception, I agree). That's too bad, since only the complex impedance gives a good picture of the audio response of the pickup. The resistance only tells you how many windings the pickup has (assuming you know wire gauge used). One can compute an approximate frequency response from the R, L, and C values obtained.

I'm not sure where one finds bridges these days. Angela Instruments has had test equipment in the past, but they're sold out right now. Ham fests, guitar shows, ebay, etc., might turn up something. You do need to make sure it's calibrated, though.<font size="1" color="#8e236b"><p align="center">[This message was edited by Dave Mudgett on 19 May 2005 at 10:24 PM.]</p></FONT>

You might get the impression that impedance is a complicated thing. Well it is, but for audio work you only need to know if it's low or high. A high impedance device works with 1 to 2 volts, and a low impedance device works with about 1/100 that much, so if you mismatch devices it will be real obvious right away. Other than that, it matters but it doesn't matter much.

Vern, I assume you're talking about signal transfer here. One wants the pickup impedance to be much lower than the input impedance of whatever it's plugged into, say an amp. Now, if the pickup inductance is high, the inductive impedance of a pickup could go up sharply at high frequencies. So even if the pickup-to-amp DC impedance ratio (pure resistive impedance) is low, it's still possible for the high-frequency impedance ratio to be much higher. If true, this could cause loss of high-frequency signal, over and above the filtering resulting from high-frequency inductive impedance.

Further, a pickup is not only a signal generator, but a filter with electrical parameters set by the complex impedance of the pickup. If one wants to know what character of sound the pickup has, the details of complex impedance are important on their own. For example, a pickup with high inductance tends to filter out high frequencies more. This is, I believe, part of the reasoning behind many of Bill Lawrence's designs. His humbuckers tend to be lower in inductance, which allows large numbers of windings and relatively high DC resistance, but high frequencies come through nicely. I think this is a big part of why his pickups are so popular for pedal steel. This type of design is, I believe, less popular for typical "rock guitar humbucking" pickups. Many rock guitarists want the pickup to sound dirty, so they use a high-inductance design. The complex impedance of pickups matters a lot, IMO.

Thanks for all the intricate replies, it's way over my head, and I guess I asked the wrong question, I'm only wondering how to measure, as Bobby Lee states, the DC resistance most people refer to when describing a pickup, i.e., 18.5 ohms, etc. Sorry for the error, everyone.

So Bobby, or anyone: can I get this multi-meter at Radio Shack? Does it plug into the guitar? How's it work? Thanks.

Dave, you are correct. I just didn't want to try to explain all that to someone who doesn't already know what impedance is.

Yea John you can get an ohm meter at radio shack. Hook the two leads to two wires on the pickup and measure the resistance.
Hook

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www.HookMoore.com

Thank you, Hook!!!!
I'm sure John just wanted a simple answer, not an education.

But, as mentioned the DC resistance measurement can't be compared between different brands of pickups or for that matter different pickups of the same brand if different wire is used or different magnet strengths, tightness of the windings, etc.

e.g. if you have an old Emmons PP single coil pickup at 18 K ohms DC resistance and you have, for example a single coil pickup from someone else with 18 K ohms DC resistance, they will not have the same response.

The only way to maintain consistency between pickups or between batches of pickups is to "scientifically" do it the "Bill Lawrence way".

Glad I still got my GenRad 1650-B in the attic. Someday I'll put some batteries in it and relearn how to use it. Dave M., thanks for the link to the manual. That's a big help. <font size="1" color="#8e236b"><p align="center">[This message was edited by Ray Minich on 20 May 2005 at 08:33 AM.]</p></FONT>

John, no problem, just trying to help. I taught E.E. for a long time, this is a kneejerk response. Maybe somebody will find it useful.

As Hook says, get any reasonable (e.g., Radio Shack) multi-meter, set the scale to Ohms. If there are ranges of resistance measurement, set it to the smallest range of resistance that is larger than the one you are expecting to find. If you set it too small, it will generally read 'infinite' resistance, so if that happens, just increase the range to the next larger one until you get a reading. Some modern meters are 'auto-ranging' - if so, you don't need to worry about this at all. Attach the red and black leads to the ends of the pickup leads and read it out. If the pickup is in the guitar, I usually run a cable into the guitar jack, and measure across the tip and sleeve of the other end of the cable. Make sure that the volume and tone controls are turned all the way up.

Resistance is often a useful measurement if one is talking about pickups of 'similar magnetic' properties and same wire gauge. For example, comparing traditional Telecaster or Stratocaster pickups or Gibson humbuckers with the same magnet design, the number of windings is the key variable, and that is correlated directly to DC resistance. But it often says very little about the difference in sound of pickups that have significantly different designs, as Jack says.

Dave, thanks, very simple and clear, perfect for an electronics ignoramus like me!

<SMALL>So Bobby, or anyone: can I get this multi-meter at Radio Shack? Does it plug into the guitar? How's it work? Thanks.</SMALL>

Yes, Radio Shack has a variety of models. The important thing for pickups is that you want to see resistance differences in the 10k to 25k (ohms) range.

The device typically has two leads coming out of it. You touch them to the wires coming out of the pickup.

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<font size="1"><img align=right src="http://b0b.com/Hotb0b.gif" width="96 height="96">Bobby Lee - email: quasar@b0b.com - gigs - CDs, Open Hearts
Williams D-12 E9, C6add9, Sierra Olympic S-12 (F Diatonic)
Sierra Laptop S-8 (E6add9), Fender Stringmaster D-8 (E13, C6 or A6)</font>

John, yes Radio Shack has what you need anywhere from about $25 up..Most "multi meters" today are digital auto rangeing and easy to use..

Give me a call if you need help and I can help you with measuring the DC ohms..

I recommend an inexpensive one as about the only difference in the higher priced ones is they are a bit more accurate..But + or - 10% or so is more they good enough for this purpose..



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Jerry Wallace/TrueTone pickups-2001 Zum: D-10,8+6, "98 Zum: D-10,8+8,Nashville 1000,Session 500 ,Session 400 head only amp,Tubefex,ProfexII, Artesia, New Mexico
http://www.jerrywallacemusic.com

John, this one will do what you need and a little more...



Radio Shack

...like check your AA and 9V batteries...

(Cute how they set the display to show the price: 19.99)

If you are trying to measure a pickup in a guitar that has volume and tone controls by using an open plug in the output jack you may not get a correct reading; anyone can feel free to correct me if I am wrong about this...

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<font size="1" color="#8e236b"><p align="center">[This message was edited by Mark Herrick on 20 May 2005 at 11:44 AM.]</p></FONT><font size="1" color="#8e236b"><p align="center">[This message was edited by Mark Herrick on 20 May 2005 at 11:45 AM.]</p></FONT>

Thanks again, Bobby, Jerry and Mark!