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Old 1st May 2004, 03:10 AM   #1
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Default Low-impedance pickups

Hi all,
As has been much discussed on this list, traditional electric guitar pickups have fairly high inductance, series resistance, and self-capacitance. As a result, they have a self-resonant peak frequency within the guitars frequency range (i.e, it affects the guitar sound). In addition, these characteristics make the pickups sensitive to the effects of cable capacitance and amplifier input impedance. Finally, the required high input impedance amplifier makes them more prone to pick up hum and noise through capacitive coupling to any stray electric fields in the environment.

I'm not sure why this design was originally chosen - most likely because the tube amps of the time had limited voltage gain available, and consequently required a fairly high output voltage from the pickup.

About a decade ago I built my own electric guitar, because at the time I was the prototypical starving student and had no money to buy one. I started with a throw-away arch-top acoustic guitar that had split down the top, from which I cannibalised the neck and tailpiece (nothing else was useable). Anyway, I made my own pickups from bits of scrap mild-steel strip, scraps of ceramic magnet from a damaged pocket-radio loudspeaker, and other odds and ends. I used some fine gauge wire that I happened to have (no idea what gauge it was, but it was hair-thin), and wound about two or three thousand turns for each coil (I made two pickups). By sheer chance and through my complete ignorance I ended up with low-impedance (around 2 k Ohm, IIRC), low-output pickups.

When complete, the guitar had a very clean, shimmery tone, sounding more like an acoustic steel-string guitar than like the typical electric guitar. Running a finger along the round-wound strings produced a shimmery tone that clearly had more high-frequency output than typical guitar pickups. Due to the low output voltage, I had to crank up the gain of whatever I was plugged into, but it wasn't hard to get some distorted tones when I wanted them (I had a diy preamp and two or three diy distortion/overdrive units).

Some years ago there was a popular TV commercial featuring Les Paul playing a few notes and a chord or two. I was struck by the clean, bright tone he got from his guitar, which reminded me a little of my home-made guitar (though Les Paul's guitar had a far more beautiful tone and sustained far longer). Not long ago, I read somewhere that Les Paul preferred to use "custom, low-impedance pickups" on his personal guitars.

I was just wondering if anyone else knew more about Les Paul's "low impedance" pickups, or had ever used low-impedance pickups on their guitar. I'd also like to know about any pros or cons to either approach. To me, the idea of a low-impedance pickup which imparts very little coloration to the guitar sound is appealing - that way, the player can get whatever tone he/she wants by appropriate distortion/filtering/eq in the amp and appropriate playing technique, rather than having the pickup decide the guitar tone for you.

Thoughts? Experiences? Observations? Anyone?

-Flieslikeabeagle
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Old 1st May 2004, 10:14 AM   #2
sreten is offline sreten  United Kingdom
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Join Date: Nov 2003
Location: Brighton UK
The issue here is really low inductance, which goes hand in
hand with low output, though thicker winding wire which
reduces source resistance can help matters.

I don't know much about jazz guitar pick-ups, which I assume
would follow your line of thought. Electric guitar pickups are a
different matter altogether and here high output and thick tone
rule for the Bridge pickup. Middle and Neck pickups vary a lot.

sreten.
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Old 4th May 2004, 01:13 AM   #3
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Well, assuming a given magnetic field, the output voltage is proportional to the number of turns in the coil. The inductance, on the other hand, is proportional to the *square* of the number of turns in the coil. And the resistance is proportional to the number of turns, and inversely proportional to the wire gauge (fatter wire = less resistance).

So it would seem that a given reduction in the number of turns (say 50% less) would reduce the output voltage by only 6 dB (one half), but reduce the inductive output reactance to one quarter of its previous value.

My original idea was basically this: a simple active 2nd order lowpass filter can duplicate the resonance of a guitar pickup, and all its parameters (Q, resonance frequency) can adjusted by the twist of a knob or flick of a switch.

So it would seem that a low-impedance single-coil pickup (and I agree with you that this primarily means a low inductance, since the inductive reactance dominates the pickup impedance) in combination with an active second order lowpass filter in the guitar preamp would be very versatile, essentially being able to duplicate the sound of just about any single-coil pickup placed in the same physical location on the same guitar.

Things get more complicated for humbuckers, since they use the algebraic sum of two voltages derived from two different but nearby points on the guitar string. I'm not sure if that sort of rolloff can be replicated electronically.

Still, it's a very tempting idea, no? A pickup that has a flat frequency response of its own, so that along with its preamp it can be dialed in to sound like any pickup you want...

-Flieslikeabeagle
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Old 27th July 2010, 12:01 AM   #4
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Fast forward six years - I just recently heard about the Epiphone Les Paul Ultra II, a solid-body electric guitar which incorporates a third pickup called a "Nanomag" mounted right at the end of the fretboard.

This third pickup (a creation of Shadow Electronics) appears to be a low-impedence one, which we can infer from the ad copy that claims it has a wide, flat frequency response. There is also some onboard electronics that appears to be pretty much what I described in post #3 - there is an audible bass resonance which is designed to simulate the main acoustic resonance of an acoustic guitar's body. It sounds as though there may also be some processing to add some acoustic guitar "shimmer" to the tone, similar to what's in the various acoustic guitar simulator effects pedals on the market.

-Flieslikeabeagle
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Old 27th July 2010, 01:03 AM   #5
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Location: Western Sydney
Interesting, though the 'audible bass resonance' would probably drive me crazy. I'm not a fan on of on-board electronics.
I think pickups are a matter of compromise between lower z to get a clearer tone, and enough voltage out to get the overdrive one wants. (I use single coils in series).
Once I can figure out how to mount them, I'm going to try a Gretsch filtertron knock-off (GFS) on my Epiphone wildkat. These lower-than-usual z humbuckers have a nice clear sound. Not keen on the P90 sound in neck posn.
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Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency
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Old 27th July 2010, 02:32 AM   #6
benb is offline benb  United States
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Yes, you're surely right, the high-impedance large-coil-of-wire pickup design was likely chosen to give significant voltage output into a high-impedance-input tube amplifier grid circuit, saving the extra cost of another tube amplifying stage (or step-up audio transformer) that a lower impedance design would need, and this of course became the standard design of pickups for electric guitars. That design with the high inductance and self-capacitance of all that wire causes a self-resonance of the coil around 5 to 7kHz which of course affects the tone, and became part of the identifiable sound of an electric guitar. The design is a many-decades-old lashup, but it's still what "sounds right" today. Actually, this is the first use of such a pickup:
Frying pan (guitar) - Wikipedia, the free encyclopedia

I HAVE heard about the Les Paul model with low-impedance pickups, it's called the Les Paul Recording model (as for a recording studio, lower noise and electrostatic pickup of hum due to the lower impedance and such). If you google:
Les Paul recording guitar
you'll find out just about everything you want to know about this special, not-so-popular model of the Les Paul.
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