Monday, 11 June 2012

Humbucker coil tap with passive shunt, or Shuntbucker.

I have been fitting the pickups to my guitar build and they came with five wires. A shield wire and green ground wire combined. The coil tap connection red and white together and black for hot. It occurred to me whilst wiring it all in that simply shorting out the bottom coil to ground to coil tap whilst giving a good single coil sound, was a bit simplistic and short sighted. There has to be some use for the output of the now shorted coil other then dropping it to ground.

The easiest mod would be to short it to ground with a capacitor. This way I should get the brightness from the top coil but with some added bottom end and a small amount of hum-bucking from the bottom coil. If you plot the impedance (AC resistance) of say a 47nF (0.047uF) and 100nF capacitor you will get the following impedance values.

If  I presume that the _average_ impedance of each coil would be say 3k then you can see that with the 100nF capacitor (red plot) half of the output of the shunted coil will be lost via the capacitor at 500Hz.  Without some complicated measuring I can't be sure exactly what the impedance of the coil will be at a particular frequency. I do know how the capacitor works though and it had to be worth a go. I went with 47nF (blue plot) in the end and am very happy with the effect on the bridge humbucker. Below is the diagram showing the two coils from a humbucker and how the capacitor shunts the bottom coil.
Shuntbucker with frequency dependent shorting of unused coil.

It has a distinct change when tapped as you would expect but the pickup has a lot more balls, more P90 than strat sort of sound. For the bridge pickup this is ideal for me as I never found a use for a single coil bridge position. After trawling the net I did find that this is not a new idea, coil cut was a term I found amongs some others. Considering the simplicity of the modification, and the potential in using more complicated passive networks I am a little lost as to why the vast majority of sites will just short out the one coil and concentrate on additional phase and series parallel switching.

I think the major pickup manufacturers have also not actively brought to the fore this simple modification or even re-voiced there own pickups with a passive shunt so that when taped you get something better sounding. Combining a capacitor, inductor and resistor network across the lower coil could tailor curves into the pickup and tailor the response beyond wire gauge, turns and ad-hoc parasitics. It seems a vastly unexplored path, maybe it needs a name people can identify with, so maybe coining the term Shuntbucker would help it stick in the conscience.


I have found that as per the follow on article “Pickup Frequency Response Measurement.  Humbucker, Single coil and Shuntbucker” this really needs a series resistor with the capacitor to make this work. The issue is that the capacitor and coil inductance are too resonant and the transition results in the mid-range being swamped.  I have found that a 47nF and 3k3 resistor across the bottom coil works well for me.


  1. I like the way you think!
    Always wonder and ask why!? The answer usually is just because and thats how we've always done it. When in fact its like primate research- monkey see monkey do... after all it is a proven low risk formula that has worked well for the other guy that they copied their designs from?
    Amazing, how much guitar stuff that is considered the holiest of holy mojo is attributed to old, low tech, engineering and antiquainted production processes. I doubt much of the traditional and classic designs weren't based on state of the art or performance back when such products first debuted. More likely from a bottom line with the priority of qty of units produced vs tooling costs. Remember electric guitars were originally manufactured as a novelty more so than crafted as a fine instrument.
    Nowadays, everyone is trying to book a way back trip to the magical good old days when made in America meant something. Alot of marketing successes fueled with great mojo and superstition. Interestingly the greatest explainations for some of the best of the best are always that one unit that had the perfect storm of inconsistiencies to make for that one off revered hallowed tone perfection.
    Today they are marketing designed imperfection and inconsitiency at a premium. The cruder oversimplified worn and "relic'd!
    Ok I will admit that I am a tech junkie that likes to over complicate and reinvent the wheel, but I somehow have a preference to keeping my guitar simple. Just plug the amp in turn it up to eleven and go.... No stomp box or digi effects rack. Just dime all the controls and play guitar. I remember alot of cool switches dials and buttons added to guitars at the end of the 70's and it usually turned out that given all combinations most players would rely on just one or two preferred settings. A classic player and a icon guitar of notability would be BB King and his Lucille with the Gibson Varitone Switch. Which is a gang of rotory switched capacitors.
    It do wonder why some the most creative and unique designs, equipment and technology recieve the least credibility and respect. Come on Roos is vaccum amp tech really so superior. Given todays semiconductor tech cant we really build a superior amp if we wanted? Peavy kinds comes to mind as a underappreciated innovator. Tom Scholz and even Dimarzio isnt fully given creds..... I.D.K. ? Just a couple of thoughts.
    On the technical side of passive tuned circuits... Is there any sort of reason or negative quality like suceptibilty to certain resonant harmonic generation or possible characterjstics that may effect the temperament of the tonal scale or intonation of the instrument. Or maybe make it recieve secret low wave signals from nuclear submarines or large corona discharges as the strings build up vibration and high negative ions from strings parastically couple with the output transformer creates a dimensional gateway back in time to 1948 but only during peak sunspot activity? (ok sorry im getting carried away kinda delerious from not enough rest...working hard in the sun lately)
    Seriously i do like the way you think, and question as to why. Why hasnt this been exploited?
    YOUR TIMING IS PERFECT Ive been considering a project that i wanted to try and wind and build my own pups ... Now im more motivated with the addition of another one of your fine recipes. I love that you always back up your theory with modeling and have charts for reference. You are awesome!
    How about your guitar of the future .....
    " featuring patent applied for 2112 roosville shuntbucker pickups"
    Hell Yes!!!

  2. Ha! I did look at Shuntbucker pickups (one google hit) but it’s a subjective, saturated market and any design will get ripped off pretty soon. I think it would just be heading for pain. I do think valve amps are better for guitar; you don’t have to worry as much about a safe-operating-area as you do with transistor design. Valves just run out of steam, but will stay there all week. The true sound from a valve amp is the calming nature of the output transformer and limited response of the speaker, it’s a complicated arrangement.

    I think there is a lot of real innovation in the market, it’s just lost under the noise of everything else. As you say the past is a familiar country. I did do the tech route and spent all I had on a Korg A3 DSP in 1988 but I have eventually come back to a guitar, wire, combo approach.

    If you wanted to pull a humbucker apart; Coils. machine or scatter, but I can’t find a mixture of both. I would like multiple taps within each coil so I can add passive shaping to different sections within a coil. I would like to see armature shapes looked at to see if you can get more output whilst maintaining a weak field. In a Strat pickup a lot of the magnetic field is just between adjacent pole pieces and possibly isn’t optimised. I would like to try grounding pole pieces by a high impedance and not shorted to ground so I potentially extend the treble. I can reduce this later, but I can’t add what isn’t there. Conductive foam would be an easy engineering fix. The length of the adjustable pole pieces, sometimes they are flush sometimes they extend a long way behind the pickup. It’s not an exhaustive list.

    I did make an electronic pickup about five years back using a GMR sensor chip (I did some work on a while ago on magnetic field sensors for an under car secure alarm system for car parks). It was flat from DC to MHz but was very power hungry and even with bandwidth limiting, very noisy. Currently I have reciently recovered an old air pump motor with speed control but it just begs to be made into a coil winder, that and I have just been given a pair of old PRS humbuckers to play with…..

  3. I find this an interesting idea, but I am questioning the resulting behavior. The L of the coil and the switched C constitute an LC tank -- a passive oscillator with a natural resonating frequency = 1 / (2 π √ L C). With a 3H coil and a 47nF cap, the frequency would be about 420 Hz -- deep in the audio range. I'm guessing that this configuration isn't oscillating if anyone is having success with it. I am going to hook up a scope -- and an amp -- and see/hear what this looks/sounds like.

    Has anyone put this configuration to work? Any reports back?

    1. Hi,

      If you look at the follow up ( next) post to this I do cover this point. As you correctly note, the simple inclusion of a capacitor is too reactive and it requires additional series resistance to prevent passive gain (resonance) and to smooth the transition between humbucking and single coil operation. One other note is that you will need to reduce any tone control capacitor value to prevent early frequency rolloff.

  4. This comment has been removed by the author.

  5. Thanks so much for this shunt info!! I love modding guitar wiring but I usually come to my solutions by partially by understanding and partially by luck. I'm trying to understand the wiring above. I thought when you split a coil you typically route the red/white pair to ground which leaves one coil off. If I shunt (w/ cap/res) the red/white wire pair wouldn't that just manipulate the sound of the single coil that is working. I just soldered up my first attempt at one hum, one tone, DPDT (hum, shunt w/ 0.1 mF cap, 22K res). These were the closest value I had to your specs. Do you think this would work? I'm getting output but both settings sound the same. Plus I think my set up isn't even humbucking right now.