Pickup Frequency Response Measurement. Humbucker, Single coil and Shuntbucker.

Well the shuntbucker has been in for a while now and I am still trying to get my head around how good it sounds. As it happened I did a pickup swap and quick setup for a PRS Tremonti SC (the cheap one) last week and was paid with the old pickups, which was more than I wanted. Anyway I now have a pair of ex PRS Korean generic humbuckers measuring about 10.1k DC resistance to play with.

I thought it would be the perfect test vehicle for a frequency response measurement comparing normal humbucker wiring, the normal shorting of the slug coil for single coil type response and the 47nF shuntbucker mod. First though I would need a stable and repeatable frequency measurement kit. It took a few swipes as you have to energise both the pickup coils with opposite magnetic polarity and it needed to work from low Hz to high kHz. Anyway to cut a long story short, I soon had a rig using a frequency generator, an OPA540 as the coil driver and an energiser coil. The output of the pickup was connected to a 100k resistive load and measured on an oscilloscope. This was a slight limitation as I would like to have measured at 500k but it’s the input impedance of the FET scope probe I used. To eliminate ambient noise the measurement was taken as an average of 100 consecutive measurements. The output in mV is plotted below. I have since reduced the drive amplitude as the output was higher than expected.

The blue plot is the normal humbucker. The red plot is with the slug coil shorted to ground which is normal practise for coil tapped pickups. In this configuration you acn see that the resonant peak has shifted to a higher frequency giving the expected single coil sound.  The green curve is the shuntbucker mod with a 47nF capacitor to ground across the slug coil. As this is directly injecting a magnetic field you won't see the inverse behaviour caused by fat strings producing a higher magnetic signature than say the thin top E string, which helps flatten all this out.

From this you can see that the 47nF worked as expected and has adopted the humbuckers low end frequency response. What is evident is that using a straight 47nF has produced some resonance with the coil which is why it appears to have a higher output compared with the humbucker configuration at 400Hz. You see the effect again at 800Hz as the kink in the otherwise smooth curve.  By adding a series resistance the resonance can be modified and a much cleaner transition from humbucker to single coil can be obtained. At the high end the shuntbucker follows pretty much the higher resonant peak of the normal single coil configuration. Hence a shuntbucker is like a single coil but with hairy balls.

 I think I will need to measure the spot frequency impedance of the humbucker and single coil configurations to work out the ideal shuntbucker capacitor series resistor. I could just try it out but I think the data will be useful. It could be I end up with two resistor/capacitor nets and possibly look at pulling the high frequency resonant peak about.