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Here's some information and links on an interesting technique for controlling the resonant frequency of a pickup by electronically varying the load capacitance with pot-controlled feedback to one side of the capacitor.
The basic idea is from the late 60's and derived from, of all things, the Thomas/Vox original wah pedal (Cry Baby) design.
As you might recall, the Vox wah pedal is able to get a varying resonant filter from a fixed inductor, by varying a potentiometer mechanically geared to the pedal position. R.G. Keen provided an excellent analysis of this in the article:
The Technology of Wah Pedals
Is this relevant to pickup and guitar electronics design? You bet. Alex Troshnev in 2005 came up with a not widely publicized circuit for varying the resonant frequency of a guitar pickup using the same idea:
Built-In Guitar Amp - SAGE (Smart Altering Guitar Electronics)
While switched capacitor methods such as the Stellartone Tonestyler are in the market, and very useful, Alex's circuit allows continuous tuning of the pickup characteristics over a usefully wide range.
My own simulations and experiments show that for low-Z pickups, this type of circuit can provide about 1.2 - 1.5 octaves of range with a Q value of up to a little under 2 with a single fixed capacitor and a low parts count circuit. The tradeoff is that it is hard to get Q values more than about 2 (6 dB peaks. This may be just a limitation of my version.)
However, even with a Q limited to about 2, the circuit looks like it can emulate most traditional passive single-coil and humbucker resonant peaks when the passive PUs are in-circuit and loaded by controls and cable capacitance, just by turning the knob, or worst case by switching another capacitor in or out to change the adjustment range of the pot.
I haven't prototyped Alex's original circuit with high-Z pickups but don't have much doubt the method would work as he describes.
Alex's circuit represents the first attempt I am aware of to provide a continuous control over the pickup's real resonant peak frequency as an onboard control in the instrument. (As opposed to simulating the response with active filters, etc.)
This could provide an onboard tone control which is vastly more musically useful than the "vanilla" passive tone control we all love or hate, and in many cases don't use.
I am a bit concerned that someone will try to ring-fence this idea with patents. Hopefully this will assist more people to be aware of this technique as prior art and therefore most likely public domain, to preserve the ability of experimenters and small builders/manufacturers to explore and refine.
I can't comment on whether someone has tried to patent this or has successfully done so to date, so hopefully someone can help out with knowledge or a search.
Take care,
Charlie
The basic idea is from the late 60's and derived from, of all things, the Thomas/Vox original wah pedal (Cry Baby) design.
As you might recall, the Vox wah pedal is able to get a varying resonant filter from a fixed inductor, by varying a potentiometer mechanically geared to the pedal position. R.G. Keen provided an excellent analysis of this in the article:
The Technology of Wah Pedals
Is this relevant to pickup and guitar electronics design? You bet. Alex Troshnev in 2005 came up with a not widely publicized circuit for varying the resonant frequency of a guitar pickup using the same idea:
Built-In Guitar Amp - SAGE (Smart Altering Guitar Electronics)
While switched capacitor methods such as the Stellartone Tonestyler are in the market, and very useful, Alex's circuit allows continuous tuning of the pickup characteristics over a usefully wide range.
My own simulations and experiments show that for low-Z pickups, this type of circuit can provide about 1.2 - 1.5 octaves of range with a Q value of up to a little under 2 with a single fixed capacitor and a low parts count circuit. The tradeoff is that it is hard to get Q values more than about 2 (6 dB peaks. This may be just a limitation of my version.)
However, even with a Q limited to about 2, the circuit looks like it can emulate most traditional passive single-coil and humbucker resonant peaks when the passive PUs are in-circuit and loaded by controls and cable capacitance, just by turning the knob, or worst case by switching another capacitor in or out to change the adjustment range of the pot.
I haven't prototyped Alex's original circuit with high-Z pickups but don't have much doubt the method would work as he describes.
Alex's circuit represents the first attempt I am aware of to provide a continuous control over the pickup's real resonant peak frequency as an onboard control in the instrument. (As opposed to simulating the response with active filters, etc.)
This could provide an onboard tone control which is vastly more musically useful than the "vanilla" passive tone control we all love or hate, and in many cases don't use.
I am a bit concerned that someone will try to ring-fence this idea with patents. Hopefully this will assist more people to be aware of this technique as prior art and therefore most likely public domain, to preserve the ability of experimenters and small builders/manufacturers to explore and refine.
I can't comment on whether someone has tried to patent this or has successfully done so to date, so hopefully someone can help out with knowledge or a search.
Take care,
Charlie
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