Electro magnetic guitar pickups? "True Active Pickups"

[Havaden]

Does anybody know if this is done before?
A true active pickup using a regulated electric magnet instead of ceramic/rare earth magnets?
I did a quick search but couldn't find anything.
Mabye there is some noise generated from the coils that makes this unviable?

[ashcat_lt]

I can't think of anything like it and don't have much to say, but if I want to watch this thread. 

[slacker]

That's basically how things like the Fernandez Sustainer and the Ebow work, except the generator is an amplified version of the guitar signal so you get a changing electromagnetic field that keeps the strings vibrating.
I don't know what the point would be of using an electromagnet just to make a pickup though or if it would even work. I suppose you could change the field strength maybe allowing you to change the characteristics of the pickup?

[Transmogrifox]

I suppose you could change the field strength maybe allowing you to change the characteristics of the pickup?

That is one advantage that comes to mind.

My intuition tells me it would take a good deal of juice to match the magnetic field strength of permanent magnets used in a traditional coil.  Ever try to pick up a nail with a magnet made with a 9V battery?  Not likely to be a battery compatible system.

Interesting idea anyway.

[R.G.]

There are issues.

Where does the signal come out? On the same winding driven by the generator? Or another winding not shown?

If there are two windings, there is more complication and expense. There is also the issue of how tightly coupled the signal winding is to the "generator" winding. If the coupling is "tight", then generator noise rides right through on the signal line, just as it would on a single-coil setup where the generator drive runs the same coil as signal out appears on.

Tight coupling is also a problem in terms of impedance. If the generator is a "voltage source" low impedance, then the signal generated through a sense coil is heavily attenuated by the low generator impedance. You can get around this by making the "generator" be an active current source with a high impedance, but these circuits tend to generate a lot of hiss/noise.

If there are two windings and the coupling is loose, then by definition the signal generated is much smaller, as the coupling is through the magnetic field for both signal and the exciting coil.

It can be made to work. There is a significant amount of discovery and development needed to make it work well. Not to mention that you have to step over the line that requires an electrical power source in the guitar.

[ashcat_lt]

Yeah, what he said. 

Just for fun, I have heard about the idea of using the string itself as the signal carrying wire.  Its motion within the field of them magnet induces a current through it, and "all" you have to do is take the output from across the string.  There are (more or less) obvious issues with implementing this on most guitars, not the least of which being that even with all six strings in series, the overall resistance just isn't enough to add up to much signal voltage worth mentioning.

There's also that electrostatic pickup technology like in the Wurlitzer pianos where the tines float above a charged plate or bar creating a capacitor whose value changes as the tine vibrates, and that creates a changing voltage between the two.  Big problem there is that in order to get a useful signal from it, there has to be a REALLY BIG voltage applied to one or the other.  In a Wurlitzer, that's about 170V.  Definitely don't want that inside my guitar!

[R.G.]

There are also single-turn pickups. These use a (big) single turn of heavy wire, usually cut from sheet copper or aluminum, and feed a transformer immediately to try to convert the small voltage upwards.

Might be something there, but I wouldn't want to have to design the step-up transformer for that. Wide-ratio transformers have big problems keeping good frequency response.

[Transmogrifox]

If you want to patent the idea you have a lot of r&d (research and DISCOVERY) ahead of you

Best way to start is to gut a pickup and remove the magnet....

I'm in favor of starting with the generator and signal coil being one in the same.  The first step is proof of concept, then from there you're dealing with noise, signal levels and frequency response until "it be good".

[Rob Strand]

There a lot of energy stored in a magnet and you are getting this for free for years.  If you want to create an equivalent electromagnet you will be burning up batteries.   Mind you charging-up a lithium ion battery these days is widely accepted.    (Maybe we can design one that catches fire by itself and call it the Jimmy Hendrix model.)

[Alex83tele]

I'm not an authority on active pickups, but the only kind I've ever seen use an op amp (or something like it) to amplify the signal coming directly from the pickup coils. By energizing those coils, there can no longer be current induced from the vibration of the string across the magnetic field. It seems to me that coiling another wire along side the electromagnet's power coil would only produce a massive buzzing sound. If you made an electromagnet and incased it in nonferrous material about the size of a pickup magnet, then coiled wire around that, it might work. But as I understand it, some magnets are considered too strong to use in pickups because they have adverse effects on the strings regarding intonation and sustain, etc. As others have mentioned already, your getting free power from the magnets. I'm not suggesting that a pickup can't be designed using an electromagnet, but I don't see how it could function like a magnetic pickup on steroids. I would think it would have to be an entirely new design, like piezoelectric or electrostatic pickups are, but neither of those were intended to replace magnetic pickups. As much as people complain about the high impedance signal from guitar pickups (me included), they do a wonderful job of capturing the sound of a vibrating metal string. But who's to say there isn't a better way to do it.


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[bluebunny]

Maybe we can design one that catches fire by itself and call it the Jimmy Hendrix model.

LOL.  And sponsored by Samsung, perhaps? 

[amptramp]

Many years ago, loudspeakers were designed with a field coil rather than a permanent magnet.  The field coil was often used as the smoothing choke for the power supply of the radio it was in.  Coupling of audio to the magnet was not an issue because the speaker impedance was about 4 ohms and the coil might be anywhere from 2000 to 4000 ohms DC resistance and much higher impedance.

An electromagnet in a pickup, unless there was some way of shielding it, would have voltage induced on it and it would have to be driven from a high-impedance (current) source.  It would allow a variation in tone by changing the current.  Note that the pickup is a high-impedance output of several thousand ohms so you can't get away with much noise in the field current.  The guitar will require a power supply.  If the field coil is also to be used as the output coil, then the noise is directly added to the output.

[PRR]

> Coupling of audio to the magnet was not an issue because....

... there was 16uFd on both ends of the field coil.

The impedance difference is little impediment because it's mostly Turns Ratio. You can couple a 8K plate winding to a 4r speaker really good.

[Havaden]

I suppose you could change the field strength maybe allowing you to change the characteristics of the pickup?

Well that's my point with the regulated electro magnet. I wonder how it would sound.
You could change the magnetic force or even invert it. And allso as mentioned before.. you could probably hook it up as a sustainer driver too.

[Havaden]

I did a quick experiment sacreficing a cheap chinese strat pickup.
At 36Vdc and 35A I was not even able to move a steel rivet pin, wich even the smallest piece of crushed magnet would move with ease.
That being said, im not sure what the pins inside the pickup are made of. (probably some cheap nickel filled steel)


Experiment reasult:
1 - The "Driver coil" would have to be special made with a solid steel core.
2 - One would need an external power source for the guitar. (Mabye a stereo jack and a special made power supply stomp box signal/power splitter would work?)