Idea for circuit to attenuate single coil hum

[Morocotopo]

Just a thought... how about a notch filter at 55Hz or whatever your mains frequency is, and a couple more at Freq*2 and Freq*3 to get some of the harmonics, slightly tuneable with a pot from, let´s say, 50 to 60 Hz to get exactly the hum freq, and a depth pot to fine tune? I believe making very sharp filters is difficult in the analog domain... not much experience with that. Will the filters provoke phasing problems? Eat away too many surrounding frequencies? Maybe some company already makes (made) such a thing, or some of you, resourceful DIYers...

[R.G.]

Yep, it's been done.

Your thoughts about it are right. The filter has to be really, really sharp to get good results; it has to be tunable to get good accuracy; sharp filters have big phase shifts; digital can do it better, but introduces its own problems to be solved; and there exist some solutions, although not in the form you mention.

First, it is possible to make a single coil guitar be remarkably lower hum by rewiring and shielding it well. There is a web page somewhere that has instructions on this; I followed them on my Strat and the hum went down to near-inaudibility. I'll try to remember the page.

Next, there is a hum canceller design I've seen where the device generates 60Hz, 120Hz, 180Hz, 240Hz sines with variable amplitudes and phases, all knob-controlled. It goes into an amp input, and cancels both the guitar hum and the amp hum. The problem is that any part of the hum coming in that varies can only be statically cancelled, and if it varies - like if you move the guitar around  - the hum size and phase varies, and the knob setting for cancellation is no longer correct.  I was actually about to design up something based on that til I remembered that guitarists don't always sit very still in one place while they play. 

[Paul Marossy]

First, it is possible to make a single coil guitar be remarkably lower hum by rewiring and shielding it well. There is a web page somewhere that has instructions on this; I followed them on my Strat and the hum went down to near-inaudibility. I'll try to remember the page.

www.guitarnuts.com ?

[Mark Hammer]

Another approach uses "dummy" coils.  Any magnetic pickup coil operates in two ways.  It senses disturbances in its electromagnetic field produced by the strings and any other nearby metallic objects.  It also operates like an antenna, sensing ambient sources of "radio" signal.  One of those is 60hz fluctuations produced by any of a variety of AC sources: fluorescent lights, transformers, etc.

A hum-rejecting pickup that uses two coils, provides two such "antenna" that sense the ambient 60hz hum equally, but in opposite phase.  When combined, those two signals cancel to greater and lesser degrees, depending on matching of the signal levels. "Dummy" coils do the 60hz-sensing, but don't sense the string because they have no magnet.  Ideally,  hum is sensed at equivalent levels when the dummy and string-sensing coil are close to each other, but they don't need to be.  The Suhr backplate system puts/laminates a coil around the perimeter of the tremolo-spring cover-plate, providing a single coil that recives a relatively equivalent 60hz signal, regardless of which pickup you might be using (i.e., surrounding them all means it isn't "nearer" to any single one).

This approach does not introduce any of the phase issues that RG noted.  Of course, a stompbox doesn't require any routing of the guitar, or changing of pickups, where placement of a dummy coil may necessitate some physical change to the instrument in some instances.

On a long drive, once, I thought it might be useful to produce a multi-tapped dummy coil, with selectable tap-points and a dipswitch to play with to get the optimum cancellation, and match the dummy to different pickups.

[arawn]

yep www.guitarnuts.com

I can play my start in front of the computer with no audible hum even with the flourescents on!!

[Paul Marossy]

Another approach uses "dummy" coils. 

Yeah, some of Suhr's guitars use that system.

[Mark Hammer]

Another approach uses "dummy" coils. 

Yeah, some of Suhr's guitars use that system.

I tried one out about a half dozen years ago at the Montreal Guitar Expo, and made a point of standing right beside the transformer side of a big amp head.  Not dead quiet, but more than quiet enough for gigging or recording.

[ashcat_lt]

Dummy coils have their own issues.  Specifically, they are rather large inductors.  So are standard pickups.  Wired in series with the pickup, it will dull the treble, making it sound more like a Humbucker.  In parallel, it will actually make the thing brighter. 

Shielding is really the first place to start, as noted above.

[Paul Marossy]

Another approach uses "dummy" coils. 

Yeah, some of Suhr's guitars use that system.

I tried one out about a half dozen years ago at the Montreal Guitar Expo, and made a point of standing right beside the transformer side of a big amp head.  Not dead quiet, but more than quiet enough for gigging or recording.

Yeah, it's way better than the standard noisy Strat pickups. Although, no amount of shielding seems to help cheap single coil pickups be any quieter. 

[Mark Hammer]

Dummy coils have their own issues.  Specifically, they are rather large inductors.  So are standard pickups.  Wired in series with the pickup, it will dull the treble, making it sound more like a Humbucker.  In parallel, it will actually make the thing brighter. 

Shielding is really the first place to start, as noted above.

The nice thing about the Suhr approach is that he doesn't approach it as a magnet-less pickup, but as an antenna.  From what I understand, there are not that many turns onthe backplate coil, and the inductance is fairly low.

But you're quite correct in pointing out that dummy coils are not always a panacea; you don't get something for nothing.

[Paul Marossy]

Oh, the ever elusive panacea. 

[Morocotopo]

You know what´s funny?

I don´t own a single coil pickup guitar!!!



But the challenge still stimulates me. How about this: A simple circuit that inverts the phase of a copy of the signal and mixes it with the original, to cancel it. Of course it has to differentiate noise from useful signal, so an envelope detector has to be added. That would detect useful signal and "duck" the inverted signal. Doable? It´s basically a noise gate, but instead of silencing signal it silences an inverted copy. Reinventing the wheel? Hmmm.

[Paul Marossy]

Reinventing the wheel? Hmmm.

Yeah, probably. But it might be an interesting project to try.

[Joe Hart]

I don´t own a single coil pickup guitar!!!

Shame on you!
-Joe Hart

[StephenGiles]

You know what´s funny?

I don´t own a single coil pickup guitar!!!



But the challenge still stimulates me. How about this: A simple circuit that inverts the phase of a copy of the signal and mixes it with the original, to cancel it. Of course it has to differentiate noise from useful signal, so an envelope detector has to be added. That would detect useful signal and "duck" the inverted signal. Doable? It´s basically a noise gate, but instead of silencing signal it silences an inverted copy. Reinventing the wheel? Hmmm.

I own 3 single coil guitars 

[Paul Marossy]

I have a couple of guitars that have single coils, but I almost never use the single coils. I always go for the humbuckers.