my homemade BPSSC

[madzub]

I know there's been discussion about the Suhr-Illitch patented Silent Single Coil system, though I don't know if anyone has gotten around to testing their own version.

Well, I followed the patent and built my own to appoximately those specs. I wound an approximately 8 inch diameter coil of about 200 loops of 34 awg wire, shielded it (very necessary) and wired up a couple of 5k trim pots with a cap on one of them. It's stuck to the back of my hardtail strat, and I'd say it kills about 80 percent of the hum.... More accurately, with the trim pots properly adjusted it kills all of the low frequency hum, but still a fair bit of higher frequency noise gets through.

More on the trim pots: The patent shows 10k ohm trim pots with parallel resistors of unmarked value. I used 5k pots and still all the action occurs near the end of the pot. I suspect they use a 1k or 2k resistor in parallel with their 10k trim pots just to make them log-tapered, so you could do the same or just use a 1 or 2k trim pot and i'm sure it would be fine.

Upon adjustment, I found that there was a sweet spot on the trim pot where there was the highest cancellation of hum; going below this introduced more low frequency hum, and going above this introduced more high frequency hum. The trim pot with the cap has a barely noticable effect, as far as i can tell.

I just found Joe Gwinn's comments on calculating the size/winds of a noise cancelling coil, basically area * turns = hum sensitivity, however he also mentions that core material factors in, though doesn't say by how much.

My pickups are a 5k tele pup and 5.75k strat pickup, 42awg wire. Area is approximately 1 inch square, times about 6000 winds, so hum sensitivity not including the factor of core material, is about 6000.

Noise sensing coil is about 6" by 10", or 60 inches squared, and about 200 turns, therefore it's hum sensitivity is about 12000, and being an air coil their is no added core factor.

So Joe, what is the added factor of 6 alnico slugs?

I imagine it's better having a larger than necessary noise sensing coil, since the parallel trim pots are used to bring its hum voltage down to the right level to match that of the pickups.

So it's not perfect (not sure if it's the design or my version of it or both), I'd say the guitar is overall about 80% quieter with no noticeable change in tone whatsoever as far as i can tell, but the level of noise present still fluctuates depending on the position of the guitar; perhaps this is due to the noise sensing coil being on the back of the guitar and could be improved by having the coil on top, surrounding the pickups.... I might try this out.

-Andy

[madzub]

the noise sensing coil, by the way, measured about 125 ohms.

[Peter Naglitsch]

Thanks for sharing. I have a lipstick pickups guitar build coming up (just need to learn to wind them lipsticks, have trashed a few already...) and I was thinking about using a noice canseling coil like this so its great to hear that it works pretty good for you

[David Schwab]

I wound an approximately 8 inch diameter coil of about 200 loops of 34 awg wire, shielded it (very necessary)

Why would you need to shield it? You want it to pick up hum. The original is not shielded.

[big_teee]

So is the coil wired in series?
What was the Patent # of the Shure Device in question?
Terry

[marku52]

wouldn't he mean *electrostatically* shielded? ie, wrapped in copper tape or aluminum foil, otherwise it would be a big addititon to the electrostatic noise proportion. And the equivalizing (is that a word) of the turns X area product implies that what is being cancelled is the magnetic proportion of the noise signal. hence you can "shield" the sense coil and still have it work.

As long as you dont put it in a mu-metal box....

[Dave Kerr]

Hi Terry - patent #7259318

[David King]

I think I've read around here that the Alnico isn't much different from an air coil. I'm also confused by the electrostatic shielding of the dummy unless the pickup is similarly shielded. I'd drop the turns*area of the dummy down to 6000 and try it unshielded.
Would the gauge of the large-area dummy coil need to be any larger than 42 awg?

[madzub]

I'm pretty sure we're dealing with 2 different types of noise here, the electrostatic noise and the 60-cycle hum.... We want the coil to pick up the 60 cycle hum so as to cancel that of the pickup, but the other type of noise is the same stuff that your electronics cavity picks up if it's not shielded, the stuff that goes away when you touch the grounded strings. I do believe the patent mentions shielding the coil also.

Yes it is wired in series... in parallel there would be a serious volume drop, as the resistance of the air coil is only 125 ohms.

Using larger diameter wire for the air coil means less resistance is added to the circuit, therefore it has less effect on the tone of the pickup.... adding 125 ohms of additional resistance to a 6000 ohm pickup is not going to make much of any difference, but if the air coil were wound with 42 gauge wire it would have somewhere around 1000 ohms resistance (just throwing a rough estimate out there), so think of the difference in tone between a 6k and 7k ohm pickup, it's significant. But between 6k and 6.1k, not so much difference. The patent suggests using any wire at least 2 steps thicker than that of the pickup, so if the pickup uses 42 awg, the air coil should use 39 or thicker. In its specific example it uses 200 winds of 36 awg wire in an 8 inch diameter coil, measuring 250 ohms. I think the only reason to not use thicker wire is for size restraints.

I might try tapping the coil halfway, around 100 wraps, and see what difference it makes, but i'm guessing it won't make much difference considering the compensation of the trim pots.

[madzub]

more on shielding: You can wrap a single coil pickup in copper foil and ground it and it will be somewhat quieter than without, but will continue to pick up the 60-cycle hum; shielding will never block that. The electrostatic noise is not phase dependent, as far as i know, so having two coils out of phase will not cancel that type of noise.

When i first wired up my air coil, i had copper tape on only one side of it. There was that electrostatic noise that went away when i touched the strings, making my body part of the shield. I added copper tape to the other side so as to completely wrap the coil in foil, soldered it up, and that noise was no longer there, just like when shielding the electronics cavity.

[Mike Sulzer]

more on shielding: You can wrap a single coil pickup in copper foil and ground it and it will be somewhat quieter than without, but will continue to pick up the 60-cycle hum; shielding will never block that. The electrostatic noise is not phase dependent, as far as i know, so having two coils out of phase will not cancel that type of noise.

When i first wired up my air coil, i had copper tape on only one side of it. There was that electrostatic noise that went away when i touched the strings, making my body part of the shield. I added copper tape to the other side so as to completely wrap the coil in foil, soldered it up, and that noise was no longer there, just like when shielding the electronics cavity.

Both types of hum occur at 60 Hz and harmonics. The hum the loop is designed to cancel is from magnetic fields. The hum that the shield is designed to keep out is from electric fields. Shielding the loop will not stop it from picking up hum from magnetic fields; that is, it will not stop it from doing its job. You cannot cancel electric fields with this set up, only magnetic, and so you might as well make sure that it does not pickup electric fields. But it is pretty low in impedance, and so it might not pick up so much in some cases.

[madzub]

I've made some more adjustments to the guitar (i have some odd wiring going on inside) and it's now even quiter than previously reported, in fact it's nearly dead quiet... of course, the noise level still fluctuates a bit as the guitar changes positions, just as it does normally, but overall the noise is nearly completely gone, and the tone is still bright and transparent as it ought to be. I might try building a new coil and placing it on the top of the guitar, so that it would be in the same plane as the pickups, and i suspect that this would cure the directional fluctuations.

Any idea if eddy currents would be an issue in the hum-sensing coil? I don't imagine it would matter, since eddy currents, afaik, mainly affect the resonant frequency, which, in the case of this low impedance coil, would be well above the hearing range. Any other opinions?

[David King]

I doubt there'll be any eddy currents if the coil is far enough away from the strings and the single coil. I'm guessing that 1/8" or a 1/4", 3-6 mm will suffice.

[madzub]

I wound a new noise sensing coil, this one larger in diameter and sitting on top of the body, surrounding most of the strat body shape minus the upper horns, approximately 12 or 13 inches diameter, with about 70 or so winds (estimated; coil measured about 55 ohms). Works well, i think a bit better than the last one, probably due to it's location on top of the body which is closer to being on the same plane as the pickups.

It cancels more hum for the bridge pickup than the neck pickup, probably because the bridge pickup is located nearly in the center of the noise sensing coil unlike the neck pickup which is out near the edge, but possibly also because it is a Tele-style pickup with the shielded steel baseplate, whereas the neck pickup is an unsheilded Strat-style pickup.

The position of the guitar still has a significant effect on the noise, just like how you can normally position a single coil-equipped guitar in such a way so as to eliminate most of the noise. However, the low frequency hum is completely (or nearly so) cancelled, whereas there is still significant higher frequency hum in most positions. I will try shielding the coil in aluminum (it's currently shielded with copper foil) to see if that might help... ?

One more thing to note with this design: If there are multiple pickups on the guitar with different outputs, the noise-cancelling coil is either going to need to be optimized for one and not the other pickup, or compromised to suit both pickups equally, or else you could maybe devise a more complicated bypass-resistor/capacitor network in order to optimize noise cancellation for both pickups. Much simpler just having equal output pickups.

[Joe Gwinn]

One more thing to note with this design: If there are multiple pickups on the guitar with different outputs, the noise-cancelling coil is either going to need to be optimized for one and not the other pickup, or compromised to suit both pickups equally, or else you could maybe devise a more complicated bypass-resistor/capacitor network in order to optimize noise cancellation for both pickups. Much simpler just having equal output pickups.

Or, one can provide each pickup with its own noise-cancelling coil in series, and then switch pickup+nc-coil as a unit.