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Post by pablogilberto on Feb 2, 2020 7:17:12 GMT -5
I have seen a lot of people doing some experiments with Dummy Coils.
Basically, the goal is to provide a hum cancelling property on single coil pickups by adding another coil to it.
The idea is similar to how some "Noiseless" SC pickups work.
There are technically two coils but only 1 has a magnet core. The other 1 is a pure coil with no magnet at all.
To get the best hum cancelling result, the two coils should be matched as possible.
That means, same size, wire, number of winds, RLC values, etc.
Similar to HB, as you increase the difference/mismatch between the two coils, the hum cancelling property are reduced.
The dummy coil should be wound in reverse.
I am thinking about this also.
Assuming you have a regular single and you want to put a dummy coil to complement it and reduce/eliminate the 50Hz/60Hz hum/noise, your first goal is to create the closest possible coil (same size, wire, number of winds, RLC values, etc.).
The next thing is connecting it.
You can connect it in series or parallel.
This will surely reduce/eliminate the 50Hz/60Hz hum/noise.
BUT by doing this, you will also create another problem of altering the total RLC values of the SC pickup.
This is like putting a filter.
Thus, changing the overall sound.
If connected in series, the R and L values will be doubled, while the C will be halved.
In theory, this will have an effect shifting down the Resonant Frequency.
If connected in parallel, the R and L values will be halved, while the C will be doubled.
In theory, this will have an effect shifting up the Resonant Frequency.
I'm thinking of a technique which can be employed which will result to hum cancelling properties while retaining the original sound of the pickups.
Any input or suggestions?
Thanks
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Post by reTrEaD on Feb 2, 2020 12:44:52 GMT -5
To get the best hum cancelling result, the two coils should be matched as possible. That means, same size, wire, number of winds, RLC values, etc. Similar to HB, as you increase the difference/mismatch between the two coils, the hum cancelling property are reduced. , I'll agree that the 'frequency response' of the two coils should be as similar as possible. But I think superior results can be obtained if the coils are radically different in terms of RLC. More on this, later. The dummy coil should be wound in reverse. Reverse-wound or reverse-connected. Either will achieve the same goal. guitarnuts2.proboards.com/post/62685/threadI'm thinking of a technique which can be employed which will result to hum cancelling properties while retaining the original sound of the pickups. If the dummy coil has much lower resistance, inductance, and capacitance than the string-sensing coil, it will have very little effect on the tone of the string sensing coil when the two coils are in series with each other. If the dummy coil is more sensitive to external em fields than the string sensing coil, the hum-canceling can be put in balance by partially shunting the dummy coil. (potentiometer in parallel with the dummy) Several years ago, JohnH held a discussion regarding an idea of using a backplate coil that you might find useful. Hum-cancelling with a backplate coil. |
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juster
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Post by juster on Sept 13, 2020 1:25:26 GMT -5
I was thinking of a stereo output, tip is real pickup, the other is dummy. You build a small box with a stereo jack input and mono output. The box contains a 9V battery and a simple mixer circuit. The box has a knob with a slight gain trim for the dummy, maybe $25 for parts. $12 in parts for the guitar, very little work since you are using a pickup with ceramic magnet removed. The guitar works as normal with a mono jack, the dummy is shorted.
The mixer could be anything with well isolated inputs, such as a differential amplifier: The box would look like this, with a stereo tip (which plugs into the guitar):
Normally, you use the guitar as is. If there is a noise problem, 'scuse me while I whip this out.
Pros: No interaction between dummy coil and anything else on the guitar, easy to implement, cheap, noise level can be adjusted to near zero, multiple guitars and one box, you could even have the coil inside/on the box if you don't want to modify the guitar. Best of all, it will work just fine.
Cons: You have a box hanging off your guitar (you could tape it to the guitar strap with a 1' cable), batteries needed.
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juster
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Post by juster on Sept 13, 2020 12:30:24 GMT -5
I ordered a swamp ash telecaster yesterday, and I was thinking of routing room for a large dummy coil under the bridge. But that would involve digging holes in my new guitar, and the final product might even be unsatisfactory. But a stereo output, I could have a small dummy coil made from a pickup on one amp channel and the pickups on the other. That seemed easier. The old Vocal Canceller circuit in a plugin box like an Orange Squeezer would give me the functionality of an Alembic active guitar with very little trouble. So this circuit would work fine, a switch to disconnect the right (dummy) output would act as bypass for humbucking settings. The vocal canceller removes any signal that is the same on both channels, that's what we want. You could double up the values of R1/R2 and R3/R4/R5 for higher input impedance. RV1 could be preset, or a knob on the box. A switched output socket would provide a battery on/off when not in use. So that is an easy way to have a dummy coil with no routing that has no effect on guitar tone. Although it isn't the usual approach, and won't suit everyone.  |
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Post by ashcatlt on Sept 13, 2020 13:37:37 GMT -5
It would also be nice to have the dummy coil in as close to the same place as the real coil. That’s why HBs tend to work a little better at humcancelling than, say, the combination settings on a Tele or Strat, and why those noiseless pickups usually have the dummy coil on the same bobbin as the sensing coil.
With the differential circuits depicted in this thread, it’s tough to get acceptable noise specs with high enough input impedance for a passive guitar. You’d be best off putting a buffer on each coil, and then going into this diffamp. Note that since this is a diffamp, you would actually not want the dummy coil inverted with respect to the sensing coil because 1 - (-1) = 2.
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juster
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Post by juster on Sept 13, 2020 19:42:33 GMT -5
Agreed, a JFET buffer on each input would help. I'm open to suggestions for improved electronics.
Edit: I just realized that I could rout a scratchplate, cutting about 1/4" of the edge, glue a large 1/4" coil around the edge, and spraypaint the whole thing. Scratchplates are cheap, and the coil would be large enough to require fewer turns. If you had a body in bad shape, you could rout a groove round around the body, stick in a huge coil, add filler and spray paint the thing. The hugeness of the coil would give you a low enough resistance to not change the tone much.
Comment: A lot of people have a buffer or boost pedal at the start of the chain. An active dummy coil box would replace this, so it wouldn't add much noise.
I might just connect the outputs of 2 Tillman JFET preamps to see if that would work as the entire low noise/hi impedance mixer circuit.
Edit again: (Telecaster specific) For the active dummy coil I can use a coin-sized 3V cr2032 battery, which would fit in the rout for the neck pickup wiring. I would cut a slot in the pickguard so I could replace the battery without unscrewing anything. I can have a "defeat everything" switch beside it. The dummy coil and electronics can be attached under the pickguard, so I just swap back the original pickguard if I don't like it. So a battery compartment without routing.
I can also mount a large 0.20mm wire coil on top of the guitar body with no routing just to see if I like it, the wire is $5. Maybe $25 for the active scratchplate including a coil from a pickup.
Edit even more:
I will 3d print a plate that has a circular groove to hold the large dummy coil and will attach to the Telecaster strap button and to the bridge. Because the Tele has no room internally. This will snap on to the body, and the wires will go into the bridge pickup cavity and thence to the wiring with a push/pull for in series/shorted out. The plate will act as a winding bobbin. Once I have the plate printed, maybe 2 hours work to wind and wire up the coil. |
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Post by JohnH on Sept 14, 2020 15:57:07 GMT -5
Interesting ideas! Out of various concepts that I've seen, I like the idea of the large backplate (see Iltich backpalte) or scratch plate coil best, as a purely passive idea. The large coil area leads to far fewer turns, less wire and hence low added impedances for minimal tone change. Essential: Dummy coils must be in as near as possible the same place and on the same axis as the main coils. It doesn't take much unbalanced hum to become such that it wasn't worth bothering to do it at all. If you are using pickup-sized dummies, then yes some active electronics will help, to avoid tone loss. For jfet buffers, id suggest a source follower type, which have x1 gain rather than Tillman which is a common source and amplifies. This thread might help: guitarnuts2.proboards.com/thread/3150/jfet-buffer-cable. After buffering, the differential amplifier seems like the right circuit rather than a simple mixer. Unless you build the whole shebang into the guitar, I quite like the idea of just putting in the dummy coil and taking it passively to the ring terminal on a TS jack. You can plug in a normal cable and its just like a normal circuit, the dummy is shorted and not involved. But then use a stereo cable into your floor box to balance and mix the real and dummy coils. Its a good way at least for a prototype since you can mess around with the box and tweak it until its right without having to keep opening up the guitar. |
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Post by newey on Sept 15, 2020 14:30:42 GMT -5
Edit: I just realized that I could rout a scratchplate, cutting about 1/4" of the edge, glue a large 1/4" coil around the edge, and spraypaint the whole thing. Scratchplates are cheap, and the coil would be large enough to require fewer turns. If you had a body in bad shape, you could rout a groove round around the body, stick in a huge coil, add filler and spray paint the thing. The hugeness of the coil would give you a low enough resistance to not change the tone much. Ilitch has all three types- backplates, pickguards, or a version to be permanently routed into the guitar under the pickguard. They also have their systems for Teles, Jazz basses, and other guitars with backplates. www.ilitchelectronics.com/ |
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Post by antigua on Oct 12, 2020 14:50:21 GMT -5
It would also be nice to have the dummy coil in as close to the same place as the real coil. That’s why HBs tend to work a little better at humcancelling than, say, the combination settings on a Tele or Strat, and why those noiseless pickups usually have the dummy coil on the same bobbin as the sensing coil. Is a dedicated HB really better and noise cancelling than RW/RP? Another cause could be that pickups often have mismatched specs these days. I think the noise source would have to be in very close proximity to the guitar for a few inches to make a big difference. The thing about stacks, I'm sure that's more about the convenience of having the primary and the dummy in a neat little package that's easy to install. Having the dummy co-axial with the primary is not so great. |
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Post by ms on Nov 8, 2020 15:50:02 GMT -5
The method I came up with several years ago is:
1. Make the dummy as near to identical to the pickup coil as possible. (This makes it easy.)
2. Make the dummy active. It feeds a FET source follower which can run on low current because it only has to swing the hum voltage, not the signal. (Although it does have to swing the signal current, but that is small for a H Z pickup.)
3. Connect the ground lead of the pickup to the source of the FET (AC coupled) instead of ground. It sees a very low impedance to ground.
So the voltage on the source is the hum voltage from the dummy coil, and it appears in series with the pickup and so it cancels the pickup hum. I forget the details, but it works fine and does not affect the sound from the pickup to a any significant degree. You do need a battery, but life is long since the current can be low. I forget how low.
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