Dummy Coils - Theory help (then maybe application)

Lester

Senior Member
Joined
Mar 18, 2007
Messages
1,003
Reaction score
488
Starting on the theory side...

I thought I understood how a humbucker worked... two coils, one reverse would, back to back. Hum generated by both coils with opposite hum/sine waves, sine waves cancel each other (always wondered why this didn't cause the canceling of other frequencies that I want, like the notes I play, but that's a side question/tangent).

I also know that you can have the same arrangement with two single coils and one RWRP forming an extended humbucker on a two-pickup, single coils guitar.

My issue: I thought that in either case the magnetic coils needed to be in physical alignment for it to work. Same plane, rough physical proximity, just one coil RWRP. But, as I've started to look into using a dummy coil (no magnets or core), I've seen them stuffed in all sort of locations with little concern about physical placement/alignment.

So, first question: Does the humbucking effect work simply by having the RWRP in the circuit? Would it work the same (as an example) if I stuck the dummy coil in a non-metal box with a couple jacks and plugged into the box from the guitar and then went to the amp?

Second question: Let's assume you have a humbucker or two single coils (one RWRP) already doing humbucking. You put a dummy coil in the line. Does the dummy now re-generate hum that your humbuckers filtered out?
 

ErictheRed

V.I.P. Member
Joined
Dec 8, 2009
Messages
6,783
Reaction score
8,888
Just real quick to answer your side question:
The induced electromagnetic field due to the movement of the strings depends on both the magnet polarity AND the direction of the windings. This is obvious, because the magnets and windings work together to create the pickup. By making the magnets 180 degrees out of phase AND the windings 180 degrees, you are back in phase! Hence there is no cancellation of the signal from the strings (actually there is a small amount because nothing is perfectly in/out of phase, but you get the idea). The string signals from each coil add up constructively.

On the other hand, the direction of induced current that is noise is ONLY dependent on the direction of the windings. The magnet is not in play here at all, your external noise sources are not being generated by the magnet, they have their own other sources. Therefore, since the coil windings are 180 degrees out of phase, all of the extraneous noise is cancelled (or most of it is). The noise signals add up destructively (interfere with each other to cancel out).
 
Last edited:

Lester

Senior Member
Joined
Mar 18, 2007
Messages
1,003
Reaction score
488
Just real quick to answer your side question (I don't have much time):
The induced electromagnetic field due to the movement of the strings depends on both the magnet polarity AND the direction of the windings. This is obvious, because the magnets and windings work together to create the pickup. By making the magnets 180 degrees out of phase AND the windings 180 degrees, you are back in phase! Hence there is no cancellation of the signal from the strings (actually there is a very small amount because nothing is perfectly in/out of phase, but you get the idea). The string signals from each coil add up constructively.

On the other hand, the direction of induced current that is noise is ONLY dependent on the direction of the windings. The magnet is not at play here at all, your external noise sources are not being generated by the magnet, they have their own other sources. Therefore, since the coil windings are 180% out of phase, all of the extraneous noise is cancelled (or most of it is). The noise signals add up destructively (interfere with each other to cancel out).
Thanks, makes sense.

On to the core questions.
 

ErictheRed

V.I.P. Member
Joined
Dec 8, 2009
Messages
6,783
Reaction score
8,888
I don't understand the way that your other questions are phrased very well. Yes, you could have a second dummy coil that has no magnet in it that would provide him canceling, but if that coil is mounted in your guitar then it's a waste of space and you'd might as well use it with a magnet to get more of an output. I suppose that you could somehow pry the magnets out of the middle pickup in a Strat for instance, to see if the in-between positions would still him cancel. They should, though you'll have less output.

The noise cancellation is less dependent on physical geometry and proximity. The physical geometry and proximity to each other in a regular humbucker is so that you get more usable output in your signal. You could theoretically put other dummy coils in your signal chain somewhere that would also cancel some noise, but nothing is free and you'll be adding inductance and will have issues with resonance and the like, without the benefit of adding more actual signal. So you might cancel some lower frequency noise and then pick up more higher frequency noise. You'll lose some high end due to the additional inductance in the cable that a big coil of wire produces. Think of this as "pointless inductance" in a way, as it's not helping to actually produce a signal at all, such as a winding in a pickup is.

The humbucker windings and geometrical arrangement maximize output and signal to noise ratio, while also providing him canceling. The dummy coil takes away highs, doesn't help with more output, lowers your signal to noise ratio, possibly introduces unwanted resonance, but also yes, should hum cancel as well.
 

Roger66

Senior Member
Joined
Mar 4, 2021
Messages
214
Reaction score
129
Hi. I have a walnut elite 83/84 strat which came stock with a dummy coil between the bridge pickup and the bridge. The 3rd pot was devoted to just turning up and down the dummy coil. I noticed almost no difference in hum, bit it did sound slightly fatter when the coil was turned all the way up. Well, I was not impressed at all with the actives in it, tone-wise. Nor was I that happy about the battery cover plate on the back when they redesigned the trem to not have to have a back plate to save the pretty wood. The locking nut was non-invasive, but silly and unneeded IMO. So, I tore out all of the electronics, and the pickups (I still have) and dummy coil out, took the locking/fine tuner off, made a custom pickguard out of 3 plies of walnut veneer, and set it up like a mustang with 2-SD quarter pounders and a 3 way switch for them. I was young. I dont know why.
What I ended up with was fantastic! MUCH better than all the active and dummy coil nonsense. I dont think fender used a dummy coil again. I could be wrong, but I see no use for one. I appreciate that it is clever in theory, but I personally find them unnecessary, like I think fender did, and everyone who gets a tone that is just fine without them. I'm not gonna go as far as saying they are a fad or gimmick, but oops, I just did! I do think that they are aptly named. Incidentally, the wood pickguard really improved the tone, I think.
When you tapped on it with a pick, it made a much richer sound through the amp. Tapping with a pick on a plastic pickguard, it makes a quite.....'plastic' sound through the amp. Also, when I tore the guts (I still have) out of my Eric Clapton 'Blackie' to create a David Gilmour 'Blackie' the active circuit board inside was just like my walnut 83/84 elite, but, yup, without a dummy coil.
I hope this helps.
 

Lester

Senior Member
Joined
Mar 18, 2007
Messages
1,003
Reaction score
488
I don't understand the way that your other questions are phrased very well. Yes, you could have a second dummy coil that has no magnet in it that would provide him canceling, but if that coil is mounted in your guitar then it's a waste of space and you'd might as well use it with a magnet to get more of an output. I suppose that you could somehow pry the magnets out of the middle pickup in a Strat for instance, to see if the in-between positions would still him cancel. They should, though you'll have less output.

The noise cancellation is less dependent on physical geometry and proximity. The physical geometry and proximity to each other in a regular humbucker is so that you get more usable output in your signal. You could theoretically put other dummy coils in your signal chain somewhere that would also cancel some noise, but nothing is free and you'll be adding inductance and will have issues with resonance and the like, without the benefit of adding more actual signal. So you might cancel some lower frequency noise and then pick up more higher frequency noise. You'll lose some high end due to the additional inductance in the cable that a big coil of wire produces. Think of this as "pointless inductance" in a way, as it's not helping to actually produce a signal at all, such as a winding in a pickup is.

The humbucker windings and geometrical arrangement maximize output and signal to noise ratio, while also providing him canceling. The dummy coil takes away highs, doesn't help with more output, lowers your signal to noise ratio, possibly introduces unwanted resonance, but also yes, should hum cancel as well.
@ErictheRed : Yes... all your assumptions match/confirm with what I've read. In this end-application I'm looking at, this would be for noise reduction with single coils used alone. Known tempering of the top end (probably not an issue for me). Also, yes, people doing this ARE removing the magnets & core and using it as a coil only. And thanks for the info on the advantages inherent in the humbucker close proximity coil. That makes me feel better about my earlier understanding of a humbucker.
 

ErictheRed

V.I.P. Member
Joined
Dec 8, 2009
Messages
6,783
Reaction score
8,888
Hi. I have a walnut elite 83/84 strat which came stock with a dummy coil between the bridge pickup and the bridge. The 3rd pot was devoted to just turning up and down the dummy coil. I noticed almost no difference in hum, bit it did sound slightly fatter when the coil was turned all the way up. Well, I was not impressed at all with the actives in it, tone-wise. Nor was I that happy about the battery cover plate on the back when they redesigned the trem to not have to have a back plate to save the pretty wood. The locking nut was non-invasive, but silly and unneeded IMO. So, I tore out all of the electronics, and the pickups (I still have) and dummy coil out, took the locking/fine tuner off, made a custom pickguard out of 3 plies of walnut veneer, and set it up like a mustang with 2-SD quarter pounders and a 3 way switch for them. I was young. I dont know why.
What I ended up with was fantastic! MUCH better than all the active and dummy coil nonsense. I dont think fender used a dummy coil again. I could be wrong, but I see no use for one. I appreciate that it is clever in theory, but I personally find them unnecessary, like I think fender did, and everyone who gets a tone that is just fine without them. I'm not gonna go as far as saying they are a fad or gimmick, but oops, I just did! I do think that they are aptly named. Incidentally, the wood pickguard really improved the tone, I think.
When you tapped on it with a pick, it made a much richer sound through the amp. Tapping with a pick on a plastic pickguard, it makes a quite.....'plastic' sound through the amp. Also, when I tore the guts (I still have) out of my Eric Clapton 'Blackie' to create a David Gilmour 'Blackie' the active circuit board inside was just like my walnut 83/84 elite, but, yup, without a dummy coil.
I hope this helps.
Active circuits will work differently than passive ones, they have their own powered buffers and filters and sometimes preamps and on and on.
 

freefrog

Senior Member
Joined
May 21, 2011
Messages
1,515
Reaction score
926
@ErictheRed Known tempering of the top end (probably not an issue for me).
For the record, a (series) dummy coil will also lower the output level a bit, because it doesn't add only "useless inductance" but also useless resistance - if it's a standard dummy coil with the same specs than a regular PU, of course.

Also: I've used old pickups as dummy coils and/or wound my own coils for that. On the basis of this personal experience, I'd say that the most interesting and easiest to build are "Ilitch" coils: I've wound a bunch of these and they work well, at least with Fender style single coils.

Not enough time for more explanations so, do a Google search: there's detailed threads about that on various forums.

Good luck in your experiments...
 

Roger66

Senior Member
Joined
Mar 4, 2021
Messages
214
Reaction score
129
Active circuits will work differently than passive ones, they have their own powered buffers and filters and sometimes preamps and on and on.
Good point! I did not take that into consideration. I have always had an active dislike for guitars that need a 9V battery. It's just the principle, I guess. I guess I take the 'tube only' thing to the extreme. I have a tube tape echo. A tube reverb, and tubes in my English Muffin, though I rather suspect there are some transistors in it, like my Electric Mistress. I have tube hifi's as well. I build, fix and mod tube amps. So I am totally biased towards tubes. There are some transistors that do sneak in, though. As for the dummy coils, I have never had a guitar with so much hum, even through 5 gain stages, that I needed a dummy coil. Those who do might want a dummy coil, should make sure that the shielding is adequate. A kill switch also comes in handy if it hums right before and after you play a song. And turn off the beer lights yourself when you play at a bar.
I am not peeing on dummy coils, and their users. I am just glad I never needed one. Also, going out of phase helps a lot. I dont think dummy coils help the tone any, but I could be completely wrong about everything I say. That is always the case. I like it when someone points out I am wrong, because I just learned something.
 

ErictheRed

V.I.P. Member
Joined
Dec 8, 2009
Messages
6,783
Reaction score
8,888
Wow I thought that this was just a curious idea that Lester had, I had no idea that people actually do this! If you're having serious noise problems you can shield your guitar, use an isolated power supply, a noise gate, ferrite bead, etc. This would be about the last thing that I would think to try!
 

Roger66

Senior Member
Joined
Mar 4, 2021
Messages
214
Reaction score
129
Wow I thought that this was just a curious idea that Lester had, I had no idea that people actually do this! If you're having serious noise problems you can shield your guitar, use an isolated power supply, a noise gate, ferrite bead, etc. This would be about the last thing that I would think to try!
That's what I was saying! I see no point at all. They dont help tone any.
 

Roger66

Senior Member
Joined
Mar 4, 2021
Messages
214
Reaction score
129
That's what I was saying! I see no point at all. They dont help tone any.
Its a band-aid solution. It treats the symptoms, not the disease. And it has unwanted side effects, but I can see that sometimes it is a last resort. I heard of a trick where on the tube socket, you put a tiny wire on the plate, and wrap it a few times around another pin wire, leaving an air space. I forget which pin wire you wrap it around. Maybe the filament wire or the B+ voltage. It's out there, somewhere. It's a trial and error process. When I was young, I used that trick with some success on an old Marshall without a master volume on the sockets I added for 2 more 12AX7's to get 4 more gain stages. It brought down the hum a bit.
I also grafted a Hammond reverb out of an old organ into it and put the tank at the top in the same amp. I still have it but it likes to blow the auxillary filament transformer I added to handle the extra tubes. Last time I blew it, I havent gotten around to fixing it. Which brings up the point that switching the filaments to DC like Marshalls instead of AC like Fenders can result in less hum. With AC, the filament wires should be tightly twisted together between tube sockets and must not touch other wires in the amp. The tightly twisted AC filament wires also hum cancel.
Theres lots of stuff that cause hum. Especially in high gain amps. Any sort of neon lights, old tv's, fans, pumps, etc can make them hum.
Sometimes the last resort IS a dummy coil. Gotta go.
 

Lester

Senior Member
Joined
Mar 18, 2007
Messages
1,003
Reaction score
488
Well, for me this is on my "mod platform" guitar. It's experimental. I've heard it can cut hum. I've heard it can also cut tone and/or reduce output. But I have a spare coil / pickup that has no real value, so I thought I might try it and see what happens. Just poking around the concept for now. However, I needed a little more info to understand it.

Also, FYI, GIbson apparently added a dummy coil to the BluesHawk guitar (also along with a Varitone type setup). It came into play with either single coil selected. So, it has some history. But info is limited.

So, moving on to the next question on the theoretical side (and once again, ignoring possible downsides):

If you had a humbucker and you added ONE dummy coil, you'd be adding back the noise, correct? The dummy would actually generate noise?
 

Roger66

Senior Member
Joined
Mar 4, 2021
Messages
214
Reaction score
129
Yeah, you would need 2 dummy coils for that. This is turning into a very amusing thread.
 

Roger66

Senior Member
Joined
Mar 4, 2021
Messages
214
Reaction score
129
You are correct. You would actually need 2 dummy coils.
Back in the day, Amps didn't have that much gain. Mine don't have preamp volumes.
I submit that vintage guitars from the day didn't need to be shielded as well. Maybe. If you totally shielded a guitar, I wont hum with single coils. Using a dummy coil with a humbucker is a waste of time because they are HUM BUCKERS already. I really don't know why dummy coils are a big deal. Another way to cancel hum, which is actually fun is to have an out of phase option. If you run 2 single coils out of phase, it bucks the hum as well. Like if you wired the middle pickup out of phase on strat, say, bridge and middle, or neck and middle, would buck hum and any pickup by itself would sound the same as it always had. I love out of phase!
Well that's all for now, and I really feel for you guys who have too much hum. Oh and if your amp hums by iitself, no dummy coil will fix it. I think that more shielding cannot hurt, and a killswitch, maybe a pullpot to do that. Also a noise gate is a good idea as well. I know nothing about a ferrite bead, but i ca imagine how it could help. The last guy mentioned that. He had some great points.
 

Lester

Senior Member
Joined
Mar 18, 2007
Messages
1,003
Reaction score
488
Thanks. I'm not planning on running one with the humbucker, just looking to cement my understanding of the electronics.

I do have the phase option already, so I can humbuck the two singles as a pair. The only noise is when running one of the pickups in single coil mode. That basically what the BluesHawk did - dummy engaged with either single coil option and disengaged for the pair (apparently RWRP on one) using a couple extra poles on the pickup switch.

I already have fully grounded shielding of the entire control cavity. It might be helping but it's not stopping it. Might be my proximity to relatively high voltage power company lines. I'll be experimenting with that too. But, even if you shield, the pickups are still mostly exposed.
 

freefrog

Senior Member
Joined
May 21, 2011
Messages
1,515
Reaction score
926
That's what I was saying! I see no point at all. They dont help tone any.
1) A dummy coil with regular specs (like a pickup without mag(s)) has indeed a negative impact on tone.

But it can be compensated. For instance, the pickups of the Blueshawk were not normal P90's. they were P90's with AlNiCo rods, giving them a lower inductance and more "touch sensitivity" thx to their stronger magnetism. These specific pickups in serieswith the dummy coil work(ed) well.

With normal P90's, some high resistance pots would also have diminished to some extent the negative impact of the dummy coil on tone.

I'd even say that the weaknesses of dummy coils can be changed in tone shaping tools...

For example, the "normal" dummy coil that I've in a Strat has been selected for its specs: once in series with regular Strat single coils, it gives them the resistance & inductance of first generation / old school noiseless stacks (like the DiMarzio HS2). That's what I use when I want a "Swedish shredder" tone from a Strat... and without noise. :)

The dummy P90 that I've in a Hamer with P90's has a different function : I've added a LR network in parallel to the harness, in order to obtain the specs of minihumbuckers when the dummy coil is enabled in series with the pickups. So, once this circuit involved, my P90's are not only less noisy BUT also close (tonally and volume wise) to the mini-hum's that I've in another guitar.

IOW, I've took advantage of the limitations of dummy coils.... and it works well for me. YMMV.

2)Albeit it doesn't work so well with P90's, an "Ilitch" air coil has almost no influence on tone with Fender style PU's... Reason: it has a tiny DCR & inductance since it relies on a wider area to be "noise sensitive"...

Such air coils are expensive if bought directly to Ilitch (or as a "Suhr BPSSC")... but as I said, they are easy to build...and to stuff in Fender style guitars or in any axe with a wide enough pickguard / backplate.

Here is a topic about that: https://www.tdpri.com/threads/diy-large-hum-cancelling-coil.670923/

I don't know this guy... but I've done the same kind of things and his topic avoids me long explanations.

3) Currently, I've here guitars with regular SC's, noiseless "fake" single coils, real single coil + small dummy coils & real single coils + "Ilitch air coil (large hum cancelling device)...

My main strat hosts noiseless pickups. My second Strat has the dummy coil described above.

Why? Because this "band aid" solution efffectively helps us when we play on stage next to neon lights, for example... :cool:

It's probably for similar reasons than dummy coils have apparently been used by a few folks a bit less anonymous than me as reported below...

https://www.thegearpage.net/board/index.php?threads/srv-seen-this.2067224/page-3#post-28992997

https://www.premierguitar.com/diy/mod-garage-ritchie-blackmore-stratocaster-mods

All that being said as a non argumentative testimonial: I'm just summing up a personal experience in order to explain how and why I use dummy coils... and if this sum up allows to share some ideas useful for some other musician(s), that's fine.

FWIW. De gustibus et de coloribus non est disputandum. I wish you all a nice day.
 
Last edited:

Roger66

Senior Member
Joined
Mar 4, 2021
Messages
214
Reaction score
129
Tree Frog has explained some really good uses for dummy coils, Which I hadn't even thought of. Absolutely, when a neon light is nearby, you will need one, and when other EM interference is present. Tree Frog obviously knows his s--t ! He has tricks to compensate for the admitted tone loss which I can dig.
I stand corrected. Dummy coils ARE useful for unfriendly EM environments.
A lot of us don't know what TF knows. I usually go out of phase or just use a guitar with humbuckers. Tree Frog has managed to keep his single coil tone in high gain
Unfriendly EMF conditions.
I love it when I'm wrong, thats how I learn new stuff.
Thanks Tree Frog! Now to uncover your true Identity, (maniacal laughter)
 

Roger66

Senior Member
Joined
Mar 4, 2021
Messages
214
Reaction score
129
Starting on the theory side...

I thought I understood how a humbucker worked... two coils, one reverse would, back to back. Hum generated by both coils with opposite hum/sine waves, sine waves cancel each other (always wondered why this didn't cause the canceling of other frequencies that I want, like the notes I play, but that's a side question/tangent).

I also know that you can have the same arrangement with two single coils and one RWRP forming an extended humbucker on a two-pickup, single coils guitar.

My issue: I thought that in either case the magnetic coils needed to be in physical alignment for it to work. Same plane, rough physical proximity, just one coil RWRP. But, as I've started to look into using a dummy coil (no magnets or core), I've seen them stuffed in all sort of locations with little concern about physical placement/alignment.

So, first question: Does the humbucking effect work simply by having the RWRP in the circuit? Would it work the same (as an example) if I stuck the dummy coil in a non-metal box with a couple jacks and plugged into the box from the guitar and then went to the amp?

Second question: Let's assume you have a humbucker or two single coils (one RWRP) already doing humbucking. You put a dummy coil in the line. Does the dummy now re-generate hum that your humbuckers filtered out?
Answer to Q1: I dont see why not if you use only.one single coil at a time, but I would see a big problem, when 2 or 3 single pickups are used, you might have to use 2 or 3 dummy coils. Ok, I see why not now.
Q2: yes it would be a nightmare. Its like you used a really long unshielded cord. It would add hum big time.
 


Latest Threads



Top