dummy coils?

[Supernautilus]

I love the sound of the Phat Cat in the bridge of my Offset Tele. But the hum is outrageous. Especially lately since I've been exploring more high gain sounds.

I started looking at maybe using a dummy coil to quiet it down, but I have no experience with it. Anybody here use dummy coils? It is worth it to try and make one and wire it in? Or nah?

 

[Mincer]

Sounds like a candidate for the Phat Cat Silencer.

 

[Supernautilus]

Sounds like a candidate for the Phat Cat Silencer.

Ha, I should have guessed someone would mention that one. I probably should have mentioned that I need to go cheap on this one. I’ve made several big purchases lately and I need to chill out for a bit lol. So no new pickups for now.

That’s ok. I have humbuckers I can swap in if I can’t figure out the dummy coil thing. No worries. Just figured I’d ask.

 

[beaubrummels]

You just need the dummy coil to be the same resistance as the main pickup, just like balanced coils in a humbucker. If it's a little off, then you'll get a little hum coming through.

 

[freefrog]

I mount dummy coils of all kinds for decades. They work reasonably well with some pickups. Mostly with those hosting rod magnets (like the specific P90 mounted by Gibson in their Blueshawk guitar, including a dummy P90 for noiseless operation)....

P90's are more difficult to tame: they have a high inductance and the flattened surface of their coil is structurally more sensitive to noise.

The dummy coil used must have the same number of turns than the noisy pickup but also the same "area" [EDIT - Unless a much wider area allows to use less turns... That's the idea behind Ilitch coils].

If one puts a reverse wound P90 in series with a noisy one, it works as a humbucking coil but gives a super high inductance (in the 15H range), making the sound super mid centric / dark.

If one puts the dummy coil in parallel, it's the contrary: the inductance is divided by 4 and the sound becomes thin with less volume. Like a devitalized Fender single coil... One can put some capacitor(s) in parallel with the whole and rise the pickup(s) the closest possible to the strings to compensate these downsides. Some say that's what SRV had in his Strat. But again, it works better with rod magnets...

It's possible to obtain a correct DCR + a correct inductance from a dummy coil in series with the noisy pickup + an inductive/resistive filter in parallel with the whole assembly. I have an experimental circuit like that in a guitar... Dummy coil + filter being enabled or disabled, I measure similar DCR and inductance + similar resonant peak. It works but still with a slight drop of output level and a subtle alteration of sound - P90's sound more like mini-hum's when I enable my experimental silent circuit.

If you can put the widest possible coil under the body or pickguard of the guitar, consider building yourself an Ilitch coil: you can use thick wire for that (38AWG, if memory serves me), making it easy to hand wind on any frame, and albeit it requires more turns than for Fender PU's, it's potentially the most efficient / less tone corrupting solution.

If you use a cheap pickup as a dummy coil, keep in mind my explanations above... Also and counter-intuitively, don't hesitate to put the dummy in series with the pickup on the hot side rather than on the ground side traditionally used for that. It might diminish a bit the tonal alteration noticed.

 

[Supernautilus]

Ok I didn’t know you had to match the DCR with the dummy coil. I was just thinking of using a cheap Tele pickup I had laying around. But I guess that won’t work. Ok, no worries. Thanks for the heads up guys.

 

[freefrog]

Ok I didn’t know you had to match the DCR with the dummy coil. I was just thinking of using a cheap Tele pickup I had laying around. But I guess that won’t work. Ok, no worries. Thanks for the heads up guys.

No, what you have to match is the number of turns if the form factor is the same. It obviously gives a same or similar DCR if the coils are identical and wound with the same gauge but DCR alone means nothing here. Example: for a same number of turns, a Tele neck PU will measure 6.9k if wound with AWG 42 and 8.6k if wound with AWG43...

Buy a cheapo P90 with ceramic magnets. Remove its magnets. Keep its other parts (screw poles contributing to noise sensitivity). Wire it out of phase with the Phat Cat. Put it in your electronic cavity in a position cancelling the noise (flip it upside down if it increases noise at first). It should do a decent humbucking job but with the downsides explained in my previous post.

 

[idsnowdog]

You can easily create a dummy coil with a cheap ceramic single coil. Just remove the magnet and slugs then wrap in aluminum flashing tape.

 

[freefrog]

Dummies are really guitar/pickup dependent, IME... Enough to make me think there's ONE specifically adapted kind of dummy for each situation. they can be mounted on the ground side, on the hot side, in the electronic cavity, in additional routing pools, parallel or perpendicular to pickups, behind the body, in series or parallel...

Sometimes it's useful to check the dual resonant peaks created by dummy coils in series; depending on their inductance and stray capacitance, this dual resonance can comb filter some desirable frequencies or, conversely, promote selectively annoying frequencies, darkening the tone OR giving a drone effect (since a dummy coil can behave like a VariTone beyond the main resonant peak because of its own parasitic capacitance). Increasing or decreasing the inductivity with core materials helps to control this effect, as does the use of external capacitors.

It's also better IME to experiment with magnetic cores in order to find the optimal sensitivity to noise: removing slugs, screw poles and keeper bars is not always a good idea since these parts are what make a dummy coil as noisy as a real pickup and are therefore what ensures noise cancellation OOP if the two coils used have the same properties... for the same reason, wrapping a noisy coil in shielding material can actually be counter-productive.

A fascinating thing is what Les Paul did with his Gold Top... He did put a first dummy between pickup under an extended pickguard, for the bridge PU (this dummy being apparently wound with thicker wire). Then he did enlarge the switch cavity to lodge a 2d dummy coil for the neck PU... But this time, he swapped the neck P90 for a DynaSonic - which seems logical since, as I said, pickups with rod magnets are sonically less altered by noise cancelling bobbins...

Pics below, for the record.

 

[marcello252]

Pics below, for the record.

amazing, in modern days such a chisel work would be considered outrageous.

 

[uOpt]

Dummy coils in a passive guitar or an active guitar with "only" one preamp change the sound dramatically since you combine the two coils' electrical properties.

However, you can avoid that by giving each coil its own preamp and combine them after the preamps.

 

[freefrog]

amazing, in modern days such a chisel work would be considered outrageous.

I've done far worse: the experimental guitar that I use for testing pickups has a hole though its body in bridge position. And a vast routing in the back, to host big air coils (à la Ilitch).

 

[freefrog]

For the record, the last Ilitch coil that I've built and put in series with the ground side of two P90's caused a deviation of 0.5dB only at resonant peak thx to its super low DCR and inductance... And I'm pretty sure this deviation could have been even less noticeable thx to a better choice of the capacitor in parallel with the air coil.

The sensitivity of this coil was due only to its considerable dimensions. It didn't cancel ALL the noise and Ilitch systems are more directional than others. But they do a pretty good job when it comes to noise reduction.

Such big air coils are easy to build : a few turns of thick wire don't take a long time to wind and are not fragile. Their only problem is their size...

For those who'd want a smaller dummy coil with low DCR/inductance but high sensitivity to noise, there's what Kinman did: using a laminated core to avoid eddy currents in an underwound bobbin nevertheless extremely sensitive to noise. I've not yet dissociated such a noise sensor from a Kinman pickup to test it separately / with other SC's but it should be an interesting experiment...

 

[Supernautilus]

Offtopic: I don't know if there any any Hitchhiker's Guide fans here. But I can never hear the word "eddy" and not think of the "eddies in the space time continuum" part lol.