Dumny coil for PJ bass

[CarlosG]

Hi!
I have old PJ Fernandes bass. It has dimario replicas which sound good. I have full shielding, pickups under cover are shielded too and i grounded polepieces.
P Has two coils so are quiet. But Jazz is single cool and has 50-60hz noise.
I would like to make a dummy coil. Is it best to do it in series or parallel with the jazz?

to get 100% noise removal is it enough that the resistance matches? can I use a guitar single cool, or one P coil, or is it best if it is Jazz? should it fit in the potentiometer chamber

 

[freefrog]

Generally, dummy coils are wired in series with pickups. But you can try parallel wiring if you want.

No, equal DCR doesn't suffice to cancel the noise totally. What is needed for that is the same number of turns on a same total area.

But a decent noise cancellation can be obtained from many coils, actually.

The trick is to use a dummy coil as noisy as the single coil to tame, then to wire / position both RW comparatively to each other.

Dummy coils can be "tuned" to be more or less sensitive to noise, by using more or less turns of wire, core material (slugs) or not and so on.

All that being said, a large noise sensing coil in the Ilitch fashion is something to consider : it requires only a few hundreds of turns, can be wound by hand with thicker wire than a guitar pickup and will have the least impact on the tone thx to its very low DCR / inductance... the only downside it that it's... large.

Possibly useful link (evoking things that I've done myself more than once: it works):

https://www.tdpri.com/threads/diy-large-hum-cancelling-coil.670923/

FWIW. HTH.

 

[Chistopher]

What's the advantage of using a dummy coil over just getting a noiseless J pickup for your application?

 

[Mincer]

What's the advantage of using a dummy coil over just getting a noiseless J pickup for your application?

While I understand that some people might have cost concerns getting specific pickups in their region...finding a dummy coil that 'sort of' works and the time it takes to experiment doesn't sound really great, either. I would probably play as-is and look for a stack design used and try to snag it.

 

[Chistopher]

Yeah, if you're looking at other options too, for $80 a blackouts bass preamp can give you bass boost and cut, treble boost and cut, and it makes your pickups noiseless.

 

[zionstrat]

Yeah, if you're looking at other options too, for $80 a blackouts bass preamp can give you bass boost and cut, treble boost and cut, and it makes your pickups noiseless.

Someone could easily write two books about what I don't know about preamps however, I was surprised to see your comment and I don't understand how a preamp can reject single coil pickup noise.

In my mind, if a single coil picks up noise it's in the signal and all a preamp would do is amplify that.

Can you explain the process.. I wonder if it could be the answer to some difficult noise encounters.

 

[Chistopher]

If you want to do your own deep dive, look up "common mode noise rejection of a differential amplifier"

Basically a differential amplifier has two inputs, and any signal that isn't present in both inputs is amplified. For a guitar circuit, the two inputs are a noisy guitar signal on one input and the second input is just noise. Because the noise is common to both inputs it doesn't get amplified.

 

[zionstrat]

Wow that could open up some doors! There's been many times in the past where I have had to fight hard for analog noise rejection because I apparently missed the boat on this approach.

So how would you wire it? Do you run the pup into one input and leave the other side unconnected as the noise source?

What about the noise signal itself? Different pickups pick up different noise signals depending on how they're wound. What makes the noise side of the circuit see exactly the noise that is captured by the pup?

And could you dig more into the wiring? I once worked on a bass that exactly the problem the original poster referenced. The p base pick up was totally quiet and the J base was unusable in high RF environments.

In that case I'm assuming you would run both pups into the preamp? And I'm assuming the preamp would give you panning between pups just like an active bass?

Thanks again for an interesting thread.

 

[freefrog]

I've no experience at all with the Blackout preamp for bass.

I know of active single coils achieving a low noise thanks to fewer turns of wire in a shielded frame and whose low output signal is then directly boosted by the onboard preamp.

I know of active electronics with op amps whose input and inverting input receive each the signal of a coil, allowing to buck the hum: AKAIK, most EMG's work like that, including SC sized products which are actually stacked humbuckers.
If memory serves me, the preamp of Duncan Blackout humbuckers still applies this idea, albeit in a more refined way (with transistors before the op amp).

I don't see how an op amp could make a single coil noiseless without another coil for that. Maybe I'm too old, too tired or too annoyed by some current issues to get it. I'd be glad and grateful to read an explanation about that.



All that being said, finding a dummy coil that 'sort of' works passively to cancel the hum is actually not that hard IME. I'd even venture to say that any cheap single coil can be used as a dummy coil with some success once deprived of its magnet(s). It alters the tone of single coils, but not more than first generation passive stacks.

IMHO because IME, what is tricky is not to buck the hum: it's to find a way to do that without lowering the output and without altering the resonant frequency (that a crude dummy shifts down, not without creating a kind of unrequested dual resonance. This feature can be noticed with first gen stacks as well and even with advanced noiseless designs, if they are not tweaked properly to cancel capacitive mismatching ).

I know of two ways to obtain efficient hum cancelling + practically unchanged tone and output with existing single coils:

1-a large passive noise sensor with only a few turns, wired in series in the Ilitch fashion: it leaves the DCR and inductance of single coils almost identical, can be paired to its own tone filtering network in order to avoid HF noise and works well enough to be present behind one of my guitars with P90's.
After decades of researches about guitar PU's, someone like Mike Sulzer is still digging this very same idea, BTW. See his posts 4 and 16 in the following topic. It's still a variation on the Ilitch recipe...
https://guitarnuts2.proboards.com/thread/10687/reducing-field-ferrite-core-coils

2-an active noise cancelling circuit pairing a dummy coil to an op amp, as with the Ernie Ball device already evoked by Mincer in other topics here. Below is an excerpt of the related patent:
https://www.talkbass.com/attachments/eb_silent_circuit-png.2032353/


FWIW - Non sensical rambling from an old fart a Sunday morning...

 

[zionstrat]

Dude you're ramblings help my brain incredibly!

Much appreciated!

 

[Mincer]

I always appreciate the ramblings, too, even if I have to read it a few times to 'get' it.

 

[uOpt]

Hi!
I have old PJ Fernandes bass. It has dimario replicas which sound good. I have full shielding, pickups under cover are shielded too and i grounded polepieces.
P Has two coils so are quiet. But Jazz is single cool and has 50-60hz noise.
I would like to make a dummy coil. Is it best to do it in series or parallel with the jazz?

to get 100% noise removal is it enough that the resistance matches? can I use a guitar single cool, or one P coil, or is it best if it is Jazz? should it fit in the potentiometer chamber

You really need to match the existing pickup. Even the shape and dimensions of the coil should be similar.

You won't preserve the sound either in series or parallel with a passive setup. To preserve sound with a dummy coil you need two preamps (or for each coil).

 

[freefrog]

In practice and as I said above, a satisfying noise cancellation can be obtained from various coils.

Some advanced noiseless designs rely on low DCR noise sensors, measuring around 1k or even 400 Ohm only. In such cases, the sensitivity to noise is increased by cores made of magnetically permeable materials (laminated steel for the 400 Ohm noise sensor aforementioned: lamination avoids eddy currents but the use of a totally metallic bobbin makes it way more sensitive to noise with way fewer turns ).

The Ilitch recipes does even better than this on the basis of a simple principle: increasing the area of the dummy coil makes it more sensitive to hum, allowing to use less turns of wire...
Here is the related patent: https://patents.google.com/patent/US7259318B2/en

Albeit a bit "directional" in noise detection, an Ilitch style coil properly wound and wired bucks the hum reasonably well while modifying the resonant peak of less than 1dB. Hence a virtually unchanged tone. And it's easy to build: a few hundreds turns of AWG36 on a wide frame can be wound by hand very quickly... The main challenge is then to find where to put the large coils obtained. They work well behind giant backplates IME. I'm less convinced by rhomboid coils like those that Ilitch sells to be hidden in the electronic cavity of Gibson guitars... YMMV.

EDIT - The rhomboid coils aforementioned are in fact the subject of another patent filed by Dave Ulbrick, and describing the Ilitch system as "prior art". ;-)
https://patents.google.com/patent/US9589553B2/en