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Question relating to electric guitar pickups and capacitance .

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Ash Small

Fri Feb 05 2016, 01:07AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I've been puzzling over this one for ages now

Suppose you have a typical single coil electric guitar pickup, no humbucker or anything just a simple single coil pickup, as used by Fender, for example

Now, suppose you wind it for, say, 10 Henries, using AWG 43 wire, but tap it at every Henry, so you end up with a multi - tapped pickup wired to a ten way switch, for example, so you can change the inductance of the pickup just by turning a knob .

What I want to know is, supposing the switch is set to one Henry inductance, does the capacitance of the unused windings contribute to the capacitance of the coil, or do I only need to consider the capacitance of the windings currently switched into the circuit?

DerAlbi

Fri Feb 05 2016, 03:42AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

First thing crossing my mind: "a tap every Henry". How would you do it?
Lets say youve got an AL-value of 10mH. So you need 10 turns for 1H. For 2H you need only 14 turns. So your Taps would be: 10 turns, 14 turns, 17 turns and so on. Strange: the coil fomred between turn 10 and turn 4 can only have 160mH.
Somehow the model of the coupled inductor is in the way here.

But answering your question concerning the parasitics: it depends on the coupling.
If the coupling is bad, then the parasitic capacitance decreases, because the turn-2-turn capacitance wont do much of the unused turns. You are basically left with a piece of wire thats acting as capacitance against earth - might be bad on its own.. but...
If the coupling is strong, then you wont gain much. The parasitic capacitance is a load to the "transformer" which is reflected to the soruce according to the winding-ratio. So using only half the turns still gives you the full parasitic capacitance.

Ash Small

Fri Feb 05 2016, 04:08AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Thanks for the reply Der Albi, I thought the answer might be along the lines of 'neither one or the other' , I'm just going to have to build a test rig and make some measurements.

Off the top of my head, without checking references, I think these type of pickups require in the region of 6-8 thousand turns for a few Henries, although, as we all know, there is more to it than just the number of turns.

Fender's own pickups can vary from less than two Henries on some more modern pickups to nearly ten on some of the early ones from 65 years ago.

radiotech

Fri Feb 05 2016, 10:09AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

What kind of switch could you design to open and close such a small potential with no
wiping potential, and to work reliably after having been bounced all over the land by a
traveling musician .

Design the switch, to engage multiple windings in series parallel, so each step uses ALL the copper.

At the amplifier end, do the same with an input transformer.

What will happen, is the project will produce a 'voicing' switch.

Subtle as it is, if the coil draws power from the strings, it damps them.

Here is a scheme for maximum copper usage of a winding, from long ago.

1454666939 2463 FT175305 Ne 171c Strapping

Ash Small

Fri Feb 05 2016, 11:52AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

You've some excellent ideas there RT. I did read somewhere that putting a capacitor in parallel with the switch, and maybe a resistor too, would eliminate such problems.

This isn't a new idea, you can already buy something similar. I'm just considering combining it with some other ideas I have.

Apparently the lead from the guitar to the amplifier can have significant capacitance. Would it be better to put the impedance matching network on the guitar side of the lead ?

EDIT: String pull is a fascinating subject in itself

radiotech

Fri Feb 05 2016, 07:12PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

The best solution is to incorporate a preamp into the pickup and phantom-power it
from the amplifier, or from the studio mixer itself. Then lead capacitance vanishes
and the signal can 30dB higher.

See

Ash Small

Fri Feb 05 2016, 08:16PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I don't really want to use active pickups if possible, but I do now have an idea involving a rewound vintage interstage transformer with multiple switchable primaries

Signification

Sat Feb 06 2016, 05:44AM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

I am not familiar with such "pickup coils" at all. But what would happen if you shorted out all the unused windings via the taps above the desired inductance? For example, if you want 4H, short "ALL" the other taps--not just 4-10, but 4-5, 5-6, 6-7, 7-8, 8-9, 9-10?

Or a switch that connects tap 4 to the circuit and completely isolates the rest of the coil at tap 5, leaving the remaining 6H coil floating and/or shorted?

Sigurthr

Sat Feb 06 2016, 06:49AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

Those shorted sections just appear as shorts, loading down the output signal. It's the same as if you had a power transformer and shorted all the windings you didn't use.

@Ash what significance does stray capacitance have on the guitar signal anyway? It's all AF range, so aside from maybe some weird resonances all you'd see are phase changes right? If your amp is voltage driven (or specifically, if the amp's preamp is) what significance does phase changes have on the output.

I like the idea of multiple varied impedance taps, it would be interesting to hear the differences in sound. But I wonder what would happen if you were to include two variable capacitors and a bypass switch into the build? One varicap in parallel, one in series, with the series one able to be bypassed (shorted) with the switch. You should be able to play around with capacitance then to see if there is any noticeable changes in sound. You wouldn't even need to incorporate them into the guitar body, just make an effect box out of them. I can see the series capacitance would need to be hefty in order to not cause a severe high pass effect, but the parallel capacitance could be on the order of what your suspected inter-winding capacitance is, and that wouldn't be large at all.

Ash Small

Sat Feb 06 2016, 02:43PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Sigurther, it's pretty much exactly what you describe as 'wierd resonances'. That's basically all that differentiates the tone or sound of one guitar compared to another.

The tone circuit on an electric guitar uses capacators pretty much the way you describe.

Any LCR circuit will resonate at some frequencies, and tend to block others.

Any guitar pickup has a resonant peak, frequencies around this resonant peak are enhanced, while frequencies above this tend to be blocked. Frequencies below this point just sound like you would expect them to.

I'm trying to keep this as simple as possible.

Stray capacitance or added capacitors will lower the resonant peak.

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