Use of Coil in Tone Control (like Capacitor)

[congo77]

Hi.

I wonder if anyone has tried using coils in tone controls for guitars. I want to be able to cut bass in the same way as I'm cutting treble. My thought is that if I replace the capacitor in the treble control with a coil (inductance) then I would get bass (bass cut) control. Is there anyone who has tried this? Size of coil (mH)? Any info? I use single coil pickups. ( I don't want to use a Capacitor in series, it's the adjustment ability in paralell I'm looking for).

Greetings and Thanks

[UnklMickey]

hi Congo77,


WELCOME TO GN2!



inductors have definitely been used in guitars, and that goes back decades.

gibson had the "Varitone" circuit that used a rotary switch, coil, and 5 different capcitors as a mid-cut circuit. the rotary switch selected which capacitor, was used, to select the frequency the cut was centered on.

they apparently are now using it on the blues-hawk.

the original varitone used a 1.5 Henry coil, the ones in the blues-hawk are using a 7.2 Henry coil.


my guess is the coil you'd want to do a bass-cut would be larger. maybe about ~ 10 Henry.

but that's just a guess.

i'll bet if you ask real nice, JohnH will plug some values into his Pspice program and come up with the optimum value.

unless you have a better idea, you might want to design around a corner frequency of 150~200 Hz.

the lowest note on a standard 6-string guitar is ~82.5 Hz.

one octave above that is ~165 Hz.



unk

[congo77]

Thanks for your reply unklmickey. 10 Henry seems to be a big coil. Where can i find such? Does it have to be aircore or is a ferrite or transformer core OK?

[Mini-Strat_Maine]

Welcome to GN2.
Among those making tone controls with LC or LCR networks (I think that's the teknikul term) are Torres Engineering and Bill Lawrence Designs.
Somewhere around here I've got a Torres PowerBucker harness with the coil on one tone pot, but I dunno if it's marked for value. (And I'm too lazy to go find it and look for markings, plus it's probably Highly Classified Info. ) You can see a pic of the coil on the Torres Super Midrange Kit here: www.torresengineering.com/supmidandton.html Click on the thumbnail for a larger image.

Bill Lawrence (AKA Willi L. Stich) makes the Q-Filter, which I think goes back to his days with Gibson. IIRC, the idea was used on the Gibson L6-S. Those coils are around 900 mH, I believe. This pic won't help much, with the parts all boxed up, but here it is: www.billlawrence.com/Pages/Pickup_Window/Q-Filter.htm. The 1973 schematic for the L6-S (sometimes L-6S) is on the Web, but doesn't show the value of the coil.

So, just how did inductance end up as 'L' and current as 'I', anyway?

[UnklMickey]

congo77 said:

Thanks for your reply unklmickey. 10 Henry seems to be a big coil. Where can i find such? Does it have to be aircore or is a ferrite or transformer core OK?

it does seem large, but the lower the inductance, the higher the frequency for the corner. (the other way to use a lower inductance, would be to lower the impedance. more complicated, don't go there.)

i'm not gonna swear to that value, it's just a guess.

you won't have to ask JohnH, for verification. i sent him a PM, and asked for him to look at this.

but be sure to thank him, if he does have the time to help.


an air-core inductor of that size would be huge, physically.

so it would have to be a ferrite, or more likely a mini-laminated. kinda like an modem or interstage transformer.

you won't need to worry about core saturation, like you would with a crossover for a loudspeaker.

the currents involved here are puny.

ChrisK mentioned a source, way back when, for some inductors:

guitarnuts2.proboards45.com/index.cgi?action=gotopost&board=wiring&thread=1123091479&post=1123726610





unk

[JohnH]

OK - pay attention kids, electronics practical demo coming up!

Below is a test circuit, in its neutral position. On the left we have a humbucker, with a 6H inductance, 12k resistance and some self capacitance. (L1, R1, R2 and C1). On the right is the cable and amp, with a 1nF cable capacitance and 1M input resistance (C2and R5). A nominal 1V signal is feed in, swept from 80Hz to 10kHz.

Our prospective bass control is a 10H inductor, wired with a 1000k resistance, like a tone control in minimum cut position. Such a coil would have some resistance, so I've assumed 10k for that, making 1010k total resistance. No bass cut yet, but you can see the resonant peak and drop off, caused by the pickup interacting with the cable.





Below, it is with max bass cut, leaving just the new coils self resistance of 10k. You can see a general reduction in lower frequencies, and also a lower volume. BTW, the vertical scale, from say 100mV to 1V is 10dB of voltage, double that for power(=volume). Hence we are seeing about a 3dB voltage cut in bass, which is 6dB in volume





Next, for theory, here it is with a perfect inductor with no self resistance (1 ohm). It shows a clearer corner frequency, at about 200Hz. No need to send Unkl back to school.




With all of these, the new coil has big effects on resonance, and also takes out quite alot of overall volume.

A few weeks ago we had another thread on this, using a capacitor circuit as a bass cut. This works at least as well, and is alot smaller to build. It is done with a pot wired as a volume control, with a fixed resistor to ground (here 220k), and a treble bleed cap to let the high frequencies through as the pot turns down the bass.




cheers

John

[UnklMickey]

Cheers indeed!

that Pspice stuff is quite nice.

it's nice to know my s.w.a.g. was somewhere in the ball-park.

i'm gonna take another guess and say that the internal resistance of the coil will be around 500 ohms. but i have much less confidence in that being close.


John, just out of curiosity, does your program have an option for a log scale (dB) for the vertical?



Thanks a ton John!

you know, picture, thousand words and all that.



unk

[JohnH]

Most welcome -

The plots above are log on both scales, which I think is most useful. Its an option though, you can also plot to linear scales in pSpice

John

[ChrisK]

Thanks unk.

I don't have to remember which PC (or city) that I left the info in. The inductance values in my post were derived from the impedance.

Yeah, caps are a whole lot cheaper and smaller.

My bulk order of Fender blend pots came in. These are really 0 Ohms in the center detent position. I got 250K and 500K. These are linear pots on ether side of the detent. No problem, I'd written an Excel "curve a fier" program that lets me add fixed resistors across the end lugs as well as from each end lug to wiper and plots the resultant curve. I can "taper" away.

I ordered them from Fender since I'd measured the blend pot in my Urge II bass and knew it did what I wanted. I never could get StewMac to confirm (in logical exact terms) that their blend pots were such, and not just pan pots (both signals are -3dB in the detent position) which have aggregate equal power curves.

Least I continue to wax verbose, I got these to do real pickup blending, the true 0 Ohm detent positions are critical to my blend pot design that supports parallel as well as series blending (or I need a 3PDT switch).

Also, I'm doing a low pass/high pass tone control in one pot. From the detent to CCW, it's a standard cap to common. From the detent to CW, it's a cap in parallel w/ the pot, in series w/ the output.


And Q.E.D., (unk) there's the Generalized Impedance Converter (GIC). It's the subject of several doctoral thesis' on the web, but it's been in my design vocab since the early 80's. Aboot every 15 to 25 years, things get discovered again for the first time.

A. B. Williams text on filter design give a reasonable treatment thereof. The beauty of a GIC lets one use standard passive filter recipes of various types (Butterworth, Bessel, Elliptical, etc.) and orders, and eliminates the need for actual inductors.

[JohnH]

chrisk said:

Least I continue to wax verbose, I got these to do real pickup blending, the true 0 Ohm detent positions are critical to my blend pot design that supports parallel as well as series blending (or I need a 3PDT switch).
.

Chris - Id be very interested to know your arrangement for series and parallal blending with those pots? Any chance you could post that?

cheers

John

[sumgai]

Doug,

.....
So, just how did inductance end up as 'L' and current as 'I', anyway?

L = Heinrich Lenz, who formulated several laws about electromagnetism during the early and mid- 1800's.

I = Intensity, the amount of electrons flowing through a conductor.

Off-Topic
Many of the names found in electrical/electronic work do not seem to follow any kind of a convention, with the possible exception that more and more, we're seeing the honoring of past inventors, experimentors, and mathemeticians, all fellows who were considered "eccentric" in their day, but were vindicated by history. Still, some original names remain, such as intensity for number of electrons flowing at a given moment.
/off-topic


sumgai

[sumgai]

And before anyone castigates me for appearing to have forgotten.....

Hi, and a big to congo77!

Seems like your question(s) have been copiously answered by capable people, so I'm gonna stay out of this one. ;D


sumgai

[Mini-Strat_Maine]

sumgai said:

L = Heinrich Lenz, who formulated several laws about electromagnetism during the early and mid- 1800's.
I = Intensity, the amount of electrons flowing through a conductor.

Okay, thanks.
I consider a day wasted unless I learn something, and it's a very good day when I learn something without personal injury, property damage, or catching hell for something. ;D

[sumgai]

.....
so I'm gonna stay out of this one.

Nah, can't do it! ;D

Anyone begin to understand yet why I'm advocating 'switched capacitor' filters for tone controls? With the advent of nearly micro-miniature amplifying elements, and extremely low power requirements, it's fast becoming plausible, not just possible, to start sticking such circuitry into our guitars. In short, an ideal replacement for large, and/or impossible-to-find, and/or expensive coils.

If you're not up to speed on this one, and providing that you're intrigued, may I suggest you peruse this PDF file: www.tinaja.com/glib/hackar2.pdf (This is a big file, so it may take a few moments to load. When it's done, scroll down to page 59.)


sumgai

[sumgai]

Doug,

L = Heinrich Lenz, who formulated several laws about electromagnetism during the early and mid- 1800's.
I = Intensity, the amount of electrons flowing through a conductor.

Okay, thanks.

I consider a day wasted unless I learn something, and it's a very good day when I learn something without personal injury, property damage, or catching hell for something. ;D

Hell's Bells, I consider the day an excellent one if I find myself still above ground! ;D

You're welcome.



sumgai

[ChrisK]

'Cept that a GIC ain't switched, but is a continuous (analog) function.

JohnH,

It's actually going into the "Demented TeleBlender" in the schematic arena........when I get to it.

[Mini-Strat_Maine]

sumgai said:

Hell's Bells, I consider the day an excellent one if I find myself still above ground! ;D

Well, yeah, that's a good way to start and end the day.

That PDF is huge. I'm going to try and print out just that section. Maybe I can build one'a them Butterfingers filters.

[UnklMickey]

johnh said:

Most welcome -

The plots above are log on both scales, which I think is most useful. Its an option though, you can also plot to linear scales in pSpice

John

well, duh. you'd think i would have noticed that.

it's marked in volts, rather than dB. but it's similar enough to dB to use as is.


unk

[sumgai]

If anyone else is interested, I'll extract the pertinent portion from that huge file, and post it for reading and/or downloading.

Lemme know.


sumgai

[congo77]

Hi.

I thank you all for your interrest and activity in this matter, I got my question answered. A special thanks to John H for showing what really happens. Thanks.

Congo77