Is there a way to passively cut a selectable frequency?

[rjb]

Ok, what do you exactly want to build?
|
this very complete tone control should be converted to 9V single rail and used with TL062.
Or you could ditch the bass/treble stage and leave just the parametric mid ...you should use the first op amp as a buffer anyway:

Some day, I might try building that Rod Elliot circuit (or just the parametric mid section) for use with an Epi Dot.
Guess I'd either have to saw a hole in the back, or mount the EQ in a stompbox.
I can't speak for Daz.

[Fluoroscope 5000]

Easiest/cheapest solution like that is a Wilde ‘Q-filter’ for ~$30 shipped in the states. It works best on a ~4H+ pickup with a low ~200pF capacitance load for a variable peak from 1~6kHz with a dip down to ~700Hz. It acts as normal tone knob from 10~4, and lowering the knob from 4~0 widens the mid-dip while sweeping the peak up. It’s a very useful tool. It can work fine on somewhat lower H pickups down to ~2.5H, but the lowest and highest peaks are higher, so it mainly just softens the upper-mids and adds a very high end sparkle at 0. Higher cable capacitance will lower the variable range, which also lowers the resonance at 10, so it’s more limited. The Q-filter is a specially constructed 1.8H “air” coil of very low impedance for a balanced sound and no audible noise. A cap and resister are included. You can change the ~700Hz peak a bit via the cap value, and adjust the bass level at 0 via the resistor value. It’s a passive LCR network - - no batteries needed.

[rjb]

Easiest/cheapest solution like that is a Wilde ‘Q-filter’ for ~$30 shipped in the states.

How did we forget the Wilde Q-filter? Good application advice there.

But are you sure the inductor is an air coil? I've never seen that mentioned at the Wilde site, and the photo suggests it's wire wrapped around a bolt in a plastic box (like a non-humbucking version of the choke used in the Bill Lawrence- designed Gibson L6-S).

There used to be more explanatory information at the site, but IIRC, there are two basic ways to use the boxed inductor:
1) Replace the cap in your tone circuit with the inductor. Now as you lower the tone control, you add inductance in parallel with the pickup's inductance- thereby raising the resonant frequency.
2) Replace the cap in your tone circuit with the inductor in series with a capacitor. Now you have a single-frequency notch filter with depth adjustment, very similar to the "DIY Varitone" circuit linked in post 2- less the rotary switch. (As noted above, BL does some low-end tuning with a resistor in parallel with the cap).

Here's one wiring diagram.
Switch down: Normal tone control
Switch up: Inductor in series with parallel combination of cap & resistor.


EDIT:
Links to some more Q-Filter info, including a detailed explanation by Fluoroscope:
http://music-electronics-forum.com/t29037/#post257577
Q-filter
Bill Lawrence Website
http://www.gibson.com/Files/schematics/L6-S.PDF

PS: I referred to the Q-Filter as a "non-humbucking version" of the choke used in the Gibson L6-S. It appears that the Q-Filter comprises two shielded inductors wired in humbucking configuration.

[Fluoroscope 5000]

https://www.youtube.com/watch?v=YY6wwAhvirY

Yes. It isn't actually an air coil...my bad. It's a very small amount of wire wrapped around a doughnut-shaped ceramic clamp (called a “pot coil” in certain circles) tightened via the bolt to vastly increase the inductance while retaining very little resistance - - critical to the specific functioning. I believe what happens when wired as a Q-filter is: as the inductance on the high end above the cutoff point of the cap is lowered, the resonance point goes up as the midrange is further reduced. It’s a much beloved little tool. Close to what the OP requested, but not a notch filter.

You can get a sort of notch filter with two pickups in parallel & out-of-phase, and with a cap in series on one of them. I have a guitar wired to adjust the notch from 2.5~4kHz or so by blending one of the pickups via a pot. The mutually canceled harmonics between the pickups are of course then reversed. It’s not a compete notch, but it can be adjusted for sweeter or harder edged sounds. It works well as a simple crossover when in phase. I made a ~15 minute demo just trying to highlight the sound of each position I liked. Check the Details button at upper right: https://drive.google.com/open?id=0Bx...khoYnl3ZHVmSG8

I think I need to switch to a lower gauge of a slightly brighter string type on that guitar for fuller upper-mids with less of a hard click in the attack. The heavier gauge type strings I used are really better for more mellow HB pickup sounds - - where the high end rolls off below ~5kHz.

[bbsailor]

https://www.youtube.com/watch?v=YY6wwAhvirY

Yes. It isn't actually an air coil...my bad. It's a very small amount of wire wrapped around a doughnut-shaped ceramic clamp (called a “pot coil” in certain circles) tightened via the bolt to vastly increase the inductance while retaining very little resistance - - critical to the specific functioning. I believe what happens when wired as a Q-filter is: as the inductance on the high end above the cutoff point of the cap is lowered, the resonance point goes up as the midrange is further reduced. It’s a much beloved little tool. Close to what the OP requested, but not a notch filter.

You can get a sort of notch filter with two pickups in parallel & out-of-phase, and with a cap in series on one of them. I have a guitar wired to adjust the notch from 2.5~4kHz or so by blending one of the pickups via a pot. The mutually canceled harmonics between the pickups are of course then reversed. It’s not a compete notch, but it can be adjusted for sweeter or harder edged sounds. It works well as a simple crossover when in phase. I made a ~15 minute demo just trying to highlight the sound of each position I liked. Check the Details button at upper right: https://drive.google.com/open?id=0Bx...khoYnl3ZHVmSG8

I think I need to switch to a lower gauge of a slightly brighter string type on that guitar for fuller upper-mids with less of a hard click in the attack. The heavier gauge type strings I used are really better for more mellow HB pickup sounds - - where the high end rolls off below ~5kHz.

Look at this MEF post to see how I wired a G&L Telecaster to produce a passive tunable notch effect. See post number 4 on this thread. http://music-electronics-forum.com/t34987/#post326355

Joseph J. Rogowski

[Fluoroscope 5000]

Oh, yes Joseph. I noticed the 2nd harmonics myself, and may do a demo at some point. With the blend knob, I can mix in the fundamental to varying degree for a warmer, more musical sound. It’s actually better than an Octavia in that you can play chords. It works starting at about the 1oth fret up to wherever depending on which two pickups are on. The usable range moves up on the bridge & middle vs neck & middle. It of course depends on how close are the pickup volumes.

[bbsailor]

Oh, yes Joseph. I noticed the 2nd harmonics myself, and may do a demo at some point. With the blend knob, I can mix in the fundamental to varying degree for a warmer, more musical sound. It’s actually better than an Octavia in that you can play chords. It works starting at about the 1oth fret up to wherever depending on which two pickups are on. The usable range moves up on the bridge & middle vs neck & middle. It of course depends on how close are the pickup volumes.

This works best with the two volume pots wired as variable resistors rather than a traditional pot as a parallel connection of two pickups does not allow individual volume control by this traditional wiring. By using stacked concentric 500K for regular Fender type passive pickups and stacked 1M for humbucker pickups the parallel load will be about equal to the traditional pickup loading values. This works very well on a Telecaster with two control mount holes. The 4 position switch provided the three traditional pickup selections plus a series connection. The tone pot has a push/pull function to flip the phase of the neck pickup when either the series or parallel connection of two pickups is selected. The metal case ground of the neck pickup needs to be removed from the coil pickup wires and than the pickup metal case is grounded directly to the guitar wiring ground. Only up to about one tenth of a turn on either pot will create a very easily heard and variable change in notch effects. People who try this without the individual pickup volume control wiring just adjust the pickup heights to change the notch tone when they flip the phase. Stacked concentric volume pots (one for each pickup) in the same hole makes a more flexible adjustment setup.

Joseph J. Rogowski