The PTC ..revisited

[chptunes]

A few years ago (circa 2009-2010), When I first became interested in DIY, one of my first purchases was Craig Anderton's Electronic Projects for Musicians.  Inside of this classic publishing is a very simple circuit [a building block, really].. a 'Passive Tone Control', with selectable Resonant Frequencies and Depth Control.  I tried to build it and use it.. clumsily.  After I figured out how to build it correctly, I moved on to thinking about it's application.  It's a useful and fun 'Notch Filter' in Passive form, but with poor impedance qualities and very low output.



Over the past few weeks, I've revisited the PTC.  I worked up a MOSFET Booster in front, and tagged a JFET Buffer on the end.

After tweaking and listening [and more tweaking], I'm happy with it.



Questions/suggestions/criticisms are welcome.

-Corey

[chptunes]

This prototype endured lots of poking and revising.. there's even a few components on back of the Perfboard.




-Corey

[goldstache]

I'm bread boarding this tonight.  Let ya know what I think.
Thanks!

[goldstache]

Cool. In certain spots it acts like a pickup modeler.  I still have the spst on the center tap as an option.  Trying to determine an implementation. 
Where did you end up?   
Any expansions of the idea chptunes?

[Giglawyer]

Looks very cool...but I too am struggling for an application.  Maybe tie it in with a boost circuit?

[samhay]

Looks like you are missing a cap before the pulldown resistor on the JFET gate.

[Mark Hammer]

I made something a half-dozen years back that I labelled the "Hippy Dippy" (a nod to George Carlin's "Al Sleet, your hippy-dippy weatherman" character).  It was, in fact exactly what is described, except I stuck a Stratoblaster circuit in front of the PTC circuit.  Works great.  The nice thing about coupling the JFET booster and PTC is that the notches can introduce substantial volume drop, and the adjustable gain of the booster compensates.

[electrip]

Looks like you are missing a cap before the pulldown resistor on the JFET gate.

Should be ok, source follower biased at U/2 or whatever else voltage is on drain of the mosfet.

electrip

[samhay]

Looks like you are missing a cap before the pulldown resistor on the JFET gate.

Should be ok, source follower biased at U/2 or whatever else voltage is on drain of the mosfet.

electrip

That's what I thought until I spied R7 to ground.

[electrip]

Looks like you are missing a cap before the pulldown resistor on the JFET gate.

Should be ok, source follower biased at U/2 or whatever else voltage is on drain of the mosfet.

electrip

That's what I thought until I spied R7 to ground.

R7 (1Meg) is obsolete.

electrip

[goldstache]

Been playing guitar through this circuit and dig what it does.  I do find it to be a bit noisy with single coils.  I wanted to get my feet wet with LC filters before I tackle a pultec kit I'm assembling parts for.
Could anybody expound on these concepts to get me going?

[Mark Hammer]

I won't expound on something I don't understand, but I will expound on something I made that worked well.

If you bridge the inductor/choke, then you have a standard guitar tone control - cap through pot to ground - but with the corner frequency varying with the cap in question.  Bridge the cap, instead, leaving the inductor in-circuit, and you have a variable lo-cut.  bridge the cap and inductor, and you have a basic attenuator.  If you have variable gain, that last part isn't much use, but if you connect the common of a 3-position SPDT to the junction between the rotary switch and inductor, one outside lug to point J in Anderton's diagram, and the other outside lug to the 47k resistor that goes to all the caps, you'll have normal PTC in the middle switch position, hi-cut in one outside position, and lo-cut in the other.

That's a lot of added functionality for the price of a toggle.

[goldstache]

Thanks Mark!  That got the brain going!