Baxandall with variable mid Scoop frequency

[Vivek]

To  Rob, Phoenix, PRR, Teemuk, AMZ, RG, Hammer and so many other Gurus here

Thank you so much for your posts !!!!

My heart is full of gratitude.

[Rob Strand]

FWIW, there was actually a non-inverting tone control which was more like an active form of the James circuit.  IIRC, it only works when the "flat" gain of the stage is high.   It was in a National Semiconductor datasheet or applications note.    Perhaps a power-amp datasheet.

[Vivek]

So now I understand as follows :

2 caps near Treble pot (connected to end lugs) = James
1 cap near Treble pot (connected to Wiper)  = Baxandall


The version with one bass cap, Is that a James or a Baxandall ?

[Phoenix]

So now I understand as follows :

2 caps near Treble pot (connected to end lugs) = James
1 cap near Treble pot (connected to Wiper)  = Baxandall


The version with one bass cap, Is that a James or a Baxandall ?

That's just a variation on the bass control, but it's topologically essentially the same, and applies to both James and Baxandall. The two bass control variations provide subtly different frequency responses in intermediate control settings (outside of 0/50/100% settings). The fundamental difference between James and Bax is the topologically different treble control.

[Vivek]

So James can have a "Shift" resistor but Baxandall cannot ?

[Phoenix]

So James can have a "Shift" resistor but Baxandall cannot ?

Exactly.

[Vivek]

Therefore it appears I cannot easily modify the James or Baxandall to have a mid scoop of variable scoop frequency a la BLACKSTAR ISF

[Phoenix]

Therefore it appears I cannot easily modify the James or Baxandall to have a mid scoop of variable scoop frequency a la BLACKSTAR ISF

Did you miss these?

Ditch Baxandall, use James, replace R4 with pot wired as variable resistor. Put in opamp feedback loop if you want it to be active.
http://www.guitarscience.net/tsc/james.htm

This does exactly what you requested:

I wish to modify the active Baxandall such that when a mid scoop is created by boosting the Bass and treble, I have a method to change the position of the scoop from maybe 200 Hz to maybe 1.2KHz.

Just to make sure that I'm understanding correctly, the James circuit is passive—so at 12:00 on the t and b knobs, you get flat frequency response but it's at -15 or so dB?

Seems people have missed the idea of putting James in a feedback loop. No insertion loss, same as active vs passive baxandall.
Example circuit has mid-sweep between about 200Hz and 1.1kHz, per request.





Reverse log pot can be substituted for more common log pot if you flip it and you're happy with counterclockwise being center frequency high, and clockwise being center frequency low (which makes some amount of intuitive sense, as then counterclockwise increases bass content, clockwise increasing treble content).
R10 restricts the range of the mid frequency pot to give the desired range.

EDIT:
This plot may make it more obvious (bass and treble controls set to 100%, mid-shift pot swept from 0-50-100%

[marcelomd]

That's just a variation on the bass control, but it's topologically essentially the same, and applies to both James and Baxandall. The two bass control variations provide subtly different frequency responses in intermediate control settings (outside of 0/50/100% settings). The fundamental difference between James and Bax is the topologically different treble control.

(Mostly) off topic warning.

Thanks for that image.
I was playing with the different Bax topologies and found that the simpler implementations (only one capacitor, bass or treble) are quite interactive. Just something to keep in mind.

[POTL]

A single capacitor in the Baxandall/James circuit is not the best idea. The variation with two capacitors works better. The main difference is the independence of the controls, the second capacitor makes the circuit truly independent.