Baxandall with variable mid Scoop frequency

[Vivek]

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.

Either a switch

or continuously variable pot (Preferably not dual pot).

Idea is to get an "American - British" control.

Please suggest ideas on how to do that ?

[Phoenix]

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

[Radical CJ]

Ditch Baxandall, use James, replace R4 with pot wired as variable resistor.
http://www.guitarscience.net/tsc/james.htm

That configuration looks versatile, although comes with a big real/percieved volume drop.

[Phoenix]

That configuration looks versatile, although comes with a big real/percieved volume drop.

Sorry, edited my post just as you posted - put it in an opamp feedback loop to make it active/no insertion loss.

[Rob Strand]

If you want  preserve the shape of the curve, ie. the Q of the finite notch, then you need to scale *all* the caps up or down to shift the frequency.   From this perspective it's easier to use a Baxandall with one bass cap and one treble cap.

Think of the notch as a result of a shelving bass boost and a shelving treble boost.    Think of the center of the shelf as a moveable frequency, the highest slope part at mid-boost.   If you change a single component, doesn't matter if it's a cap or resistor,  you will simple shift the central point of either the treble or bass frequency up or down.  If for example you shift the treble frequency up the center of the notch moves up but the notch actually gets wider because the bass frequency has not move up as well.    Alternatively we could move the bass frequency up.  The notch frequency is the same as the treble case but the notch is now less deep and slightly narrower.   Neither maintain the shape and notch depth which is what you get when you scale both caps.

Some tone controls have separate switches for the bass frequency and treble frequency.  These change the bass and treble frequencies individually like I just mentioned.  If both bass and treble are on low then you get the same response shape when the base and treble frequencies are on high.   However the other settings will make the notch wider or narrow.

A moveable notch can be done with a bridge-T circuit or a Wien base parametric equalizer.   Here, you can switch either two caps or two resistors, or you can use a dual pot to adjust the frequency.    That doesn't let you adjust the level of the bass and treble though but think about it you have three controls bass, treble, and the frequency switch.   A twist on the parametric EQ is a tilt control, which could be two controls.

[Vivek]

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

What is the difference between

Baxandall

And

active James in feedback loop

?

[Phoenix]

What is the difference between

Baxandall

And

active James in feedback loop

?

They have different treble controls. James has the advantage in this case of a simple variable resistor being able to change the center frequency (though not preserving Q, as Rob points out). This trick does not work for Baxandall.

[POTL]

Contour control cuts the mids.
A similar solution is found in the Wampler Tripler Wreck circuit, this adjustment comes after the James Tone stack which is amplified by an op amp.

[mdcmdcmdc]

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? The use of the opamp is to add a buffer before the circuit and make-up gain afterwards, but doesn't affect what's happening inside the tone stack per se?

The Baxandall circuit is active, with middle position being at unity with the input and the opamp providing +/- ~15dB from there?

The active circuit is also a lot *less* interactive than the james, correct?

I plugged the values for both the orange OR-series and garnet sessionman tone controls into the TSC and it looks like the troughs of the mid-scoops are 230hz (or) and 1000hz (garnet).

[Vivek]

mdcmdcmdc, I gave you your first "Like" on this group

Please post screenshots of the james with your values plugged in. Thanks !!!

[Vivek]

Contour control cuts the mids.
A similar solution is found in the Wampler Tripler Wreck circuit, this adjustment comes after the James Tone stack which is amplified by an op amp.

Very interesting that you bring it up !!!!

I had entered this segment into LTSPICE last month since I was studying various Mid controls.

Here's the output I got:




It has about 3db insertion loss in the midband. Not bad !!!

Scoop is around 800 Hz = typical Marshall

Fender scoop is around 500 Hz

[Vivek]

Question : Why was a separate contour section used instead of the standard BMT Marshall tone stack ?

[Ben N]

Vivek, have a play with the TSC--you'll see that the mid control in the stock tone stack is a bit of a freak, highly interactive with the other controls. The contour control is a lot more WYSIWYG.

[Vivek]

Thanks Ben !

Also, the separate contour control has much deeper scoop of up to 27dB

[mdcmdcmdc]

mdcmdcmdc, I gave you your first "Like" on this group

Please post screenshots of the james with your values plugged in. Thanks !!!

Here's the orange stack:



And here's the Garnet:




I pulled these values from the schematics from the James sections at ampbook.com.

[mdcmdcmdc]

Also, just so it's a little easier to see - here are the curves with both knobs at full CW:

Garnet:



Orange:

[Phoenix]

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.

[Vivek]

I used to think that James was a passive circuit while Baxandall is the same thing thrown into a feedback loop of an Opamp.

But there are posts here taking about the benefits of the a "James in feedback loop" versus a Baxandall.


Please help me to understand the differences.

Thanks.

[Phoenix]

I used to think that James was a passive circuit while Baxandall is the same thing thrown into a feedback loop of an Opamp.

But there are posts here taking about the benefits of the a "James in feedback loop" versus a Baxandall.


Please help me to understand the differences.

Thanks.

Already explained above briefly, but here's a more complete version.

Any tone control/tone stack can be passive. But putting that same passive control into a feedback loop makes it active (capable of boost, not only attenuation). Hell, you can put the classic FMV tonestack in a feedback loop to avoid insertion loss if you want. Active also has the benefit of reduced or eliminated influence of source and load impedance on the frequency response, and any interactivity that may cause.

The reason why James is so often associated with passive, and Baxandall so often associated with active is simply because EJ James published the "Simple Tone Control Circuit" which now bears his name in Wireless World Feb 1949.
Peter Baxandall published "Negative Feedback Tone Control" in Oct 1952.
Baxandall was the first to publish about the novel idea of putting a tone control in a feedback loop. The particular tone control he chose to use now bears his name, but it also functions passively, and any other "passive" tone control also functions in a feedback loop as an "active" control.

Look carefully at James vs Baxandall, notice that (in most versions) they share the same bass control, but they have a different treble control. The James is sometimes incorrectly referred to as "passive Baxandall", which may be causing your confusion.

EDITED to add simplified schematic:

[Rob Strand]

I used to think that James was a passive circuit while Baxandall is the same thing thrown into a feedback loop of an Opamp.

Technically James is passive and has four caps.   Baxandall's contribution was the feedback version.     

Here's the original Baxandall paper.  The original paper has two caps on the bass and one on the treble.   Note the treble control is tapped;  IIRC the paper mentions removing the tap.
https://www.learnabout-electronics.org/Downloads/NegativeFeedbackTone.pdf

There's a number of bass and treble variants for the feedback type.   A while back I put up this post showing all the variants and also the different responses for the different combinations of bass and treble.  For apples to apples comparison each circuit had the same response with either bass or treble alone boosted.
https://www.diystompboxes.com/smfforum/index.php?topic=123231.msg1164845#msg1164845

[Sorry for the repeated into.  Phoenix typed his post while I was typing mine.]