Tone Stacks - Part 2 - James & Baxandall

[Chuck D. Bones]

First thing, here's a great resource on tone controls that I stumbled across after writing Part 1:

https://www.guitarscanada.com/attachments/10-tone-control-pdf.352944/

It contains the same info I'm presenting in these articles and then some.

James
E. J. James published the tone control circuit bearing his name in the February 1949 issue of the British magazine Wireless World.

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It is a passive filter that introduces about 20dB of insertion loss when the BASS & TREBLE controls are centered. Originally designed for hi-fi use, it was adopted by guitar amplifier manufacturers Ampeg (left) and Orange (right).



The pots are usually audio taper to provide a uniform sweep. In the Ampeg design, the component values are proportioned to provide a flat freq response when the BASS & TREBLE controls are at noon. TSC shows that the Ampeg tuning sets the bass / treble crossover at around 1KHz, a little high for guitar amp & pedal use IMO.



The Orange circuit is tuned lower and less symmetrically.



The James tone stack really does not need a MID control because a mid notch is obtained by setting BASS & TREBLE above noon while a mid hump is obtained by setting BASS & TREBLE below noon. R4 can be tweaked to move the freq of the mid notch / hump. In fact, R4 could be replaced with a B500K pot in series with a 47K resistor.

The James tone stack shows up in the Viceroy pedal (below), tuned similarly to the Orange tone stack. It also appears in the PCBGuitarMania Acapulco Tone and the various Revv G pedals.



The advantages of the James tone stack are large BASS & TREBLE sweeps, tweakability, minimal interaction between the controls and the ability to set the freq response to flat. The disadvantages are the relatively high component count (compared to the Fender tone stack) and insertion loss.

Baxandall
Peter J. Baxandall published the tone control circuit bearing his name in the October 1952 issue of Wireless World. The Baxandall tone stack is classified as an active filter because it requires a gain element to function. In the Wireless World schematic below, the vacuum tube serves at the gain element.

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Note that Baxandall used a tapped pot for the TREBLE control. The tap is actually not necessary as long as a DC path is provided from the grid to ground. With opamps or transistors, a DC path to the appropriate bias voltage is required. Usually, there are series resistors at both ends of the TREBLE control to limit boost and cut to reasonable levels. There is an alternate version of the treble control that replaces C3 with a resistor and puts series caps at either end of the TREBLE control (see the Acid Rain tone stack below). Sometimes the two bass capacitors are merged into one. The circuit does not need to be balanced. The caps or resistors can be deliberately unbalanced if you want more cut than boost, or if you want the cut and boost to kick in at different frequencies.

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The Baxandall tone stack shows up in several guitar pedals: Colorsound Overdriver, PedalPCB Acid Rain, Thorpy Fallout Cloud & M.I. Audio Super Crunch Box. The EQD Talons & JHS Charlie Brown have a Baxandall stack with an additional MID control crammed in. The added MID control is hard to execute properly. JHS got it right and EQD failed miserably. M.I. Audio added the MID control as a separate stage for the ultimate in control isolation. The treble portion of a Baxandall tone stack shows up in the Klon. In the Acid Rain Baxandall tone stack below, Q4 provides the gain element.

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The advantages of this circuit are large BASS & TREBLE sweeps, minimal interaction between the controls, tweakability, the ability to set the freq response to flat and zero insertion loss. The disadvantages are large parts count and the need for a gain element: either an opamp, transistor or vacuum tube. TSC does not support the Baxandall tone stack; if you need to simulate it, you'll have to use an engineering tool such as LTSpice.

 

[danfrank]

Very interesting, thanks for posting this. I always thought that the James and Baxandall were the same but I guess they're not.
BTW, the Fender "G" (not G-A) brownface amps used these type of tone controls, but only for a short time before they converted to the "G-A" brown face Amps that had a more traditional (and cheaper) tone stack, Leo Fender always wanting to save a penny or two when he could.

 

[cooder]

Excellent, muchas gracias!

 

[almondcity]

so the PedalPCB Acid Rain is basically a Big Muff without diodes with a Baxandall tone circuit?

 

[Chuck D. Bones]

There is one other important difference between the Acid Rain and a BMP: the coupling between the 2nd & 3rd stage. Instead of the typical 10K + 100nF, there is no resistor, just a 10nF capacitor. Has a strong effect on the tone. The first three stages of the Acid Rain are the same as the Noel Cornet Revu. The Acid Rain's Baxandall tone control stage is the same as in the Thorpy Fallout Cloud. Lotta very similar pedal circuits out there.

 

[Paradox916]

I’m not going to lie.. I love the stuff you cover, I have learned a TON from just reading your posts and threads while only understanding a fraction of it... it’s like drinking from a fire hose! But it gives me something to strive towards... It’s a lot like playing guitar for me I’ll never master it... I’ll just do a little better each time.

 

[fig]

Same here.

 

[Chuck D. Bones]

Very interesting, thanks for posting this. I always thought that the James and Baxandall were the same but I guess they're not.

The two circuits are related and they do basically the same thing, just different ways. In his Oct 1952 article in Wireless world, Baxandall cites the James article from Feb 1949. That tells us he studied the James design during the development of his design. Negative feedback in circuits had been around for about 30 years when Baxandall wrote that article. The genius of Baxandall's design was putting the James tone controls in a feedback loop. Circuit designers these days take negative feedback for granted, but back then, it was still a new concept.

 

[MattG]

The Baxandall tone stack shows up in several guitar pedals: Colorsound Overdriver, PedalPCB Acid Rain, Thorpy Fallout Cloud & M.I. Audio Super Crunch Box. The EQD Talons & JHS Charlie Brown have a Baxandall stack with an additional MID control crammed in. The added MID control is hard to execute properly. JHS got it right and EQD failed miserably.

Emphasis mine. What makes the JHS successful and the EQ a failure? (For reference: PedalPCB Van Pelt Drive is JHS Charlie Brown v4, and PedalPCB Chela Overdrive is EQD Talons.)

Is it possible to easily "shift" a Baxandall tone stack? What I mean is, say the sweep for a frequency is -15 dB to +15 dB, and 0 dB with the pot in the middle position. What if wanted -9 dB to +21 dB, and +6 dB with the pot in the middle position? I.e. we rarely want to reduce, and only a little when we do; but we want a default (i.e. pot at noon) bump for that frequency, and a bigger boost range?

The Tone Stack Calculator has Baxandall support now. I tried playing with the values to achieve what I wanted, and I was kinda-sorta successful, but that was by the "guess and check" method. I assume there's a more methodical way to get this result.

 

[Chuck D. Bones]

If you have LTSPice, run a .ac simulation and look at what happens to the BASS or TREBLE sweeps when MID is a 0, 5 and 10. The EQD circuit does not provide enough any isolation between MID and the other two knobs. The Baxandall circuit was never intended to be a 3-band EQ. The BEST way to do it is to put the MID control on a separate stage. Doing it the way JHS did it is "good enough" for a pedal. There is still interaction between MID and the other two, but it's manageable.

In a Baxandall tone network, the BASS & TREBLE roll-off frequencies are determined mainly by the capacitors. The resistor between pin 2 of the BASS & TREBLE pots affects the TREBLE tuning to some degree. You can put the caps on rotary switches, but that seems a bit complicated for a pedal. If you want a reasonably simple, tunable tone stack, then build two or three Vari-O-Boosts, connect them in series and voilà! You have a tunable tone stack with fully independent BASS, MID & TREBLE boost/cut knobs. We can fiddle the cap values a little to optimize the freq range and Q for each band.

I have found that in most pedal circuits, I do not need a BASS knob that has a symmetric cut and boost range. Depending on where it is it the circuit, I only need cut or boost, not both. Same goes for the TREBLE knob. Baxandall might be nice for a stereo or maybe a small mixing console, but for a pedal or guitar amp, I think there are better ways to obtain the required EQ.