1. The James/Baxandall Tone Stack.
Below is the schematic for the baxandall EQ as taken from an Ampeg SVT.
In the case of a Ampeg SVT, the EQ is driven by a 12au7 triode gain stage. The EQ has 2 controls Bass and Treble (hi and low). To summarize it is a passive EQ, meaning that the output signal level is never greater than the input signal level and in real life it is always attenuated to some degree due to the nature of filtering the signal. Thus there is only how much signal are you cutting. Baxandall EQs do have the illusion of being active, because unlike the FMV, when the controls are at noon, the EQ is ~flat, and the bass and treble operate like high and low shelving filters. Of course the EQ is not perfectly flat and can often be manipulated to be less than flat by changing the components. The Ampeg baxandall is exceptionally flat, possibly this is how the design was conceived to be as neutral as possible when the EQ is at noon. Ampeg did pull the wool over your eyes with the words Boost and Cut on the knob but this is purely their own semantic thing as the EQ is passive so the word Boost does not have an electrical meaning here.
The baxandall is probably my favorite EQ I should mention. It is primarily featured in only a few amps in the grand scheme of stuff. Ampeg SVTs, V4s, and B12s, Matamp and Oranges from back in the day, a few rare rare JCM 800 Bass units from Marshall and some others which escape me.
The Baxandall is actually 2 other circuits which are put together, one a high shelf and one a low shelf. If you take a look at the treble control and sort of blank out the bass you can probably get an idea of how it operates. if you turn the treble pot all the way, the circuit can be simplified to a high pass filter formed by the 1meg pot and the 47pF cap. This would result in a lot of high end at the output. If the treble was turned all the way down, then the circuit can be simplified to a LPF which is formed by the 1 meg pot and the 470pF cap to ground. This would have a lot off high end attenuated. However, the 47pF cap still filters which signal is going into the filter, so generally high end is only going into the control and only high end can be attenuated as little low end enters into this control.
On the bass control if the control is turned all the ways up, the circuit simplifies to a LPF formed by the 220k resistor, and the 10nF cap. The 1meg pot and 22k resistor also shape the filter but for now focus on those parts. The output would have a lot of bass as high end is attenuated while low end can go throw unattenuated. If the control goes all the to 0, the 1nF cap now forms a high end bypass where low end sees the 1meg resistance of the pot and thus, there is a HPF formed, resulting in heavy low end attenuation, and thus little low end in the output.
Now these two circuits are joined together by the 120k resistor that connects the wiper of the bass and treble controls. Think of this as a way to sum the circuit together. The resulting output which is taken from the wiper of the treble pot is the sum of these 2 filters. This circuit has minimal interaction between controls but there is some interaction.
Now the first graph of the day is the input and output of the EQ if the controls are at noon.
The blue signal is the input and green signal is the output.
So the input signal is ~0dB for all frequencies but there is some variance as the EQ loads down the tube gain stage at certain frequencies resulting in a little bit of loading. Now the green signal at the output (signal at the 10 meg resistor) is ~20dB less than the input signal. That is a lot of attenuation. Now the output also has some variance even at noon but its generally <3dB across the frequency band which is hard for one to detect if that is the case.
Now Im going to sweep the Bass or Treble with the other control being held at noon, then I will sweep the Bass and Treble together to show how the EQ works.
Here is the Bass sweep.
Blue line is again the input, while each green line represents a different turn of the pot and its resulting output while the Treble is at noon.
As you can see the sweep is quite large, going from ~-2dB to -35dB at the top of the shelving filter. Even at max bass, the output does not achieve unity with the input. There is always -2 to -3dB attenuation but that is not a ton of attenuation so its very much the best practical passive shelving filter.
Now for the treble sweep.
As with the bass there is a good range of sweep here. However the peak of the filter extends beyond 20 khz and I am not showing it on this sweep as that is beyond human hearing. The actual cutoff of the both the treble and bass controls is determined by the caps, resistors, and pot resistance values which comprise the circuit. If for example you wanted to increase the amount of high mids in the Treble control you would increase the 2 cap values on the Treble pot proportionally. So if you double or triple the values, more of the frequency band will be controlled by the Treble. The same is true of the Bass. However, too much increase in the cap values will result in more interaction between the Bass and Treble. So there is a trade off between amount of control and interaction.
The most likely reason Ampeg limited the treble frequencies is due to the fact that the active mid range extended to 3 khz in the 3 position of the mid select switch. That is right where the treble control no longer has any more control. So they were attempting to keep the controls as isolated as possible.
Finally, here are the treble and bass control swept together.
Some observations. As the controls are both increased towards max or decreased to 0, the mids are pretty much left alone. At max the EQ has a mid cut then and at minimum it has what appears to be a mid boost. This is one of the interesting things about this EQ. You can get psuedo mid boosting when the Bass and Treble backed off. However, this is as the cost of a ton of attenuation of highs and lows.
The reason the EQ has very good range is because the EQ is quite lossy. What I mean by lossy is that the EQ has a lot of insertion loss, ie, the output signal is always going to be heavily attenuated across all bands. The tradeoff with insertion loss is that you get more control if you increase it but more loss on the output. In a bass amp that is probably not a real big deal but in a guitar amp where you wanna get more clipping and distortion the result is that you need more recovery gain after the EQ to get the same level if the controls are ~ at noon.
One more important note, there is no mid control in this EQ and no easy way to put in some kind of true mid range control. That is also one of the trade offs. Of course, since the EQ has no mid cut at noon, its often interpreted as being more mid rangy then a FMV but if you do max out the controls, then a mid cut can be heard. However, I never run the EQ like this so its never a problem.
Next I will discuss the Ampeg SVT Mid Range control