James/Bandaxall question

[EFK]

Can someone explain to me what purpose the two capacitors across the legs of the bass pot accomplish in the Bandaxall stack illustrated about halfway down the page here:

Adam's Amplifiers: Tone Stacks

I'm specifically indicating caps C2 and C3. ???

[Old Tele man]

...they're there to shunt highs around the bass 500K pot when the pot is either full UP or full DOWN.

...it's Baxandahl, although James did it first.

[EFK]

Thanks! I had to ask because my James setup doesn't have them in place and Duncan's calculator gives me a weird response if I try to cut them out.

[loudthud]

When the pots are set for flat response the James Tone network acts like an all frequency voltage divider. The loss is roughly 1/11. Imagine the frequency is so low that the caps have no effect. Now the Bass pot controls the amount of signal that gets to the output. The opposite is true at high frequencies. The caps are low impedance and the treble control determines how much signal get to the output. Notice that C2 and C3 shunt the high frequencies around the bass pot so it's position doesn't matter, the voltage division on that side is determined primarily by R1 and R3. Resistor R2 isolates the Bass and Treble pots so that the voltage division around the bass pot doesn't dominate what the Treble pot wants to do at high frequencies.

You may ask why C2, C3 and C1, C4 are different values. They have a 10 to 1 ratio so they will act like 1/11 voltage dividers at mid frequencies. The James circuit typically uses audio taper pots so that flat response is obtained when both pots are in the center position.

Many people confuse the James and Baxandahl circuits. The Baxandahl uses almost the same schematic except it's in a feedback loop around a gain stage. The James is a totally passive circuit. Typically the Baxandahl uses linear pots which sets the gain of the circuit to (minus) 1 when the pots are set to flat response. You could set the gain to 10 with audio taper pots but you would need reverse log pots. There are other tricks you can use to increase the gain. The Baxandahl also uses equal value caps unless you want to skew the response.

A somewhat simplified version of the Baxandahl uses just one cap across the Bass pot and one in series with the Treble pot's wiper. You can also make a Midrange control by using both caps. See the National Semiconductor 1977 Audio Handbook for more details on that

[EFK]

Loudthud - you're way over my head however (1) I am definitely convinced to get a few small caps and try them out on the bass pot and see if I can determine a difference, and (2) so which schematic is posted at the above link? James or Baxandahl? Can you point me towards an example of the Bax circuit in the feedback loop you describe? I need something I can visualize. Thanks!

[Did J]

About this tonestack, I want to insert into my FireFly amp, does the impedance (Z) need to be matched in the circuit before the volume pot ?

I know it's a passive network, but some amp design schematics add a cathode follower to set the impedance right.

Did J

[Merlinb]

Loudthud - you're way over my head however (1) I am definitely convinced to get a few small caps and try them out on the bass pot and see if I can determine a difference, and (2) so which schematic is posted at the above link? James or Baxandahl? Can you point me towards an example of the Bax circuit in the feedback loop you describe? I need something I can visualize. Thanks!

Additionally, the Bax stack uses a single pot and one cap for the treble, where the James uses two caps. But the Bax requires an extra resistor to 'separate' the two controls, whereas the James doens't need that resistor (although it is included in Duncan's model, 'cause it's useful all the same). Nobody uses the Bax for guitar purposes, it's just not as suitable as the James.

You can see James and Baxandall's original designs here:
Vacuum Tube Design and Applications Notes
Under:
Baxandall, P. J. "Negative Feedback Tone Control,"
James, E. J. "Simple Tone Control Circuit: Bass and Treble, Cut and Lift,"

[loudthud]

Merlinb thanks for the link to that page of useful links!

Here's an example of the Baxandall. It helps to drive the circuit from a low impedance because in the boost mode the input impedance is low.

http://www.schematicheaven.com/marsh..._100w_1992.pdf