Understanding a Fender tone stack - low-pass filter

[überfuzz]

I'm tinkering with a software, LTspice,in which it's possible to simulate circuitry. Now I'm sitting here with a AB763 tone stack trying to understand how it works. I think I get most of it, but the bass control/circuit. The simulation indicates that the bass circuit is a low-pass filter, but I can't isolate it. Every time I try to isolate circuitry making up the low-pass filter that is the bass control I en up with a high-pass filter.

Could I get some help. How is the bass circuit rendered. Please boil it down the least possible components needed.

[überfuzz]

Oh, I should post the boiled down version I came up with. If it happens to be right, I still don't understand how it forms a low-pass filter.



Edit, I got it. The R1 and C1 forms a low-pass filter and C2 forms a high-pass filter with P2. I had to move around the components before I could see it.

[überfuzz]

By the way, I should stress that non of the pdfs uploaded by amp gurus, won't mention any names, had this part right.

[loudthud]

Now can you see how adding resistance in series with C1 changes it from a low pass to a shelving network thus bringing up the midange?

[überfuzz]

Now can you see how adding resistance in series with C1 changes it from a low pass to a shelving network thus bringing up the midange?

Yep! In my opinion the real stroke of genius is the low-,high-pass filter in parallel with the treble high-pass that forms a coupling. They should have gotten the Nobel prize!

[loudthud]

Yep, the way the lows can be fed into the bottom of the treble control and not be attenuated (much) because the top of the control is a high impedance to lows. Also, the way phase concellation causes the mid frequency crevasse. It's kind of something you don't notice until you stick a pot in place of R1 and tweek it in real time. If you watch the phase of the output you see it reverse as you sweep past the crevasse.

[woodyc]

Yep! In my opinion the real stroke of genius is the low-,high-pass filter in parallel with the treble high-pass that forms a coupling. They should have gotten the Nobel prize!

Well, maybe not a Nobel prize, but...

I totally agree that the stack was one of Fender's greatest innovations (among a lot of really great innovations.)

The way it worked its way into so many of his amps clearly indicates that he knew how important it was. So the fact that he didn't patent it suggests pretty strongly that he felt it was already in the public domain.

I've done a bunch of literature searches and made a several trips to the library to try and figure out where it might have originated. The best I've come up with is a D.T.N. Williamson article called Design of Tone Controls and Auxiliary Gramophone Circuits, Wireless World, Oct/Nov 1949.



By the way, the fact that it came from somewhere else does not in any way tarnish Leo's achievement. Great electronic design is not necessarily about inventing anything, its more about knowing when and how to incorporate existing art to solve problems.

[Steve Conner]

Nice detective work!

[bob p]

Not to put a damper on the celebration, I thought that the Williamson tonestack circuit would be familiar to anyone who's into classic HiFi circuits. The Williamson amplifier was the king of hifi tube gear for a long time, until it was supplanted by Hafler & Keroes' ultralinear design.

[überfuzz]

I totally agree that the stack was one of Fender's greatest innovations (among a lot of really great innovations.)

Well the Fender tone stack clearly seems to descent from the Williamson tone stack. The missing link!

[JoeM]

Fender's patent

[Enzo]

Attachments are not working it seems, how about a link to the source?

[jazbo8]

Here.
And Yeh & Smith's paper on the complete transfer function.

Jaz

[bob p]

thanks.

[woodyc]

Not to put a damper on the celebration, I thought that the Williamson tonestack circuit would be familiar to anyone who's into classic HiFi circuits...

Here's the original drawing...