Can anyone please explain the FMV tone stack to me?

[harry]

Hello all,

I have tried to find a simple explanation for how the circuit operates. I can see what it does, and what altering the values does - what I don't know is why they behave as they do. What are we seeing here? Parallel filters?



My immediate view is that there are two filters: a treble cut filter, and a bass cut filter. The break frequencies are such that they cause a mid-notch. Turning the mid control up reduces the amount of attenuation of these frequencies. The two filters are "mixed" together by the treble pot. The components are interactive, so adjusting the parameters of one filter adjusts the parameters of other filters.

Am I on the right track here?

Thanks all,

Harry

[m-fine]

Sounds like you pretty much have it. I like to think of the mid control as almost a second volume control, and the volume impacts the treble, so the whole thing becomes an interactive mess.

[Chuck H]

Yep, you've got it. I use this tone stack (different values but the same circuit) only because it's part of a previous design I use and gives me the tone I want. It's failings are many. The inevitable mid notch, the fact that you can't run affected frequencies too close or you have MAJOR interaction on the controls and the mid control becomes moot and low sweep values can also occur when trying to affect close knees/notches. Unless the standard "Fender" values (as shown) work for what you want it's kind of a mess really.

[jrfrond]

You have pretty much nailed it. See, it's not so hard, is it? It really ISN'T rocket-science!

The midrange control, or resistor in two-knob designs, controls the gain loss of the tone stack. It doesn't REALLY control mids as much as it determines how flat the stack response will be. It lessens the "scooping" performed by the treble and bass filters. Lift the ground reference here, and you lose the tone controls, but also gain about 20dB, and the preamp sounds more like a Tweed. If you are playing with the Tone Stack Calculator, see what happens when you insert a really large midrange control.

[Ken Moon]

Here's a condensed version, lifted shamelessly from the AX84 site, using the component numbering in Harry's post:

'The Treble pot acts as a balance control between the output of a high-pass filter formed by C1 and the three pots, and the output of the complex filter created by R1, C2, C3, and the Bass and Mid pots.

The Bass pot sets the lower -3dB cut-off frequency of a band-pass filter formed by R1, C2, and the Bass and Mid pots. It also affects the -3dB cut-off frequency of the treble control circuit.

The Mid pot controls the attenuation of the band-pass filter formed by R1, C3 and the Mid pot. It also acts as a variable attenuator for the tone stack output.'


A much more detailed analysis is here (starting on page 20):

http://ax84.com/static/p1x/p1-ex-theory.pdf

and in*Richard Kuehnel's*Fender*Bassman*5F6*A book.

[JoeM]

Leo's Patent

Leo's Patent