How mid controls work and how to modify this tonestack schematic?

[chumbox]

Hi All

Using the schematic below I was hoping someone could explain to me

1. How the mid control actually works in a tone stack
2. How to modify this tone stack so that the high and low are always at noon on the dial but I can have an adjustable mid control? 

Trying to get five knobs down to three for a metal pedal because I don't use the other two.



Thanks
David

[midwayfair]

This is essentially the Fender tone stack. Download Duncan's tone stack calculator if you want to play with it.

Also read Jack Orman's article on bridged notch filters. It's hard to see, but it's ultimately the same thing, with the 330nF in your example being the cap that goes to ground: http://www.muzique.com/lab/notch.htm

Reading up on notch filters will help you explain why the filter makes a dip in the midrange.

And Mark Hammer's mods to the Companion, which includes a midrange (only) control on the notch filter at the end of the circuit. Which is maybe what you should use, because it's going to be easier than replacing that tone stack with fixed resistors.

You can only have "less" mids with this mids control. As resistance approaches 0, you get a deeper notch. You have a high-pass shelf filter created by the small cap (4.7n here) and the total resistance across the treble bass and midrange pots in parallel with the input impedance of the following stage, as well as a low-pass filter created by the output impedance of the previous stage and the 2k2 driving the 330n that connects to the mids pot, and the treble control pans between those (giving you boosted or cut treble). The bass control is an adjustable high pass filter with the 330nF connected to it, with the variable resistance of the bass pot plus the total resistance of the midrange pot.

(Keep in mind too that you can only affect one aspect of the notch with this control. You can change the frequency, or you can change the amount of notch. Good mids controls usually involve two knobs, which is why it's almost universally better to just make +- bass and treble controls, because if they're done right you can get boosted mids just by turning down the bass and treble.)

[chumbox]

And Mark Hammer's mods to the Companion, which includes a midrange (only) control on the notch filter at the end of the circuit. Which is maybe what you should use, because it's going to be easier than replacing that tone stack with fixed resistors.

So looking at Mark Hammer's midrange schematic, what values would I be replacing on his midrange to match the same scoop as the one I posted?

Mine:


His:


Thanks for the great reply too, some good reading in there that I hadn't seen before.

[midwayfair]

Notch filter calc: http://sim.okawa-denshi.jp/en/TwinTCRkeisan.htm

That's a twin-T, but you can adapt it to the single filter like in Mark's schem: just remember that caps in series divide like resistors in parallel and make R3 absurdly large (100M ought to do it.)

As far as figuring out what frequency you need, experiment some ... and download Duncan's Tone Stack Calculator.

[Transmogrifox]

Notch filter calc: http://sim.okawa-denshi.jp/en/TwinTCRkeisan.htm

That site linked is a great resource.  Backing up a level gives you this overview and sim of a great list of useful filter topologies:
http://sim.okawa-denshi.jp/en/Fkeisan.htm

[chumbox]

Thanks all. The links and advice have been super helpful. Off to breadboard some. Appreciate the help.

[PRR]

The Fender-like stack works by chopping-down *everything* with (in this case) the 2.2K and the Mid pot, then adding-back the Bass and Treb.

It isn't a real good Mid control. (Why Fender replaced the mid-pot with a fixed resistor in the low-price amps.)

If you truly want just the Mid knob: hay-wire it with all three pots, adjust to taste, take the Bass and Treb pots out, measure, and replace with fixed resistors (1 for Bass, 2 for Treb).

[chumbox]

Thanks Paul.

I can make that work for sure.  Now that I think about it, maybe a better alternative would be replacing those two pots with internal trimmers so I can remove the bass and treb pots as hoped but also leave the option open for fine adjustment in the future but I will certainly trial Mark's scoop control as well.  It looks like an interesting alternative and good learning tool.

You have all given me great things to think about and trial.