Passive tone controls

[canman]

Are passive tone controls simply high/low pass filters followed by a recovery stage (or preceded by a boost stage, whichever)? 

Bass and treble make sense, but is a passive mids control a high and a low pass filter, combined so that only the frequencies between the two filters are what pass through?

[antonis]

Exactly...!!

High pass -> Buffer stage (isolation) -> Low pass form a passive Band pass filter

http://www.electronics-tutorials.ws/filter/filter_4.html

(if you interchange High & Low you create a Band reject filter..)

[canman]

Awesome, thanks!! 

Instead of starting a whole new thread, maybe I can ask here...what makes a tone control active?  Is it along the lines of the frequency roll-off in a gain stage?

[duck_arse]

gain - makes it active. I think.

[Mark Hammer]

An "active" tone control alters how gain is applied to different portions of the spectrum.  A passive tone control alters what portions of the spectrum pass through more easily.  The recovery stage then applies gain across the spectrum in even manner.

Consider the tone control in the Tube Screamer.  It is an interesting composite of the two kinds of control.  The pot is configured/wired in such a way as to alter how much treble from the clipped signal is passively "bled off".  But at the same time, it alters the gain of the tone stage, and modifies where in the spectrum that gain is applied.   As the leg of the pot connected to the inverting pin on the 2nd stage attains a smaller/lower resistance, the gain of that stage is increased, from a bit over 1x up to around 5.5x.  But because there is the .22uf cap on that ground leg of the pot, bass is cut in that 2nd op-amp as gain is increased.  At min gain (the bass position), the bass rolloff of that 2nd stage is at 35hz.  At max treble, the gain is 5.5x, and the bass rolloff begins at around 3.3khz.

[canman]

Interesting...so with the TS, the pot serves as a resistor for a filter AND sets the gain of the opamp.  Very cool!  I'll likely not mess with any kind of active tone controls, but it's good to know at least what makes it active!  Thanks guys!

[Mark Hammer]

Yep.  R7 and C5 in the drawing are a passive hi-cut, that comes after the clipping stage.  Follow the path from their junction through P2, C6, and R8 to ground.  As the portion of P2 on the way to C6 is reduced, C6 and R8 start to behave like another cap placed in parallel with C5.

Take a look at Jack Orman's interesting variations on this control for more insights: http://www.muzique.com/lab/tstone.htm

[canman]

Will do...should be an informative evening!  Thanks again!