"Tauno Voipio"
http://www.simplecircuitdiagram.com/2010/02/03/single-op-amp-tone-control/
This is the classic Baxandall tone control.
** It's not actually.
Seems like a ballsed up version with missing resistors that will be become
unstable at full treble.
There is plenty of material if Googling for
'baxandall'.
** And like most stuff on hobby web sites it is bunkum.
The "Baxandall" tone control design was published in Wireless World in
952 - it used valves. The network is different to the one in the link above
and produces variable turnover at both the high and low ends of the range.
http://www.novotone.be/_site/projets...ndall%20WW.pdf
.... Phil