An audio tone control combines both a Base control and a treble control in the same circuit. Tone controls are found on lower-end audio gear because it saves both front panel space on the unit and only requires one control knob, instead of two separate controls.
Passive Tone Control
In this case potentiometers R2 and R6 are coupled within the same potentiometer case, so adjusting one also adjusts the other [one knob, but having separate resistive wipers [Rotary Wafer Definition].
The tone control to the right just uses a simple RC filter network, with the capacitor connected in parallel with the variable resistor. The tone control circuit directly below uses a three position switch to select a different capacitor to change the tone of the signal, and is only slightly more complicated than just using a resistor trimmer. The circuit could be made with even fewer parts by only using one capacitor and a switch that connects it to the circuit or removes the capacitor, saving two capacitors.
Selectable Tone Control
Although the circuit to the right is not a Tone control, it is related to a Tone circuit. A Tone control would only consist of a single control or knob, but the circuit shown uses three separate variable resistors. However what is not depicted in the graphic is that those resistors could be ganged into a single package, basically producing a tone control. The circuit is not a true tone control, but serves to show another circuit, or filter style used to control different frequency ranges.
Note the different capacitor values used for the treble, bass and midrange sections.
Active Tone Control
This design uses an LM301 operational amplifier to control the circuit. The passive portion of the tone control is almost identical to the first circuit shown at the top of the page. However there is an additional 10k resistor added between the potentiometer and the Op Amp. Note also that resistor R3 and capacitor C4 are also no longer returned to ground, but now form part of the feedback circuit. Resistor R4 and R5 has also been combined into a signal resistor, which is now connected to the trimmer. The output of the passive circuit is now applied to the negative input of the Op Amp.
Active Tone Control
The plus input of the amplifier is taken to ground via a 68k resistor, which is only used to compensate for bias currents. Note that this example circuit also provides component values for a typical tone control.
This next topic concerns Equalizer Circuits, which are really just single-band tone controls, or use the links just to the right for more complicated 2 and 3-band tone controls.