Battery Powered Stereo Balance Control Features Low Distortion
Electronic Design - Ideas for Design / low-power, low-distortion circuit that can be used to balance the left and right headphones or speakers in battery-powered automotive or computer applications.

Better Volume Control
How to create a log pot that is better than the "real" thing

Designing With the LMC835 Digital Controlled Graphic Equalizer
National Semiconductor Application Notes,04-Nov-1995

Digital Delay Unit for Surround Sound

Digital volume control has log taper
Digital potentiometers provide a compact and convenient way to attenuate audio-amplifier signals. However, most such potentiometers suffer from at least one flaw: a nonlogarithmic step increment. To avoid this problem, a user must usually step.

Digital Volume Control

Inverted bipolar transistor doubles as a signal clamp
A number of circuits, such as level detectors and AM demodulators, benefit from a rectifier with a low offset voltage. Silicon diodes have an offset of approximately 0.6V and do not work well in low-level circuitry. A Schottky diode is a bit better...

Musical Instrument (Expandable) Graphic Equalizer (standard equalizer plus bonus distortion unit)

Passive Filters Fill the Bill At Audio Frequencies
(PDF contains multiple circuits - scroll to find this circuit.)

Subwoofer Equalizer:
The Linkwitz transform circuit is a hugely flexible way to equalize the bottom end of a sealed loudspeaker enclosure. A speaker that is corrected using this method is flat from below resonance to the upper limit of the selected driver. The low frequency roll off point is determined by the parameters of the transform circuit. Should the enclosure size be too small and cause a lump in the response before roll off, this is also corrected. A conventional active crossover netw1ork is then used to divide the subwoofer signal from the main channel signals. Note that there is also a separate spreadsheet calculator available for calculating component values for different situations not handled by the original circuit.

Subwoofer Filter:
12 dB/octave low pass filter for subwoofer, includes subsonic filter

Three Band Active Tone Control

Tone Controls With 2x NE 5532: check also PCB design and notes

Use Rotary Potentiometer To Drive Solid State Volume Control:-
Electronic Design - Ideas for Design / This design idea details how a standard volume control can be replaced by its solid-state equivalent. Digital potentiometers are often advertised as replacements for rotary volume controls. To simulate these controls, many circuits employ up/down...

Voltage Controlled Panner: variable gain amplifier circuit ..