Audio Circuits: 
Surround Sound Circuits

Allpass Network shifts Signals 90°: 04/27/95 EDN-Design Ideas 

Digital Delay Unit for Surround Sound:

Quadraphonic Amplifier: This is a four channel amplifier ideally suited for use with quadraphonic equipment such as a Sound Blaster Live card.  

Surround Sound Decoder: This surround-sound decoder is based on the "Hafler" principle, first discovered by David Hafler sometime in the early1970s. The original idea was to connect a pair of speakers  

Surround Sound Mk1:

Surround Sound Mk2: