Expansion circuit for improved stereo and apparent monaural image

DETAILED DESCRIPTION I claim: 1.
An expander circuit for a multi-channel amplifier system including left and right amplifier channels respectively driving left and right speaker units, each channel including a variable gain stage therein whose gain varies in direct proportion to the amplitude of control signals fed to control signal input terminals thereof, said expander circuit comprising left and right expansion control signal providing means having inputs for receiving respectively left and right amplifier channel signals and output terminals connected respectively to said control signal input terminals of said variable gain stages of said left and right amplifier channels, at which output terminals the expansion control signals respectively appear in proportion to the amplitudes of the sum of the signal components fed to said inputs, the improvement comprising signal coupling circuit means for coupling the signals of said amplifier channels to the input terminals of said expansion control signal providing means, said signal coupling circuit means including respective means associated with said channels for respectively coupling at least a portion of the signals therein having substantially different waveforms to the input terminals of the expansion control signal providing means of the associated channels and for cross-coupling a substantially lesser proportion of the same to the input terminal of the expansion control signal providing means of the non-associated channels, whereby the original signal separation of the two amplifier channels is substantially maintained in spite of said expansion.
2.
The expander circuit of claim 1 wherein there is provided isolating means for preventing said cross-coupled signals fed to the latter expansion control signal providing means from being also cross-coupled to the input of the variable gain stage of the latter amplifier channel.
3.
The expander circuit of claim 1 wherein the ratio of the signals having substantially different waveforms in said respective channels cross-coupled to the input terminals of the expansion control signal providing means of the non-associated channels to that coupled to the input terminal of the expansion signal providing means of the associated channels is in the range of from about 5 to 20%