"John Woodgate"
** So easy in fact, even I managed to design one.See ABSE for "20kHz Outphaser Schem ".
( Shame Adam the Google Groper will not be able)
U1 and U2 = TL074
Rs and Cs = 1% tolerance ( excepting C1)
Phase offset ( R68 -R69) = 90 degrees +/- 2 degrees, 18 Hz to 21 kHz.
...... Phil