be understood by reference to the block diagram of fig1 any conventional audio source 4 well known in the art such as a radio phonograph or tape deck will provide basic audio signals by way of leads 11 12 to output reproduction devices 5 such as speakers earphones or transmitters having signal responsive input impedance members 13 that typically are speaker voice coils a second audio channel derives the same audio basic signal and modifies it in element 6 which may be for example a filter a tone control circuit or a reverberation device output signals from the signal modifier are made available by polarity isolator 7 in a phase relationship at leads 14 15 so that they are fed through impedance member 13 in an opposite direction and for all practical purposes are fully dissipated in impedance member 13 as are the basic audio signals mixed thereby for common reproduction accordingly the impedances 13 are simple convenient mixers at a critical circuit position since the modified signal at 14 15 is not fed back into the loop formed by leads 11 12 and signal modifier input terminals 16 17 in such a way to introduce feedback distortion or oscillation which otherwise would be likely particularly if the signal modification channel amplifies the signal derived from the basic audio output leads 11 12 thus this circuit arrangement does not produce simultaneous amplification of modified and basic signals but rather isolates the basic signal to flow in one channel 18 namely through the leads 11 12 to the reproduction device 5 and further isolates the modified audio signal to flow through channel 19 without commonly amplifying both signals in the same channel should output amplification of the modified audio signal be desirable amplifiers 8 9 are inserted in leads.