Earphone adapter for aircraft radios

Claim 1

An adapter making it possible to couple two sets of earphones to the relatively low-impedance output of an aircraft navigation-communication system without adversely affecting the operation of a loudspeaker already coupled to the same low impedance output, so that a wearer of the set can listen without difficulty to the system despite ambient noise that may impair the understandability of intelligence emitted by the loudspeaker, said adaptor comprising: A. a battery-operated transistorized audio amplifier whose input circuit has an extremely high impedance and whose output circuit has a relatively low impedance, said amplifier having first and second transistors in a Darlinton configuration in which the emitter of the first transistor is connected to the base of the second transistor, said amplifier having a high gain and including a volume control; B. means including a d-c blocking capacitor for connecting the high-impedance input circuit of said amplifier to the low-impedance output circuit of said system to create a gross mismatch in impedance therebetween whereby virtually no load is imposed on the system, the resultant low level input signal fed to said amplifier by reason of said mismatch being compensated for by said high gain; C. means connecting two sets of earphones to the output circuit of said amplifier; and D. a buffer amplifier whose input is coupled to two microphones, one serving a first wearer of one set of said earphones and the other a second wearer of the other set of earphones, the output of the buffer amplifier being connected to the input circuit of the transistorized audio amplifier through a high impedance, whereby the adapter also functions as an intercom between said wearers without imposing a significant load on said system.