| 摘要 |
<p>The hearing and amplifier input circuit has a change-over switch (S) to select either an electret capacitive microphone (M) or a tele-induction coil (T)(for receiving l.f. signals via a magnetic field). The change-over switch short circuits whichever device has not been selected and thereby reduces cross-talk from the unselected device. The circuit can be attached directly to a telephone loop and used in theatres, lecture-halls, cinemas, churches etc., for the hard-of-hearing. The microphone is coupled via a resistor and capacitor to the input (base) of an amplifier (Q) whose output passes to the hearing aid's main amplifier. The induction coil is connected to the amplifier's emitter. The change-over switch effectively alters the configuration of the amplifying transistor from an earthed emitter (with high negative feedback and low output impedance) to earthed base (with high output impedance).</p> |
