is shown in Fig, I. It was adapted from a hearing-aid circuit which was developed to provide improved symmetrical clipping action. A speech waveform can be clipped to a great degree without destroying intelligibility, but any slight phase shift in the waveform components will destroy intelligibility very rapidly.
John /. Schuhz W2EEY! 1 40 Rossie St. Mystic, Connm 06355
Since phase stability is a function of the symmetry of the clipping action, it is important that the positive and negative variations of the input waveform be limited equally to a close degree. This is accomplished by the use of a pair of carefully matched diodes in the circuit
Almost any small signal diode can be used and the diodes can either be matched by testing their characteristics, or a pair of matched diodes can be purchased (for instance, a pair of JN541 diodes, sold as a matched pair for use in FM detectors, costs less than a dollar).
The circuit must operate from a high impedance microphone. It will not operate properly with some of the low impedance dynamic mobile microphones (300 to 1,000 ohms) because sufficient voltage is not developed to allow proper clipping action by the diodes. Aside from the diodes, the circuit is that of a simple transistorized audio amplifier.
The photograph shows the placement of the clipper stage in the housing of a typical mobile microphone* The components are simply assembled on a piece of vector board shaped to fit into an empty corner of the enclosure. A thin piece of foam plastic or rubber material is glued on the bottom of the vector board. The foam material is, in
Fig. L Circuit of the preamplifier clipper circuit. Potentiometer adjusts clipping level and may he replaced by fixed resistors once desired level is found.
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