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High-z-to-low-z microphone adapter

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Many commercial and home-built mobile transmitters are designed for low-impedance carbon-microphone input. The input speech circuit is usually a grounded-grid arrangement as shown to the right in Fig. 1. However, it is sometimes desirable to use a high-impedance microphone with the equipment, since a crystal or dynamic microphone has better fidelity and freedom from the hiss and "blasting" sound familiar to the carbon type.

Fig 1
Fig. 1. Microphone impedance step-down adapter. Jack J1 is the microphone jack on the transmitter's speech amplifier. All resistors are ½ watt.

Partlist fig. 1
P1Plug to mate with J1.
Q1,Q22N107 transistors.
S1D.p.d.t. switch.

The circuit shown in Fig. 1 is a matching device which will allow a high-impedance microphone to be used with the original speech circuit in the transmitter without any changes in the equipment. It uses two transistors and doesn't need a power supply, since its operating voltage is obtained from the voltage developed at the cathode of the speech-amplifier tubes. It probably could be built small enough to be installed in the microphone case or under its base. By throwing switch Si, the microphone feeds straight through so that it can be used with equipment designed for high-impedance input.

Transistor Qi is used in a common-collector circuit to step down the impedance of the microphone to match the emitter-follower transistor Q2, which feeds the signal into the low-impedance jack of the speech amplifier. The output voltage from the unit is comparable to that of a carbon microphone but is free from its characteristic hiss sound and nasal quality.

E.S. Millman, W3WNE.