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Input circuit for either crystal or carbon microphones

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The circuit shown in Fig. 1 permits feeding the output of either a crystal or a carbon microphone to the speech-amplifier tube. The closed-circuit jacks automatically convert the circuit for whichever type of microphone is used. When using a crystal or other high-impedance microphone, it is plugged into J1. In this application, the cathode circuit for the 12AX7 is completed through J2. With J1 closed (microphone plug removed) and with a carbon microphone plugged into J2, the stage operates as a grounded-grid amplifier. The second half of the dual triode is used as a conventional voltage amplifier with a gain control in the grid circuit.

Fig 1
Fig. 1. Schematic diagram of the speech amplifier designed for either crystal- or carbon-microphone input.

Vernon Phillips, W7NPV.