Microphone balanced amplifier (CB9388E)

This circuit was found in an old American documentation. The integrated circuit used is uncommon today.

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Active inductor (CB20748E)
This circuit simulates an inductor. Or operational amplifier of old type, but a configuration is...
Voltage reference with FET (CB21534E)
We found this circuit in a 1967 Electronic Design. The circuit uses contemporary transistors that...
Relay Control shield or interface (CB15483E)
To control relays safely with double insulation, we suggest the circuit in the figure. The optical...