Audible Voltmeter (CB21395E)

This circuit was found in an American publication from 1969. Transistors are from the time and can be replaced by modern equivalents.

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Filter analyzer (CB8227E)
Found in a 90s Popular Electronics, this circuit is interspersed between the filter and the...
RIAA Equalization Amplifier (CB4952E)
This circuit is from an 80's documentation. The integrated circuit is from the time and the...
Undervoltage indicator (CB9580E)
This circuit was found in a 80s American documentation. The zener diodes determine the points of...