We found this circuit with the 6A6 double triode in a 1938 Radio Handbook. The power source is not provided.
Other operational amplifiers can be used on this unit gain amplifier. The circuit is from the 1993...
This simple circuit causes an LED to light up when the battery voltage drops below the voltage set in the...
This experimental AM radio circuit was found in a 1979 English magazine. It can be fitted with a...