This circuit was found in an American publication in 1966. The circuit uses a general-purpose FET that admits an equivalent. The instrument is a 250 uA full-scale microammeter.
This circuit was obtained in a 1972 Corriente Alterna magazine (Uruguay), using the traditional pair of...
E) Using a CA3130 this preamp has excellent gain. The power supply does not have to be...
When the power is on, the 1 µF capacitor charges slowly by R1 and P1 until then on trigger...