This two-tube circuit, one being a double, was found in Radio Electronics magazine from July 1958. The distance to the microphone depends on noise and losses. The cable must be shielded.
Found in an 1868 Mullard documentation this circuit has a dynamic range of 10 Hz to 25,000 Hz....
We found this white noise generator circuit in old European documentation, but the components used...
We found this 1 A switching regulator in an Electronics Now magazine from April 1993. The...