This simple general-purpose transistor mixer was taken from an article in Electronics Now magazine in March 1994. You can choose a low-noise transistor such as the BC549.
This transistor tester was found in a publication by Gernsback from 1960. The circuit can be used...
This circuit was found at Popular Electronics. The speaker is of the greater than 100-ohm...
The circuit shown was found in a 1988 documentation, giving an indication of value in an...