This balance control for the channels of a stereo system was found in a 1969 Mullard documentation. The transistors may be the modern BC548 equivalents.
Today we have more compact solutions. This is from a Radio Electronics magazine from October 1990....
This filter was found in Texas Instruments documentation from the 1980s. The circuit uses a common...
16 commercial test circuit for tubes and transistors appeared in Radio Electronics magazine in...