This circuit uses an amplifier from National Semiconductor, uncommon nowadays. The numbers in parentheses are for the second channel. The circuit is suitable for old tape recorders.
This filter for the scratching noise common in record players was found in Radio Electronics from the...
This circuit was found in Philips documentation from 1967. The phototransistor is from that time,...
This circuit is from an old documentation but still current for assembly. Just use BC548 instead...