A 4 transistor heating aid class A amplifier schematic of the Philips KL 5500

See the Philips KL5500 paper including schematic:


A transistor hearing aid - P. Bloom, P. Bloxman - Philips Technical Review - Volume 19


http://www.extra.research.philips.com/hera/people/aarts/_Philips%20Bound%20Archive/PTechReview/PTechReview-19-1957_58-130.pdf