Join Date: Aug 2012
Location: books at londonpower.com
The schematic they showed me and that I verified by tracing the circuit back in 1981 was a bit more complex. At that time, they used six dual-transistor cans - 2N2920npn and 2N3811pnp. The circuit is essentially as above: a PNP input loaded by a four-BJT Widlar current mirror; feeding a Darlington VAS that had a current-mirror to load the input transistor and quickly discharge the base of the output; constant current sources for the input comprised of dual BJTs with a cross-collector degeneration resistor; current source load for VAS; mosfet output as above.
A friend and I showed them an alternate way to lay the amp out and an alternate circuit using all-discrete BJTs. Maybe they saw the cost savings of dumping the duals?
Their original schemo was on a folded piece of paper with a pencil-drawn circuit on it. The circuit follows standard lines but I suspect they borrowed the details from someone else. A few years later the original designer left and was replaced by a young Asian guy who could also design tuners and suchlike allowing them to expand their product range.