OK back in the 80s i was right into building high power amplifiers, the bigger the better and that meant WATTS of power... mainly for the Disco 'Industry'... DJs would find some understanding person who'd build them a plinth with two turntables, a basic mixer, a huge amp internal and maybe a coloured light show plus some dodgey big speaker bins set up!
There were plenty of designs around and i must have munched through 100s of 2N3055s... the go to transistor of the day...
I'd an interest in HiFi because CDs had recently arrived but were horrendously expensive and no one had a player.
Then one of the hobby magazines published a design using MOSFETS and the game changed. The cost of making WATTs of Audio Power came down and for a while i was munching through quite a few of the new 'weird' things... ahhh and blowing up a few i might add! (they self oscillated and even made a mess of the upper VHF radio band if you didn't watch it)...!
I read articles in HiFi magazines on how the MOSFET gave a better tone, similar to tubes (valves). I honestly did not notice that aspect but i recall seeing the hundreds of AudioPhools spouting their preference for Bipolar transistors only.... which i thought strange because even then the holy grail were tubes....
Then my career changed, bye-bye audio stuff hello TV Broadcast cameras.... (another story).
Cut to today and someone brings me a knackered Tuner-Hometheater Amp made by Sony that had been to so many parties. It was being used as straight amp or sometimes the FM radio was simply left running. They used it on two speakers only, joining them to the FRONT marked terminals. Worked for years said the owner.
This is a 7.1 surround sound thing meaning there's 7 main speaker terminal sets and another driver for a SubWoofer and it all weighs a ton. Obviously dropped a few times too going by the dents. I class it as genuine dumpster stuff but the owner 'really needs it".
It randomly goes "SPLATT!" shuts down, puts up the words in in very dim display ("RECOVERING").
So i tore it down and discovered where what must be 90% of the worlds aluminum reserves have ended up. The thing's heatsink is MASSIVE. And bolted to it a pile of fat transistors and i go hunting on Google and find the pdf for these transistors.
I made the assumption the output drivers would be huge Bipolar Specials or MOSFETs like when i found that sort of trend in the mid 1980s but NOPE.... they are DARLINGTONS!!!
Talk about live under a rock. Darlingtons in my day were always power-switching of loads like driving ignition packs on 4 cylinder motorcycle engines or H-Bridge arrangements on 3 phase motors... I never saw an audio circuit using Darlingtons because they would have lots of extra noise and AudioPhools would start on 10,000 rpm head spins over the shocking sound quality....
So here i am looking at a row of big 2SB1647/2SD2560 TO-3P 150V 15A devices.... 7 individual channels big push-pull driver output stages... Man what i could have made for the DJ guys back in the John Travolta/BeeJees 'Saturday Night Fever' days if this stuff had existed...