"Alex Pogossov"
In another thread Phil Allison commended Quad 405.
Its typical schematic is he
http://schematic.narod.ru/Audio/semi/Amp/Quad405-2.gif
** Err - I commented on the Quad 306.
Assuming that Phil knows how it works, and how R38, L2, R6, R20||R21 form
a perfect bridge, blah-blah-blah, here is a question:
What is the purpose of diodes D5 and D6 connected in series?
** I have seen a few 405s for repair ( the PCB electros fail after about 20
years) but make no claim to be an expert on them.
But seeing as you asked, here is my take: D5 and D6 ensure output
transistor TR9 remains biased off at idle and at low output current while
permitting some current flow through R38 into the load.
Facts to keep in mind about the 405:
1. The output transistors operate in pure class B, ie with zero bias
current.
2. TR7, the "class A" stage transistor drives the upper output transistor
TR9 direct while TR8 is the driver for the lower one, TR10.
3. The bridge network allows the class A stage to drive the load direct when
the output transistors are biased off, so virtually eliminating crossover
distortion.
4. It really worked - Quad were able to use slow, rugged NPN output
transistors in the original 405, have no bias adjustment or thermal
compensation for same and yet get vanishingly low THD and crossover
distortion.
5. Later incarnations of the "current dumping" topology simply added extra
NPN output transistors in parallel with the first pair to increase power and
current output. No special matching of these transistors was required since
there is zero bias.
I would prefer to reduce distortion in the output stage by design, rather
than making the power stage non-linear and then rely on the bridge.
** How boring.
I have always been impressed by smartness (non-boringness) of Quad design,
but would not be relying on the current dumping for performance.
** Good thing you don't have to then.
But Quad sold over 200,000 405s plus large numbers of 306s and various
relatives making them the highest selling hi-fi power amps of all time.
.... Phil