Another way of making the output section with class A predriver

[JensRasmussen]

Hello all,

I stumbled across this schematic while looking though some old service manuals. It is the output section of a 150W power amplifier.


I like the way it’s put together and thought I would share it for interested to use as inspiration for a new design.

\Jens

[destroyer X]

The pain of a heavy class A to.... Join us Jens!, you have a lot to help!... came to us.... class A crazy guys.

No matter your intention, this gift is for me.... everyone that open their mouth to talk about class A, i feel as talking directly to me...and i became happy.

Thank you, my wish, thousand guys came here... full of ideas... with courage to expose them.... will be the best thread from long time.

Carlos

[phase_accurate]

I don't see anything special within this topology (apart from looking "very Philips").

Regards

Charles

[JensRasmussen]

Charles,

No Philips in here

What else do you see?

\Jens

[Upupa Epops]

What is device named as IC 1 ?

[JensRasmussen]

IC1=MPSA13

Datasheet here

\Jens

[Steven]

Indeed, a bit unconventional. Output stage is normal darlington, but bias setting is via resistors R94+95. Temperature compensation via IC1 and TR28, making a temperature dependent current sink. IC1 senses the current through R91. This results in a temperature dependent bias voltage across R94+95.
I don't see the benefit of this compared to a fixed current sink and Vbe multiplier as bias voltage source, though.
Another unusual thing is the driver, TR23, which is an emitter follower instead of the more common common-emitter stage (VAS). This might result in a-symmitrical driving of the output stage. The emitter follower can supply more current (positive edge) than the current sink can sink (negative edge).

Steven

[jam]

The schematic is probably from a B&O amplifier................my guess is probably the Penta powered loudspeaker.

[JensRasmussen]

Hey Jam, you know your vintage stuff

\Jens

[sajti]

Originally posted by Steven
Indeed, a bit unconventional. Output stage is normal darlington, but bias setting is via resistors R94+95. Temperature compensation via IC1 and TR28, making a temperature dependent current sink. IC1 senses the current through R91. This results in a temperature dependent bias voltage across R94+95.
I don't see the benefit of this compared to a fixed current sink and Vbe multiplier as bias voltage source, though.
Another unusual thing is the driver, TR23, which is an emitter follower instead of the more common common-emitter stage (VAS). This might result in a-symmitrical driving of the output stage. The emitter follower can supply more current (positive edge) than the current sink can sink (negative edge).

Steven

Mr. Hood used this type of thermal compensation in some desings. Not so common, but logical...
TR23 is separate the output stage from the VAS. Someone believes, that is reduce the distortion in the output stage, due the low impedance driving.

sajti