Why these resistors

[Vostro]

I've been looking around and this topology has been around a long time,
But can someone please help me understand why Yamaha add R31/32.

Thanks

[Ketje]

Probably they had some stability problems ,R31 brings down the open loop gain.
For R32 a gess.The feedback loop includes R15 inducing a small part in the input.To balance that R32 returns some output to the other side of R15.Why from that point (c of Q23) and not directly from the output,perhaps experiences indicated that point the best.
Mona

[sbrads]

The bit I don't like about the endless variations of the current mirror in the collector of the input transistor circuit is that there's hardly any DC across the collector load to swing AC around. My rule of thumb shows 0.6v or less across Q6. It might work but it just seems so non-optimum to me.

[Ian Finch]

For comparisons and interest, what model Yamaha amplifier is this taken from?

[JMFahey]

there's hardly any DC across the collector load to swing AC around. My rule of thumb shows 0.6v or less across Q6. It might work but it just seems so non-optimum to me.

That's the way it's supposed to work.
In fact, don't consider Q6 a gain stage but a sophisticated diode.
The *big gain" stage is Q23 and its active load Q24.
Q2's collector "might" be tied straight into Q23 base, like in the basic Lin structure.
Everything between them is added in an effort to linearize that stage, is not a separate or proper gain stage.
The more you linearize that, the less global NFB you will need.

[Vostro]

If I simulate the two circuits posted, then the one with the 2 xtra resistors, simulate with about same gain bandwidth but with much higher THD-1000, 2nd 3rd harmonic way up, and 5th higher aswell.

Originally Posted by Ian Finch

For comparisons and interest, what model Yamaha amplifier is this taken from?

The circuit posted is about the same as Yamaha A-S300, A-S500, A-S700, AX592, AX596 and many others, it seems to be used in many of there amps over many years now. The first three mentioned are still in production, and in my opinion are very nice.

The A-S1000 and A-S2000 are totally different.

Regards