Dear forum members,

What is the purpose of the two series diodes in this amp?

Thanks and regards

 

They limit the voltage to 1.2V at the base of Q6. That gives 0.6V at the emitter of Q6.
That limits the current through Q6 to 0.6V/18Ω (R28) which is 33mA.
Without it an overload protection stage ( which may or may not be present, and may be off screen to the right) would destroy Q6 if it was activated.

 

Thanks Ian0, the amp does not have an overload protection circuit - only a fuse. Also, the sound impact is like a compressor/limiter, given that this is the power stage of a mixer M911 by Acoustic Control

 

Ops, this might be an overload protection. Anyway I am more interested in the sound impact of the two diodes.

 

Yes.That’s an overload protection circuit, but not one that would blow up the VAS transistor.
So my explanation is not the answer. Another possibility is that it is done to prevent the amplifier from going into hard clipping, because the output circuit will take a long time to recover, and during the recovery time, there can be a shoot through problem where both top and bottom output stages get turned on at the same time. That is usually destructive.

 

It looks to me like those two diodes compensate for the two base/emitter drops in Q7 and Q8.
Showing just a snip of a circuit is an excellent way to confuse those able to do an analysis of what the whole circuit does,, by the way.

 

MisterBill2, agree, the compete amp circuit is in post#4.
By the way, "Acoustic Control" has put the series diodes in the driver stage here. Now I am more confused by the VAS connection and the role of D1 and D2

 

It looks to me like those two diodes compensate for the two base/emitter drops in Q7 and Q8.
Showing just a snip of a circuit is an excellent way to confuse those able to do an analysis of what the whole circuit does,, by the way.

No, they are just a clamp. They should only conduct if the long-tailed pair is completely out of balance. However, the circuit voltages are wrong.
R4, R5 and R7 bias the LTP to 16V, that means the tail current is 98V/25k = 3.9mA. with the LTP balanced that is 1.96mA down each side, which is enough to bias the diodes into conduction. No wonder it doesn’t sound good.
In the second circuit D1 and D2 clamp the bootstrap circuit so it cannot exceed the positive supply.

 

In this circuit the four diodes certainly are a clamp. Rather a harsh clamp , but it may save transistors. Evidently the protection of the amplifier is required.
It might even be an attempt to simulate some of those 1960's severely over-driven tube-type amplifiers. I remember the sound, it was a part of that music. The 50 watt amplifier delivering about 75 watts and into hard flat-topping on the poor 6L6 stage.