Quasi N30 ClassA amplifier

[AndrewT]

Hi,
have you seen Quasi's proposal for a ClassA push pull BJT amplifier using his obligatory Quasi (NPN) output stage?

Go and have a look for N30-classA @ http://www.adam.com.au/cgpap/QuasiWeb/index.htm

[quasi]

Originally posted by AndrewT
Hi,
have you seen Quasi's proposal for a ClassA push pull BJT amplifier using his obligatory Quasi (NPN) output stage?

Go and have a look for N30-classA @ http://www.adam.com.au/cgpap/QuasiWeb/index.htm

The cat's out the bag now!!! Just when I thought it was safe to .....(enter own activity words).

Cheers
Q

[Graham Maynard]

The Quasi class-A might place T6+T9+T10 at risk if a loudspeaker impedance dip blows a 2A output fuse.

I have experienced plain old mechanical fuse failure (not electrically blown) so I always check my circuits with first one fuse and then another pulled from the circuit.

Cheers ....... Graham.

[AndrewT]

Hi Graham,
could you explain the failure mechanism?

I still find fault finding (or operation outside spec parameters) more difficult to understand than "normal" operation.

[Graham Maynard]

Hi Andrew,

If the output transistors lose power, the drivers will exceed their SOAR rating and fail via the output base-emitter junctions+load.
This could lead to VAS failure via supply+47R without any current limit.

Fuse failure should remove supply from VAS + output driver supply unless current limited or reverse diode linked to ensure pull-down.

Cheers ....... Graham.

[AndrewT]

Hi Graham,
thanks.
But not done yet.

Does fitting the protection transistor across R12 & T6 protect only the VAS or does it also remove sufficient drive from T9 & T10 to protect them as well.

I usually scale the sensing resistor (16r+30r~=47r) on the VAS emitter to limit at about 3times the VAS quiescent current. In this case that would be just under 38mA, leaving about 25mA to feed R17 & R20 in the open fuse situation.

Does that still allow the drivers to fail?

I have let an amp play (quietly) with the fuses relaced by 10r and it never ocurred to me that the drivers may be taken outside the SOAR.

[quasi]

Hmm...I got the fuse in a pretty dumb spot. It's within the darlington configuration of the driver - output stage....dumb!

Indeed the driving transistor will be flying solo if the fuse failed and whilst glorious it's demise would be futile and quick.

I've moved the fuse.

http://www.adam.com.au/cgpap/QuasiWeb/index.htm goto N30-classA

Thanks Graham & Cheers

Q

[Graham Maynard]

Hi Andrew.

Where a VAS is current limited it is usually at 2x CCS current. This allows +/- CCS maximum drive.

Where loaded output voltage cannot follow input due to fuse failure, T2 could pull 4mA via VAS base.
If VAS gain = 50, then VAS Ie could be say 200mA, with Vce say 15V depending on driver failure modes, as might be caused by a load impedance dip at full power.

It is not likely that the output devices in this circuit would fail, but those fuses could put the drivers at risk when the amp is being driven at maximum output, esp T9 if T6 conducts at 0.2A, and T10 with 12mA on its base and a peak initial Vce of almost 30V !


Hi Q,

My browser not showing the updated circuit at present, but I am sure you have covered this.


Cheers ......... Graham.

[Dr.EM]

Has anyone built this yet? It looks interesting, I may want a different class A amp if I can't get the DoZ working. I already built a 33V DC single supply though which I doubt this can use in any means?