Doubling (or more) output stage voltage capability.

[Mr Evil]

Something I don't recall ever seeing used in an audio amplifier before, I thought it might provide some inspiration for someone. It's not something that provides any particular benefit for normal use, but may prove useful for constructing high-voltage amps.

Just like parallel output devices can be used to provide higher current, so series output devices can be used for higher voltage. I'm sure I remember there being a proper name for this, but I'll be damned if I can remember what it is. The advantage is that the output devices can be of lower than normal voltage rating. As extra devices are added so the voltage requirements are reduced: With four (twice as many as normal) each one only has half the voltage to deal with. Power dissipation is also spread in the same manner.

Attached is a schematic showing the simplest possible realization, just to make it easy to see the idea. The inner pair of output transistors is driven as normal. The outer pair is driven to half the output voltage by a simple voltage divider, so voltage across each half of the output stage is shared more or less equally by two devices. There are a number of ways to arrange the output devices by switching around N and P-channel devices, but some arrangements are rather harder to make work than others.

More to follow...

[Mr Evil]

This attachment shows a more effective way of driving the outer pair of transistors. Local feedback is applied around each one, compensating for Vgs which ensures almost perfect voltage sharing.

[Mr Evil]

Lastly, here is a graph of the output and voltages across each output device with a 1kHz sine wave input. There are five traces on there but the two pairs of output device voltages overlap precisely. Each device has sees only half the rail-to-rail voltage across it at maximum.

So there you have it. Note that these schematics are for illustrtive purposes only; they are not likely to work perfectly in practice.

[markp]

That method has been used for years and years.

[phase_accurate]

A German dealer (BRM) once sold 400 Watt amp modules using this topology. They used tons of 2N3055 and MJ2955 transistors.

Regards

Charles

[Mr Evil]

Originally posted by markp
That method has been used for years and years.

Indeed, I am aware that it is not new, I've seen it in text books before, but as I said, I have not seen it used in an audio amplifier before (phase_accurate has though!), leading me to believe that plenty of people here have probably not considered it before. Thus I hope at least to provoke thought.

[markp]

Originally posted by Mr Evil

Indeed, I am aware that it is not new, I've seen it in text books before, but as I said, I have not seen it used in an audio amplifier before (phase_accurate has though!), leading me to believe that plenty of people here have probably not considered it before. Thus I hope at least to provoke thought.

Lots and lots of amps use this topology, like the old Luxmans, Tigersaurus, to name just two.

[DigitalJunkie]

I think the Leach "Double Barreled/Superamp" uses a similar setup also.

[markp]

Originally posted by DigitalJunkie
I think the Leach "Double Barreled/Superamp" uses a similar setup also.

Yes, it does.

[Mr Evil]

Ok, ok, I give up! Everyone has learnt of this already!