Hybrid Class A / Class B design.

[GK]

G’day all.


Here is a design for a high power (500W RMS per channel) class A amplifier I’ve come up with. I’m planning to start building this amp up rather soon as the “ultimate” amplifier for my lounge room audio system.
The design is based on a differential op-amp topology with a common mode voltage servo to drive the speaker differentially (or bridged) so as to take advantage of even harmonic cancellation and slew rate doubling.
The most notable thing about it though is the use of two pairs of parallel connected Class B / Class A output stages to drive the speaker. The Class B output stages drive floating power supplies for the Class A output stages.

This makes a 500W RMS pure class A amplifier which only dissipates 320W of idle current.

I’ve scribbled a block diagram of the idea down and attached it below.

Just interested in some feedback on what other’s think of my idea, of if anyone out there has seen or come up with and/or built something along similar lines.

Cheers,
Glen

[theChris]

I seem to recall some variations on a theme. particularly a class D + class A idea where the class D amp basically generated some rail voltages for the class A output stage.

[GK]

Originally posted by theChris
I seem to recall some variations on a theme. particularly a class D + class A idea where the class D amp basically generated some rail voltages for the class A output stage.

G'day.

I actually considered using a class D amp for the supply rails, but decided aqainst it on grounds of added complexity. The extra watts dissipated by a class B stages isnt much, compared with the 320W being pumped out by the class A stages!

And a question for anyone who may know the answer - what's the typical primary to secondary winding capacitance of a typical 150VA 240V-9V+9V toroidal transformer??????????


Cheers,
Glen

[GK]

Another question for anyone with an answer......

How would a single section of a 12AX7 triode at 1mA anode current compare in noise performance to a common garden variety transistor such as a BC548 at 5mA Ic?


Cheers,
Glen

[Wavebourn]

I would go with a full bridge instead of couple of amps with a common diffamp with symmetrical outputs and strange common feedback. And I would not ever think about noises of the amp with +4 dB (or even 0 dB) input level.

12ax7 may give you more of equal gain on wider band with less audible distortions. But anyway, I would use something more powerful to drive transistors (less of them will be needed).

And... I don't think that class A and B amps driving output in series with each other (instead of when class D controls supply voltage) is a good idea.

[GK]

G'day


I would go with a full bridge instead of couple of amps with a common diffamp with symmetrical outputs and strange common feedback.


I’ve used this topology before in a bridged class AB amp with very good results. And I like to do something a bit different. The common mode feedback servo plays no role in the audio amplification, it just keeps the nominal DC potential between the outputs near zero volts, just like a DC servo in a normal amp. In a normal amp the servo is optional, but in a fully differential op-amp it is a necessity for setting the DC operating point, as the global feedback acts differentially, not with respect to a single output and ground.

Texas Instruments have a really good explanation of the differential op-amp topology here:

http://focus.ti.com/lit/an/slyt165/slyt165.pdf


And I would not ever think about noises of the amp with +4 dB (or even 0 dB) input level.


OK, I’m still interested in the noise characteristics of a 12AX7 out of curiosity though.


12ax7 may give you more of equal gain on wider band with less audible distortions. But anyway, I would use something more powerful to drive transistors (less of them will be needed).


I’m only planning on using a pair of 12AX7’s at the input – one as the differential input pair and the other as a cascode for the other one. They wont be driving the transistors – the circuit I’ve drawn up is a LOT more complex than that.


And... I don't think that class A and B amps driving output in series with each other (instead of when class D controls supply voltage) is a good idea.


Why??????
Properly implemented, I think it will work sweet. I’ve since made a significant topological change to my diagram above though – I’ll be driving the Class B stages from the output of the Class A stages instead of parallelling the inputs so as to prevent the class B stages loading the VAS stages.


Cheers,
Glen

[Wavebourn]

Originally posted by G.Kleinschmidt
G'day

Texas Instruments have a really good explanation of the differential op-amp topology here:

http://focus.ti.com/lit/an/slyt165/slyt165.pdf

Hmmm... You compensate a common mode drift; who will compensate a differential one?
I still don't understand.

I’m only planning on using a pair of 12AX7’s at the input – one as the differential input pair and the other as a cascode for the other one. They wont be driving the transistors – the circuit I’ve drawn up is a LOT more complex than that.

In such case I'd better used transistors, in my mind it is waste of tubes.

Why??????

Because they are in series, i.e. load between A class amp (without crossover distortions) and class B amp (with crossover distortions), the result is all crossover distortions on the load.

Properly implemented, I think it will work sweet. I’ve since made a significant topological change to my diagram above though – I’ll be driving the Class B stages from the output of the Class A stages instead of parallelling the inputs so as to prevent the class B stages loading the VAS stages.

Hmmmm... Power supply with crossover distortions... If will never sound sweet...

Look at your idea from a slightly different perspective.
It will actually split voltages between output transistors. You don't need your "powering" transistors work in class b, instead connect your power sources in series (they really are), and use transistors as emitter followers that regulate voltage that power your class A amp.
The same topology, slightly different arrangement. Used long time ago to produce more output voltage from opamps. However, such phase-shifted positive feedback is another story...

[Workhorse]

Glen,

Have you considered Grounded Bridge Topology so far......

K a n w a r

[GK]

A picture says a thousand words……….

I’ve just finished drawing a preliminary schematic for one channel, so here it is. This has just come fresh from my brain to the computer easel and it is not entirely complete – there are various bypass caps and other stuff still to add and there may be some errors.

The two Class B output stages and floating power supplies can be seen at the top. Immediately below in centre is the hybrid vacuum tube / BJT differential amplifier and VAS stages, either side of which are the Class A output stages comprised of multiple complementary feedback pairs. The common mode voltage and DC offset correction servo’s can be seen just off the bottom of the page

This is only about half the circuitry for one channel. It does not include the preamplifier section (which will also be a full differential opamp topology), the +/-40V Class B output stage unregulated power supply, or the +/- 50V, +230V and +/-15V regulated power supplies which will be implemented with discrete circuitry.




Cheers,
Glen

[GK]

Wavebourne:
Hmmm... You compensate a common mode drift; who will compensate a differential one?
I still don't understand.


The differential offset is dictated by the input differential pair's offset, just like the offset voltage with respect to groung is determined in a normal amplifier.
I'm using a servo to correct for that too!


Wavebourne:
Hmmmm... Power supply with crossover distortions... If will never sound sweet...


I disagree. The class A stages will exhibit a high degree of power supply rail rejection. Very little, if any measurable crossover distortion will make it from the rails to the load.