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Old 4th August 2009, 02:29 AM   #1
Bigun is offline Bigun  Canada
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Default Can you help understand limitations of ClassA/G ?

I'm wondering about the benefits of operating ClassA with low power rails and then using power-rail tracking to handle high signal levels. Most of the time I would anticipate operating within ClassA.

I put together a simple simulation to start with.

This kind of circuit is a bit of a mystery to me, and reading the threads that came up in a Search didn't shed much light on things for me. Does anyone have experience with kind of thing ?

Here's the schematic (one version is a reference ClassAB design, the other the ClassA with rail-tracking)...
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Old 4th August 2009, 02:32 AM   #2
Bigun is offline Bigun  Canada
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Here's what the output looks like

-with two different signal levels.

One signal is small and stays within the range of the ClassA stage, the other signal is larger and pushes the supply rails to track the signal.

Signal at output = green trace
Inner positive rail = red trace
Inner negative rail = blue trace
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Old 4th August 2009, 02:37 AM   #3
Bigun is offline Bigun  Canada
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The FFT 'distortogram' of the outputs at the two signal levels

ClassAB reference = blue traces
ClassAG = green traces

Here's the rub - even when rail tracking isn't needed, the output from the ClassAG amp still shows artifacts from the rail tracking circuity. If I 'disconnect' the rail-tracking feed so the ClassA amp section is operating independently then the FFT is clean.

So I conclude that the rail tracking injects too much 'rubbish' into the inner power rails. Somebody else has surely seen this before - is rail-tracking flawed or is there something I'm missing
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Old 4th August 2009, 02:45 AM   #4
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Gareth,
Quote:
So I conclude that the rail tracking injects too much 'rubbish' into the inner power rails.
To put it mildly...
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Old 4th August 2009, 03:01 AM   #5
Bigun is offline Bigun  Canada
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Hi Lumba, good to hear from you !

So, no solutions to this problem then ?

PSRR for EF output is supposed to be better than this...
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Old 4th August 2009, 03:09 AM   #6
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Quote:
Originally posted by Bigun

So I conclude that the rail tracking injects too much 'rubbish' into the inner power rails. Somebody else has surely seen this before - is rail-tracking flawed or is there something I'm missing

Quote:
Originally posted by Lumba Ogir
To put it mildly...
I think it's possible to do class G in such a way it won't have any significant negative impacts when it's not being used. So I'm not sure what's going on in your simulation.

The hard part with class G is what it does when you ARE using the added rails. All sorts of issues come up then like what's known as "rail commutation" which is limited by the speed of the power diodes. Douglas Self has written fairly extensively about class G and has both simulated and measured actual designs that have similar distortion to conventional class B designs when below the class G threshold--i.e. below 0.001% at 1khz.

The above is proof the rails need not "inject" or hurt anything when they're not being used. The performance of his class G design, below the switching threshold, is very similar to his optimized class B "blameless" amplifier. While I don't agree with everything Self writes, it's hard to argue with his published simulation and actual measurements of class G amps. And class A shouldn't change anything.
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Old 4th August 2009, 03:41 AM   #7
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But Gareth,

What is the great advantage? Reduce dissipation in the output devices, only to throw it all away in the commutating devices?

Unless there is a clear SQ advantage, I can't see much advantage myself.... You still have the problem of Vbe compression in the output EFs.

Hugh
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Old 4th August 2009, 03:59 AM   #8
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The advantage, to me at least, is a very high quality class A amp for 90+% of one's listening that doesn't heat half the house but still has enough power for listening loud when you want to. High power class A designs are horribly inefficient. They're nice in the winter in cold climates but only if you don't have a cheaper source of heat.

Crossover distortion, in a well designed amp, is typically the biggest distortion left. And the only real way to get rid of it,is Class A. So it depends on if you care about small amounts of distortion.
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Old 4th August 2009, 04:19 AM   #9
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Quote:
Originally posted by AKSA
Reduce dissipation in the output devices, only to throw it all away in the commutating devices?Hugh
When the amp operates within the range of the inner rails it is pure ClassA with some additional wasted power in the diodes used to protect the inner rail power supplies but these have a relatively small voltage drop across them so not much to worry in terms of commutating losses. The outer rails provide no current except for the idle current flowing through the ClassG driver devices.

When the inner rails can't cope, then the current has to flow through the additional BJTs that feed current to the inner rails and dissipation will climb much higher - but this only happens on transients or when there volume is cranked. The idea here is not to make a PA amp but to get the higher quality available from operating in ClassA for most of the time and yet have some headroom.

As I see this, it should in principle be possible to achieve real ClassA output over normal listening levels (for me) without excessive power dissipation because the inner rails are low voltage. And when it moves into ClassG at higher levels I'm not much worse off than ClassB.


RocketScientist - you echo my thinking; the million$ question is how to clean it up in order to realize this potential ?




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Old 4th August 2009, 04:33 AM   #10
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Quote:
Originally posted by Bigun

RocketScientist - you echo my thinking; the million$ question is how to clean it up in order to realize this potential ?
If you haven't found it already, you should start at http://www.dself.dsl.pipex.com/ampins/classg/g.htm There's lots of info there (and on his site in general). If you want even more, the 5th edition of his book might be worth buying. There's lots of other info out there, but Self is one of the few to cover class G in a fair amount of detail. He also posts here on DIYA once in a while.
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