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Old 4th November 2009, 07:13 AM   #11
Mooly is online now Mooly  United Kingdom
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Magura,
Nice link

Your comments earlier on the subjective quality deterioration you found when turning the bias current down... that's a whole new problem. That a given amplifier design sounds "different" at different bias currents while still meeting all the requirements of Class A for the expected output level. Perhaps that applies differently to different topologies/designs.

Maybe there is an ideal "sweet spot" of bias for many designs, above and below this and performance not so good.

That would require some serious double blind listening tests I think
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Old 4th November 2009, 11:09 AM   #12
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Mooly,
Quote:
That a given amplifier design sounds "different" at different bias currents while still meeting all the requirements of Class A for the expected output level. Perhaps that applies differently to different topologies/designs.
It depends rather on output device characteristic versus biasing point, but I would not worry too much.
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Old 4th November 2009, 03:23 PM   #13
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The bias current does not vary with the music as in sliding bias etc, it is determined by the max possible output for a given volume setting.
If the bias current doesn't vary, the amp doesn't play music.
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Old 4th November 2009, 05:23 PM   #14
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Hi Steve
Quote:
Originally Posted by Steve Dunlap View Post
If the bias current doesn't vary, the amp doesn't play music.
You know what I mean more than most.

An output pair in pure class B (npn,pnp... bases tied together) plays music... look no bias
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Old 4th November 2009, 05:26 PM   #15
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Quote:
Originally Posted by Mooly View Post
I am talking about pure Class A in the accepted sense of the definition.
The bias current does not vary with the music as in sliding bias etc, it is determined by the max possible output for a given volume setting.
A class A amplifier rated at 100mw or 100watts correctly meets the definition... the bias is fixed.
Then you will want to drop

"my way of thinking means the "old" definition, that the output devices remain in conduction for the full cycle"

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Old 4th November 2009, 05:50 PM   #16
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Nelson and Steve, I can see I'm not going to win on this one am I ? lol

You both know what I mean.
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Old 4th November 2009, 06:12 PM   #17
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Quote:
Originally Posted by Mooly View Post
I am talking about pure Class A in the accepted sense of the definition.
The bias current does not vary with the music as in sliding bias etc, it is determined by the max possible output for a given volume setting.
A class A amplifier rated at 100mw or 100watts correctly meets the definition... the bias is fixed.
I think I got the drift what you wanted to implement. I think you wanted to make 100W pure class A amp but you won't likely be needing 100W most of the time. When you are using it as headphone amp, dissipating 200W as heat for 100mW output is such a waste, right? But by definition, Class A amps requires output devices to be in full conduction region. Thus, high power dissipation. Let's say you wanted to put a switch to limit biasing current of output devices, how can you be sure that all output devices are in Class A conduction? Your choice is to use smaller power amps for such applications.

But putting the output devices in Class A (full conduction region) even for low power amps (such as 25W) will not make an amp low dissipation. It will still dissipates quite a lot as heat compared to what it'll output as an audio power.

Of coz, higher powered amps with many parallel output devices put in low biased setting may not work as Class A. You'll need minimum constant biasing current to put transistors in full conduction.

What kind of output devices you are trying to use? (Power BJT or Power MOSFETs)?
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Old 4th November 2009, 06:34 PM   #18
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Hi megasat16
Easy one first... it's all just a theoretical idea, no design exists or is in the process of becoming a reality. Just an idea to discuss.
I am meaning one amp, that performs as well at 1 watt with the corresponding bias for that output, as it would at 100 watts (your example) and the bias set for that output level. The only thing that alters is the (no signal) quiescent current in the output stage, and that alters in proportion to the volume controls physical position, not the signal. Knowing the max signal that could ever be applied (2 volts rms say from a CD player) puts absolute known limits on it all.
And 1 watt Class A or 10 watt Class A is very very much cooler than 100 watt Class A.
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Old 4th November 2009, 07:12 PM   #19
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Quote:
Originally Posted by Mooly View Post

Nelson and Steve, I can see I'm not going to win on this one am I ? lol

You both know what I mean.
I think you mean "the sum of the bias currents in the out pair(s) does not vary". It does go up and down for each of the pair as music or other signal is output into a load. At least that is my understanding.
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Old 4th November 2009, 07:36 PM   #20
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Originally Posted by Mooly View Post
Hi megasat16
Easy one first... it's all just a theoretical idea, no design exists or is in the process of becoming a reality. Just an idea to discuss.
I am meaning one amp, that performs as well at 1 watt with the corresponding bias for that output, as it would at 100 watts (your example) and the bias set for that output level. The only thing that alters is the (no signal) quiescent current in the output stage, and that alters in proportion to the volume controls physical position, not the signal. Knowing the max signal that could ever be applied (2 volts rms say from a CD player) puts absolute known limits on it all.
And 1 watt Class A or 10 watt Class A is very very much cooler than 100 watt Class A.
Why don't you try tapping directly into last pre-driver stage of pure Class A amps for such small output power and cut off the power (and biasing) to the final output devices?
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