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Power supply impedance and class A amplifiers?
Power supply impedance and class A amplifiers?
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Old 14th May 2017, 10:17 AM   #1
milosz is offline milosz  United States
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Default Power supply impedance and class A amplifiers?

I'm trying to picture what the power supply "sees" in terms of needing to source and sink current for the output stage of a class A amplifier.... trying to think about power supply design in terms of it's impedance at audio frequencies.

With a class A/B amplifier, the supply is asked to source and sink currents above the quiescent current level when the output stage amplifies an audio peak in class B... these varying current demands at audio frequencies makes me think that low impedance for the supply is essential for good sound in such a design. I get that.

But in class A, the output stage is always drawing the same current, and the current is either being dissipated in the output stage or is sent through the load (i.e., the speaker)... does the power supply have to sink or source audio-frequency currents in a class A amplifier? It doesn't seem so, since the current drawn is always the same....if that is the case, then there's no need to worry about supply source / sink impedance...? Could that be right? I just can't wrap my head around this.
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Old 14th May 2017, 10:57 AM   #2
scottjoplin is offline scottjoplin  Wales
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In class A the output devices are conducting through the whole cycle, it doesn't mean it's always drawing the same current, it depends upon the design
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Old 14th May 2017, 12:40 PM   #3
milosz is offline milosz  United States
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Quote:
Originally Posted by scottjoplin View Post
In class A the output devices are conducting through the whole cycle, it doesn't mean it's always drawing the same current, it depends upon the design
So, you're saying, at some point in the AC signal cycle a class A amplifier draws more current than it does at idle? Wouldn't that only be true with one of the sliding-bias arrangements, like Stasis, etc?

For example, a mini-Aleph, that's the particular amplifier I am visualizing for this exercise. Fixed bias, heavily into class A. Does the power supply in a mini-Aleph "see" audio-frequency changes in current sink / source when the output stage is amplifying a signal?
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Old 14th May 2017, 12:57 PM   #4
scottjoplin is offline scottjoplin  Wales
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As far as I'm aware the only class A amp that draws a steady current is single ended amp using a current source and then only if it is powered from a single rail supply
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Old 14th May 2017, 06:28 PM   #5
Nelson Pass is offline Nelson Pass  United States
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Power supply impedance and class A amplifiers?
Stasis was not a sliding bias design.

Amplifiers biased by constant current sources will have
constant draw (obviously), and balanced push-pull Class A
output stages also tend to have constant draw.
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Old 15th May 2017, 01:03 AM   #6
milosz is offline milosz  United States
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Default Implications of constant current being sourced / sunk from supply....

Sorry about my comment re: Stasis, I don't really know how Stasis works, my bad , I made an assumption that it was a sliding bias arrangement. There ARE sliding bias designs, meant to keep the output stage in class A as power demand increases due to the drive signal. Stasis isn't one of these so Stasis isn't what I want to discuss. Neither are sliding bias designs.

As Mr. Pass has said, class A designs with constant current sources don't have varying demands on current from the power supply, likewise balanced push-pull Class A designs.

My question relates to considerations of power supply source / sink impedance for such constant-current designs. Obviously, the DC supply needs to filter out ripple from the AC line, and it is wise to use capacitors that will also attenuate higher frequencies that might get through the power line (but I assume there's at least a rudimentary RFI filter somewhere in the path of the incoming AC power, this is just good practice.)

So let's assume the power supply filter capacitors are quite adequate to remove ripple, and that RFI is minimal on the DC output. Let's further assume that there is enough "reservoir" capacitance to help stabilize the DC supply in case of minor short-term sags in the power line.

In a class AB design it's best to have DC supply sink & source impedance as low as practical through the audio band, especially at low frequencies where instantaneous sink / source current demand is quite substantial as the output stage drives the speakers to produce drum whacks, etc.

But here we have a constant current source / sink situation, I don't see the need to think about source / sink impedance of the DC supply at all, really. What I'm asking is- is that right? With a high-bias balanced class A push-pull amplifier, there's no audio frequency currents being sourced or sunk by the DC supply? All the audio current action is happening in the output stage itself? Yes?

I'm just trying to straighten out my thinking here.
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Old 15th May 2017, 02:01 AM   #7
jcx is offline jcx  United States
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Quote:
Amplifiers biased by constant current sources will have
constant draw (obviously)
only from the rail the ccs is on

the other rail has varying current from the SE output device
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Old 15th May 2017, 08:50 AM   #8
abraxalito is offline abraxalito  United Kingdom
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Quote:
Originally Posted by milosz View Post
With a high-bias balanced class A push-pull amplifier, there's no audio frequency currents being sourced or sunk by the DC supply?
In push-pull the supply current does normally vary, even with classA. To have a static supply current demand you need an SE topology (active pass element loaded by current source) and your load needs to be in parallel with the active pass element (not, as normally shown, in parallel with the current source).
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Old 15th May 2017, 09:06 AM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by jcx View Post
only from the rail the ccs is on

the other rail has varying current from the SE output device
Quote:
Originally Posted by abraxalito View Post
In push-pull the supply current does normally vary, even with classA. To have a static supply current demand you need an SE topology (active pass element loaded by current source) and your load needs to be in parallel with the active pass element (not, as normally shown, in parallel with the current source).
that makes three of us that are awake !
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Old 15th May 2017, 09:16 AM   #10
milosz is offline milosz  United States
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Quote:
Originally Posted by AndrewT View Post
that makes three of us that are awake !
So, you are disputing what Nelson says...?

" and balanced push-pull Class A
output stages also tend to have constant draw."
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