[anastassios]

Lets make the assumption that I know exactly what pure class A is.
I did search but I didn't manage to find that much info on dynamic class A.
So if you have any knowledge or links please share.
I was wondering how it compares to pure, it's disandvantages (if it has any other advantage than power consumption -not likely I guess-), etc.

 

[Carl]

I assume it means something that is in Class A for main bulk of the music, but slips over into Class A/B for the loud bits. In other words a sugar-coated way of saying Class A/B.

 

[Clutz]

I believe dynamic class A means that the amount of biasing current varies dynamically with changes in load. Thus, the amp adjusts the amount of bias current automagically in order to keep the amp operating in class A, but not using more current than is necessary. Typically in pure class A operation, the amp has to be very deeply biased into class A such that the current demands placed on the amp by the headphones will never (or only *very* rarely) draw the amp out of class A operation and into class AB operation.

 

[a1rocketpilot]

Isn't that how Nelson Pass' Aleph current source works? By tracking the load instead of being a constant current source?

 

[anastassios]

Quote:

Originally Posted by Clutz /img/forum/go_quote.gif I believe dynamic class A means that the amount of biasing current varies dynamically with changes in load. Thus, the amp adjusts the amount of bias current automagically in order to keep the amp operating in class A, but not using more current than is necessary.

That's what I have figured out so far.
But it must have it's drawbacks or else there would be no more reason for the pure to exist.
I'm guessing the variable biasing may be causing some distortion.
How ? What kind?

 

[Clutz]

Quote:

Originally Posted by anastassios /img/forum/go_quote.gif But it must have it's drawbacks or else there would be no more reason for the pure to exist.I'm guessing the variable biasing may be causing some distortion. How ? What kind?

I take issue with the argument that dynamic class A operation must have drawbacks relative to pure class A operation, particularly on the basis of it's audio performance, based on the logic that if there weren't any drawbacks to it (dynamic class A), that it would obliviate the need for pure class A. It's possible that dynamic class A is a relatively new idea which just hasn't taken hold yet. It takes time for ideas to spread through, and be accepted by a population. It's also possible that it's a substantially more difficult to design and implement a dynamic class A amplifier than a pure class A, if only because people are more familiar with pure class A topologies.


That said, I would tend to think that there probably are drawbacks. If I were to hazard a guess, I would bet that dynamic class A amplifiers are probably less immune to power supply noise than pure class A, becauase the power supply noise might negatively impact the ability to dynamically track the current demands. I would also guess that sudden changes in current requirements might cause the amplifier to temporarily leave class A operation and move to class AB operation while the mechanism that is dynamically biasing the amplifier takes a few moments to adjust. I'm not clear on how dynamic class A amplifiers work, but my guess is that they employ some type of feedback mechanism to sense the current load, and adjust the bias supply based on it.

But that's just a guess, perhaps amb, tangent, or kevin gilmore might chime in if they know more about it.

 

[jcx]

There would seem to be little need, where a power amp could save 100+W, headphone amps only require single digit W, if that

so any extra circuit complexity or performance compromise would not be offset by cheaper heatsinking, fewer output devices and smaller power supply that would obtain at Loudspeaker power levels

the possible exception would be if battery life could be extended with superior performance by dynamic Class A bias

but the likely advantages of dynamic Class A over Class AB are again smaller in headphone amps where the output transistors can be 1-2 orders of magnitude faster than in power amps and the Class A region of the AB stage in a headphone amp may cover the drive for 90+% of listening

 

[anastassios]

Quote:

Originally Posted by Clutz /img/forum/go_quote.gif I take issue with the argument that dynamic class A operation must have drawbacks relative to pure class A operation, particularly on the basis of it's audio performance, based on the logic that if there weren't any drawbacks to it (dynamic class A), that it would obliviate the need for pure class A. It's possible that dynamic class A is a relatively new idea which just hasn't taken hold yet. It takes time for ideas to spread through, and be accepted by a population. It's also possible that it's a substantially more difficult to design and implement a dynamic class A amplifier than a pure class A, if only because people are more familiar with pure class A topologies.

Could be true of course, but (perhaps like you) don't think that's the case here.

Quote:

Originally Posted by jcx /img/forum/go_quote.gif There would seem to be little need, where a power amp could save 100+W, headphone amps only require single digit W, if that so any extra circuit complexity or performance compromise would not be offset by cheaper heatsinking, fewer output devices and smaller power supply that would obtain at Loudspeaker power levels the possible exception would be if battery life could be extended with superior performance by dynamic Class A bias but the likely advantages of dynamic Class A over Class AB are again smaller in headphone amps where the output transistors can be 1-2 orders of magnitude faster than in power amps and the Class A region of the AB stage in a headphone amp may cover the drive for 90+% of listening

To tell the truth it was a search for a speaker amp that brought about the question.

 

[amb]

Dynamic class A has been around since the mid-1970s. Nelson Pass claims the invention of this idea (and holds a 1974 patent on it), the speaker power amps he designed during his tenure at Threshold Corporation used this scheme. Many Japanese companies then made copycat designs (Pioneer, Kenwood, Technics, Sansui, and others all introduced variations of the theme, and coined names such as "Non-switching", "New Class A", "Super A", etc.).

Clutz is correct in his description of how dynamic class A works. It has a adaptive output stage biasing circuit, which measures the output current and then in turn sets the output stage bias dynamically. At low output conditions, the biasing is kept low to keep the quiescent current low. As the output current demand increases, the bias is increased to prevent the amp from dropping into class AB (i.e., the output transistors are never allowed to switch off).

Obviously, dynamic class A solves a problem associated with "pure" or static class A designs; namely, the high constant current draw and hence requirement for a large power supply and output stage heatsinks. However, static class A has an advantage in that the high constant current flow through the output stage keeps the output transistors operating at a more linear region of its transfer curve, which reduces harmonic and intermodulation distortions. Dynamic class A does not possess this advantage. There are claims that dynamic class A's continuously changing bias leads to a form of dynamic distortion, although I have not seen concrete proof one way or another. It is worth noting that Nelson Pass has not used this technique in his own designs for some 25 years now -- he said that at reduced bias levels the sound quality is compromised.

For high power speaker amps, large power supplies (big transformers, filter caps, etc.), massive heatsinks and many paralleled pairs of output transistors constitute the bulk of the cost of the amp. Thus dynamic class A is more about cost-savings (and miniaturization) than about enhanced performance, especially in the 1970s-80's Japanese mid-fi products. And the "dynamic class A" aspect was used as a marketing jingle. This is not to say that dynamic class A is without technical merit -- it does avoid crossover distortion. Whether crossover distortion is the most significant evil in a particular amp is a whole different matter.

As jcx points out, for headphone amps, static class A power supply and heatsinking issues are scaled down to a much more manageable degree, so that (at least on mains-powered home amps) the complexity of a dynamic biasing circuit is not really justified.