What does "class a biasing" mean?

[PITTM]

a new update on the portaphile site says that V2 users can send their portaphiles in to have them upgraded with class a biasing, what does that mean? what will it do for performance? i imagine it will be worth it, since i only have to pay for shipping. heres the link.

http://www.portaphile.com/ordering.html

rj

 

[amb]

http://www.tangentsoft.net/audio/opamp-bias.html

 

[Firam]

"Class A" refers to configuring an amplifier so that its output devices remain turned on all the time. This reduces thermal variation and eliminates crossover distortion, which makes the chip perform better.

That means very little to me. More explanation please.

 

[jamato8]

A number of amplifiers are class A, AB or B but mostly AB. What this means is that in class A output power remains turned on all the time to maximum while in class AB it is turned on all the time to a certain amount of power and then B comes in raising that power as needed to the maximum output of the device. If you have an amplifier that puts out 20 watts in class A it will have the 20 watts ready all of the time, which can reduce distortion and improve sound. Class AB might have 10 watts ready all the time but have 20 watts in reserve ready to rise with the output need, which is determined by the amplitude of the signal driving the amplifier. This input signal is often the preamplifier or the the volume control. the greater the input signal that is input, the greater the wattage need to drive speakers or headphones. Past the point of the capability of the amplifier you start to get into what is called clipping (distortion) which can damage speakers or headphones and doesn't sound very good anyway.

John

 

[shimage]

Quote:

Originally Posted by Firam "Class A" refers to configuring an amplifier so that its output devices remain turned on all the time. This reduces thermal variation and eliminates crossover distortion, which makes the chip perform better. That means very little to me. More explanation please.

The output section of an opamp has two types of transistors. What's important is that they behave differently, and there's some distortion when you switch from one to the other. Biasing it into class A avoids the switching by keeping it on one side all the time (via a DC offset, hence "biasing").

EDIT: To tie this into the above post, if your offset isn't big enough, it might still switch (and thus, you're now class AB/B).

EDIT2: Rereading this, I think what I wrote is misleading... when you switch between the two transistors (when the current changes direction, if I'm not mistaken), it isn't perfect, so there's some distortion. Also, the two types have different performance characteristics, so you bias it such that the better ones are used. That said, I have no clue whether you can hear this distortion or not. Certainly there are other design considerations that are more important.

 

[aseltzer144]

Some good, concise definitons of class A operation have be posted to this thread. From my perspective, as a listener not an engineer, there are some other things to consider about class A amps.

One is that, while they are theoretically optimized for maximum sound quality, they run very inefficiently since the output circuitry is always on and is prevented from switching. For a power amp that you would use with a larger stereo system this means the amp will usually run very hot and need to be properly ventilated as such. In a portable amp, I suspect they too would run a bit warmer although I don't know if the small size of the componentry makes them feel any warmer to the touch. What you will notice in an amp like the Portaphile, which runs on batteries, is that when run as class A I suspect the battery life would be shorter than what it currently is. That would be a trade off in convenience versus sound quality to consider.

The other thing I have found, from experience with power amps, is that class A operation does not necessarily translate to better sound quality. I have an old 45 watt per channel amp that runs class A and its a great sounding product. I also have a 200 watt per channel Aragon 4004 which I believe runs as class AB. Most amps at that high a power rating don't run class A. Nevertheless, it remains a fine sounding amp. The class of operation is something that you really can't easily hear. Most of the amps on the market fall into the AB class which, with proper design, means they operate class A over most listening situations and fall back to a slightly less
pristine mode of operation when stressed.

In any case, if I owned a Portaphile and a class A upgrade were offered to me I would probably go for it.