CLASS A AMPLIFIERS

Charlot

This web site goes into a bit more detail concerning the differenct classes of amplifier circuit designs http://www.knowledgeisfun.com/E/El/Electronic-amplifier.php

In general class A is much less effecient than either class B or A/B(most common design) but procduces less distortion (e.g.cross over distortion in class AB) than either class B or A/B. With the inefficency comes heat,caused by bias current, which must be dealt with by utilizing large heat sinks.This generally makes pure clas A unsuitable for high powered amplifiers. An example that comes to mind is Nelson Pass his X seriers is class A/B and ranges in power from 150 (X150) watts to 1000 (X1000)watts per channel. The XA amps are pure class A and range in power from 150 (XA150) watts to 200 (XA200) watts per channel.

Chuck

Chuck

Another aspect to consider is that Single Ended class A amps do not increase their power output in lower impedence loads e.g Pass XA series amps. Which is why you need to consider the speakers efficency in most cases when using pure classs A amplifiers.

Chuck

Unsound

As I understand it (from information gathered from the pass web site) the X series (class A/B) does double down but the XA (pure class A) does not. And as someone mentioned class A amps tend to be sweeter more detailed in the midrange.

Chuck

Chuck

Unsound

Here is the exact information fronm the pas web site I quoted.

"The XA-200 will deliver 200 watts rms into 8
ohms of impedance. This is the amplifiers class
A limit, driving the XA-200 harder or reducing the
impedance of the load will not convey any additional
power. Driving into a short will not convey any
additional power. These are not design oversights or
flaws in the XA-200, but rather a condition of the
very highly biased Class A operation. However, as
an interesting point of reference, 200 watts driving
a 87dB/1W/1m speaker will deliver a 112 dB (very
loud) average acoustic signal in a 100 cubic meter
room. Transient peaks will be somewhat higher."

Chuck

I think I read somewhere that this is acharacteristic of pure class A amps in general and that it also applies to tube amps. If I remember correctly, and I am getting old, I believe CJ told me that it was a characteristic of my premier 11a, but I am not sure. The topic came up with CJ when I was asking them questions concerning tube amps and driving thiel speakers. I maybe speaking to much to generalities and not enough to specific circuit topologies.

Chuck

Aldavis

The attached website (http://sound.westhost.com/class-a.htm) has a pretty good explaination of class A amplification form a lay persons perspective. I believe the Gryphon is designed as a push-pull topology (unsure) versus the Pass which is a single ended topology and depending on ones definition of pure class A the Gryphon may or may not fit the bill. But again I am not an expert on the subject.

Unsound I believe krell is a class a/b design

Chuck

Unsound

I went to krells web site and he states class A topology. It would be interesting to know if it is push-pull, single ended or what specific design they are.

Chuck

Unsound

That would make sense given the power output. I am sure the Gryphon is as well. It is interesting stuff as I have always found circuit design especially for audio interesting. I majored in Mechanical Engineering as I never had a mind for electronics even though I do enjoy it as a hobby.

Chuck

Unsound

I agree and I didn't mean to suggest that single ended amps couldn't produce high power what I mean't, and I may be wrong here, is that they don't produce the power output of a push-pull design e.g. Pass xa versus x series amps. And yes I remember reading the Stereophile article concerning the power output of the XA200 at least I think it was the XA200 falling short of rated output.

In general, and my logic may be off here as well, I think of class A amps (SET vs. Push-Pull) like I think of tube amps (SET vs. Push-Pull). SET tube amps typically have power outputs in the range of 3-50 wpc while push-pull varieties have power outputs in the range of 50-500 wpc. Anyway sure is fun to have these discussions as I usually learn something in the process.

Chuck

Onemug

I agree with you that in the end its the music not specifications or circuit toplogy that we listen to. And one must keep ones eye on the ball when reading and comparing specifications when trying to make a purcahse decision. But it does lead to some interesting conversations and I really enjoy the learning that goes along with it.

Chuck