Axiom Home Page Forums General Discussion Stereo Odyssey Stratos - part 2

uhhh...that was a clear as mud. That article was obviously written with an electrically educated audience in mind. Thanks for the link though.

Well Craig, since nobody has offered a superficial review of the main amp classes, I'll try it. Class A operation makes all the power output transistors carry a heavy current flow at all times, regardless of whether there's any sound being reproduced at that instant. In pure class B operation, the transistors only carry current at the instant that they're actually amplifying a part of a sound wave. One that amplifies during the positive part of a wave is shut completely off during the negative part and the other transistor(or multiple transistors)is turned on for amplifying the negative part. The net result of these schemes is that the class A is very inefficient(maybe 20%), using power continuously and runs very hot, but since the current is always "on", there's no problem in turning on when the sound wave changes polarity from positive to negative and vice-versa. The class B is a lot more efficient(maybe 60%)and runs cooler, but it has to turn on and off very quickly(all this is happening in millionths of a second)when the sound waves crossover from one polarity to another, and since it can't do it instantaneously from a dead start(i.e. no current flowing), there's some "crossover distortion" introduced at those points. The class AB(somewhat misnamed; mostly B with a little bit of A operation)allows just a little bit of current to flow at all times(but much less than in class A)so that it accelerates from idle rather than a dead start and the crossover distortion problem is reduced while keeping most of the power use efficiency and cooler running of pure class B operation.

As a practical matter, any of the preceding classes can be designed with inaudibly low distortion, so this is more about bragging rights than real-world sound quality and you can pretty much feel free to ignore amp classes. Numbers count, not letters.

Thanks John.

How about an overly simplistic analogy?

Class A amp is like the 40W light bulb turned on. It is always on, always using 40W, hence hot and inefficient.
Class B is like the 40W light bulb with a manual switch. You can turn it on and off but there is that 'lag' time between on and off.
Class A/B is like a light bulb on a dimmer switch. A little current is always flowing although the light may technically be "off" (dark room with dimmer switch set way down low) which when necessary can go up to its full 40W and down again (quickly move dimmer switch up to full position).
Class AA/B is like the previous description except that its minimum current is usually higher such that the light bulb is always at say a full 5W all the time (the Class A part) but can bump up to 40W and back down again when necessary (the A/B part).

For some definitions of terms, this is a good site.
For the amp stuff again, this page is pretty easy.

Yes, chess, good sites, and for an overly simplistic analogy I like the one in the AB definition about the three cars at the stoplight with car A racing its engine at top revs, car AB with engine idling and car B with its engine turned completely off.

spiffnme, congrats on the new amp. As a proud owner of both the Stratos and HT3 from Klaus all I can say is you will be a happy camper.

So far so good!

Great links! I really enjoyed the second one. It answered a lot of questions for me. I was wondering what Class D amps were...

Sorry for the delay...

Stratos face
Stratos in rack

Spiff, form those pictures it looks like the Stratos might interfere with your VP150. Who knows how badly those huge heat-sinks can refract sound from the left tweeter!