On Sun, 04 Sep 2005 09:25:27 GMT, Tim Martin wrote:

"Don Pearce" wrote in message
...

As far as I am aware, there are no class B amps around these days. Anybody
know of any?

Presumably this "high end audio manufacturer" makes Class B amplifiers:

http://www.norh.com/docs/amps/

"... 99% of all audio amplifiers today are Class B. Class B amplifier can be
built today so that its distortions are well below what the human ear can
detect and nearly to the point where it is unmeasurable.
Many amplifiers call themselves Class A/B. In reality, very few are. Early
Class B amplifiers had a problem known as switching delay. In a class B
design, a transistor works 50% of the cycle while another transistor works
50% of the cycle. In early class B amplifiers, there was a distortion
created between the time the devices were switching back and forth. Some
people referred to this distortion as notch distortion because there was a
notch appearance on an oscilloscope between the two waveforms.

Class A/B was created to leave the transistor conducting while the second
transistor was conducting. This created an overlap between the two signals.
The problem with this approach is that it created its own distortion called
gumming. This means that the signal would get a little fatter where the two
devices were both conduction.

Today, if you look at a properly designed Class B amplifier on a scope, you
will see no switching distortion."

I don't put too much store by that article. In fact I believe it is
entirely backwards in that many amplifiers that call themselves class B are
in fact class AB. Even a milliamp of bias current in the output devices is
enough to make this true, and holding an output pair at exactly zero bias (
no reverse bias or forward bias) is really hard - far easier to allow just
a little forward bias to ensure that the true horror of both transistors
turning off at the mid point can be avoided.

As for seeing switching distortion on a scope - well, if you could do that
you would have an amplifier of unimaginable horror. This statemenrt also
leads me to believe that this article is not written with great technical
expertise. Bujt then you say that this is a high-end audio manufacturer, so
this is no great surprise.

d