Class B is the more common design used for solid state amps, especially smaller
and older amps. The class also applies to tube amps, but only talking about solid
state amps here.

In a class B amp, the output transistors must dissipate some of the total power being
supplied to the output stage. The rest is delivered to the load. At full output, most
of the power is sent to the speaker, but about 25% is lost by the output transistors
as heat. At lower levels, the percentage of power lost in the transistors is higher.
For example at 1/2 power, 50% is lost in the output transistors.
This means that the transistors must be relatively large and mounted on a large heat
sink. This adds size and weight to the amp. Also, these amps often use linear
power supplies - 60 Hz transformer, rectifier, etc. These large transformers also
add a lot of weight.

In a class D amp, the ouput transistors are operated as switches; they are either
full on, or full off, hence the name switch mode. When a transistor is off, there
is no current through it, and the power dissipated by the transistor is zero. When
the transistor is on, the voltage across the transistor is zero, so again the power
dissipated is zero. Think of the switch on a toaster. When it is off there is no
current through the switch or the heating element. No current, no power. When the
switch is on, the switch remains cold even though it is carrying the full current
to the element.
So the class D amp approaches 100% efficiency, delivering all of the power to the
speaker and having very little lost in itself. This means smaller transistors and
heat sinks. These amps often also have switching power supplies (same principle as
the amp) and eliminate the heavy 60 Hz transformer. A much smaller high frequency
transformer is used. So lots of size and weight savings.

The classes (A, B, AB, C) refer to the biasing of the transistor or tube used in an amplifier
stage. Class A is biased in it's linear operating region. Class B is biased at or near it's
off state. In an audio amp, Class B is used if the stage is "push pull" with one transistor
handling the positive portion of the signal and one transistor handling the negative.
Class AB is biased in the linear region, but not in the center of the region like class A,
but closer to the off state (the "cutoff" point). Although the device itself is operated in a
nonlinear manner, the overall response of the amplifier stage can be linear, and will be
for audio amps. Class C is biased beyond cutoff and used in certain RF stages.

Class D is not a biasing scheme in the same sense as the others. It was simply assigned
the next available letter, if I'm not mistaken. It does not stand for "digital".

So how do you get audio out of a couple of switches?
The transistors are switched at a very high rate. If you look at the average voltage level at
the output transistors, you will see a value that depends on the relative on / off times of the
two two transistors. If each are on for equal time periods, the average will be zero. This is
what occurs during a zero signal condition. If the positive transistor is on for a longer period
than the negative one, then the average will be some positive value. And the longer it is on,
relative to the negative side, the more positive the average will be. The same is true if the
negative transistor is on for a longer time compared to the positive. Except now the average
will be some negative value.
The circuit that drives these output transistors uses the audio signal to control the relative on
and off times. The result is an on /off time for the transistors that very closely follows the voltage
level of the signal. This means that the average voltage level at the transistor output also closely
follows the audio signal. Now all you need to do is remove the high switching frequency from the
output, which is done by filtering as it is above the audio range. All that remains then is the average,
which is a perfect analog of the input signal.

Size and weight savings are one thing.

Circuit complexity and reliability are another thing, though.

Guess which type of amp is seen on our service benches more than the other these days?

Class D is cool stuff, no question about that, and if spec'ing an amp or amp stack for a touring outfit, may indeed be what I might recommend.

The Crowns, however, also come with all that software controlled stuff, Digital Comm Networking lines, and the setup, care and feeding of them, still in its infancy, is the stuff of engineering types and not our young lady sitting at home and desiring to hear some music reproduction and perhaps her own electric instruments, Band in a Box, etc.

Another issue with the software controlled amp and its communication is that, just like with other softwares, we often see the need for firmware updates and the likes, which, incidentally at this point in time often mean that the amp has to go to some authorized service center. There have also been a growing list of mods issued in the service bulletins. This may be okay for the touring company, which may buy the things by the pickup truck load in the first place and have included readily available spares. Rachael will not have that option.

Her amp will be installed in basically one place in her home and that's it.

Meanwhile, it will be subjected to powerline surges, maybe a few lightning strikes further down the powerline, possibly a couple of user input mistakes as well as just the plain enough desirability of simplicity coupled with the fact that an older design has reached maturity means to me that out of the two choices given, the QSC is still my choice.


--Mac

Is that the trend now for new amps, software control?
Are they using it even when digital signal processing isn't used?

I used to have a DVD player with a reset button!
I no longer have it, and won't buy anything else with one either (except a PC).

Am wondering whether megafiddle drives a car or truck or if megafiddle is still hitchin' old Dobbin to the buggy... <grin>

Well, software control of power amps is a trend as far as the engineering of large soundsets is concerned. It brings a level of control over certain things that was hitherto rather difficult if not impossible to implement. With more and more use of software controlled environments, with virtual mixers on pad screens using wifi communication to control house PA systems and the like, with the advent of Line Array speaker systems that depend upon Time Alignment routines, it just makes sense.

But as I've been trying to point out, all that is not needed for Rachael's two speakers in the home, so why should she have to purchase it and why should she have to be able to deal with it just to power her home speakers? The answer is that she does not have to deal with it. There also is indeed a reliability factor, any time an engineerin design increases component count, which increase can be for perfectly valid reasons if one considers the *context* for which such features are implemented, which often makes them desirable features under the right circumstances, said component count will automatically change the statistical odds of the possibilities of failure.

As far as consumer products that incorporate software control, one will be hard put to find appliances of any kind that do not contain at least a minimal processor and bit of software of some kind inside them. Old linear or analog controls have already been religated to the trash heap of history. Even the lowly Washing Machine and Dishwasher now have Digital controls in them for timing and such, there likely is not a single factory left able to build analog clock movements. And for good reason.

The Crown amplifiers with the digital comm and other features is bound to increase in utility and the robustness of such will be totally solved over time, that's my prediction.


--Mac

At least they gave you the button.

I could relate the horror stories of devices like your player that can only be reset by a trip to the factory service center, but then I'd get in trouble for mentioning brand names...

There will be a day when such devices will be so robust that everyone will take such for granted.

Amd by that time, technology will birth something else brand new and the cycle will start all over again for whatever that will be.

By the time we got carburetors that actually worked rather well from idle through the full rpm range, nobody bought any because Electronic Fuel Injection flat blew that old technology right outta the water. Today we enjoy the full Stoichiometric from darn near zero rpm to redline, even on mom's little economy model.


--Mac

For Rachel keep the amp as basic as possible. My Crowns are indeed fairly complex and flexible in their function.
I used to be an electronics tech for a very well known Yacht Co..I can't tell you how much money was spent having me come out and do the secret hand shake,i.e.,master reset of a piece of gear.

IMHO
For home audio, or studio, you'd want a fanless design.
If anything is going to fail, it's likely going to be a mechanical part, like a fan.

Class-AB may be a more traditional design, but Class-D is now the more common if you consider all application of audio amplifiers. Sound wise, there's no difference.

I didn't realize that the Crown was so complex. I figured to just 'set it and forget it'. A couple of music site user reviews talks of excessive fan noise for these units. However, some say the issue has been resolved. Now to see if my local Guitar Center has them in stock to check out the fan issue.

Thanks to everyone who chimed in! It was very helpful.