In
alt.electronics,sci.electronics,sci.electronics.ba sics,sci.electronics.design,
"Karl Uppiano" wrote:
"John Popelish" wrote in message
...
pil wrote:
"Joerg" wrote in message
...
Johan, I don't know these or many other audio books. Just one hint:
Nowadays I
would certainly try to learn about class D audio amps, the digital
kind.
That
seems to be where technology goes.
The Class D amps are certainly making many inroads in many consumer
products, but how fast it replaces analog power amplifiers remains to
be seen. For stereos sold at Wal-Mart, the transition may already be
100 percent, but I doubt you could find a 'Class D' amp in high-end
audio stores, or if so, that it sounds as good as any analog amplifier
in the store. Analog amps in that area (admittedly not a large
consumer market) will surely be around for a long time, unless Class D
amps show substantially more improvement over current state of the
art.
For the Original Poster, Douglas Self is one writer/designer of
audio power amps, and here's the relevant part of his site:
http://www.dself.dsl.pipex.com/ampins/ampins.htm
For some really interesting schematics and discussions, click on Power
Amplifier Design, then Distortion in Power Amplifiers. Read the text
while all the figures and plots load. I found the rest of his site
interesting as well. I recall reading amazon.com reviews of his book,
and the opinions were varied - some really liked him, but some
suggested books by other authors instead.
There are large numbers of hifi audio power amplifier schematics
and discussions on the Web. For some gurus and their musings and
schematics, google for Marshall Leach, Nelson Pass and Borbely.
It's debatable whether class "D" is really digital. While it's true that
internally, the signal is switched, it isn't a digital signal. It's pulse
width modulation. The pulse width is continuously variable, making it an
analog quantity.
That (punse-width modulation) is the traditional Class D approach.
Some "Class D" amps work just like sigma-delta modulators in A/D
converters. The output is either a 1 or a 0, and the time is also
quantized: the output only changes states at specific times, at the
sample rate, which is very much higher than the audio range, usually
around 2 MHz. These have advantages over straight PWM, but with an odd
disadvantage that there can be significant delays (as in a few
milliseconds) between input and output.
The amplifier you referenced is a pulse width modulator. The input is
analog, and the output is analog. Noise and distortion are not particularly
outstanding, either. It's primary claim to fame is extremely high
efficiency, approaching 90%. It might be acceptable for battery-powered
equipment or automotive audio. Conventional class A-B linear amplifiers are
still preferable for high fidelity.
This is still very true, OTOH there are amps designed for
"home-theatre" applications using Class D amps (for small size and low
heat - compare the heat output of six 25-watt Class B amps vs. Class
D).
TI's Class D audio amps claim less than 0.1 percent distortion,
which is allededlly 'fairly good' but the spectrum of the distortion
products can cause a harsher sound than an analog amp (especially a
tube amp) with the same or even a much higher distorion figure. These
Class D chips can be regarded as the 'dumbing down' of audio much the
same way as lossy psychoacoustic compression such as MP3.
TI has been notable in promoting their Class D chips for use in
home theatre '5.1' multichannel audio amplifiers. These are either a
significant advance in audio, or an abomination to sound reproduction,
depending on how good your hearing is.
Here's a web site that has some great practical designs and discussions:
http://sound.westhost.com/index2.html
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