From: "Bruno Putzeys"
Newsgroups: rec.audio.high-end
Subject: Re: Future of High-End Digital Amplifiers?
Date: 6 Oct 2003 15:16:56 GMT
Hi,
First I'd like to point out that "digital amps" is a misnomer. There
are two categories:
1. Analog-controlled class D. Switching amplifiers with an analog
input signal and an analog control system. Normally some degree of
feedback is present.
2. Digitally controlled class D. Amplifiers with a digitally generated
control that switches a power stage. No error control is present.
Those that do have an error control can be topologically shown to be
equivalent to an analog-controlled class D with a DAC in front.
The latter category initially delivered a success in the form of the
Tact Millennium. However, by their mere existence this device (and
another one of my own making, the "PPDSD" which performs marginally
better) prove that obtaining good performance from such a contraption
is largely an analog design exercise - a very complicated and
expensive one at that. After all, the distortion phenomena that stand
in the way between a perfectly formed digital control signal and a
perfect analog replica are inherently analog. Similarly, cheaper
digital class D's (such as Sony S-Master and TI's) go on to show that
at practical price and complexity levels, performance is quite
abysmal (better than 0.1% THD is unusual and be sure it ain't only
third harmonic!)
One should ask the question: would any D/A converter designer in his
right mind make it using power components? Probably not. Then how
about the old argument that digital-to-the-end is best? Well, I
think the D/A barrier should be precisely there where it allows the
signal chain to perform at its best and why should we believe that
this is necessarily right at the end? Quite obviously the concept of
a digital class D amplifier was dreamt up by DSP guys who presumed
that the signal should be kept out of the big bad analog world and at
the same time that the power stage, power supply and filter (all
analog in nature) would perform flawlessly.
The former category is a different ball game. Although most
commercially available implementations are well short of this ideal,
*analog* class D amplifiers can be made with performance figures
giving the digital variety a run for their money, at a price well
below even the cheapest digital class D's. They can have vanishingly
low output impedance right across and beyond the audio range (which
the digital ones can't!) and frequency-independent distortion (for
that "zero-feedback sound") is actually easier to achieve than with
digital ones.
So how about sound? The highish HF output impedance (caused by the
output filter) of amplifiers without post-filter-feedback (all
digital ones and many analog ones too) is responsible for their oft-
quoted tube-like warmth and air. At low frequencies the filter
impedance is low, resulting in a commanding, dynamic bass. Because
they previously had a reputation for sounding harsh (due to people
who hadn't heard them but presumed that switching couldn't mean
anything else), about every modern entry in the field was heralded as
the "first audiophile class D". Read a review about the Bel Canto Evo
or the Sharp 1-bit (which is analog btw) to see what I mean. Keep in
mind that if a device sounds radically different from what you've
held in high esteem previously, there's usually something fishy going
on.
Unfortunately, while warm and airy is nice, it isn't all you need for
real audiophile sound. I like to think audio components should sound
neutral and transparent too. A frequency response that wanders 10dB
off the line at 20kHz isn't conducive to neutrality. High THD isn't
good for transparency, especially when it goes up with frequency.
Ergo a Tact (which has flat and low THD but a nonflat frequency
response) sounds transparent but not neutral, and your average
transistor amp (which has a low output impedance but sharply
increasing THD) sounds neutral but not transparent.
Now, the technology (if you can call a circuit with 16 transistors
that) to deliver low output impedance with frequency-independent low
distortion in class D exists. Built with audiophile-grade parts it'll
frighten the pants off any high-end amp (while I'm the designer of
that circuit and thus some care reading this statement is warranted,
I do have a lot of serious folk to back up my claims). Of course,
having these characteristics it sounds more like other high-end amps
than that it sounds different, in the same vein as that the best tube
gear and the best solid state gear don't differ by miles in sound.
The upshot is:
1) digitally controlled class D: dead end street.
2) analog controlled class D: definitely the future, although you
shouldn't expect it to flatten competition from traditional solid-
state and tube amps by a tremendous margin. In the very high end
segment the three are bound to coexist for a very long time. In
mainstream gear, class D is certain to take over the scene completely,
although one serious problem remains: building a good class D amp is
an order of magnitude tougher than a linear amp, and the knowledge
required is much more diverse. It may take long before each large
company has at least one knowledgeable designer. It won't stop them
from putting class D based products on the market, but until then and
unless they buy completed amplifier modules from specialist vendors
(which eastern companies rarely do, they'd rather commit harakiri
than having to swallow their pride), they will be putting out
seriously substandard products for years to come.
Regards,
Bruno Putzeys
(Chief engineer class D audio at Philips DSL)
