"Martin "Schöön"" wrote in
message
(Martin Schöön) writes:
Try http://www.ciaudio.com/ucd_aes.pdf
Thanks, I'll study it imminently.
I have also found quite a bit of literature on
International Rectifier's web.
This Saturday was pretty gloomy weather-wise so I spent
some time class-D-surfing.
Here is something I found (you have to scroll down to the
middle of the page):
http://www.hificritic.co.uk/scene/news.aspx
Summarized as follows:
"
From a technical viewpoint it now seems permissible to:
1 Feed broad-band radio frequency noise into the power supply outlets
2 Feed broad-band radio frequency noise to the line and ground connections
3 Drive broad-band radio frequency noise into the speaker cables and
loudspeaker. (up to 500MHz with up to 2V generally measured at around 50kHz)
4 Define the output impedance using a significant passive filter, with a
result which varies with frequency and is dependant on speaker loading
5 Allow the amplifier to be marginally or completely unstable with either
high or open circuit output loading
6 Employ soft compressor clipping circuits prior to full power clipping to
prevent feedback saturation.
7 Employ high order negative feedback to improve in-band distortion figures
and low frequency output impedance.
8 Specify numerically high damping factor at low frequencies and claim that
this guarantees fine bass.(regardless of the interface to the loudspeaker or
any other property of the circuit)
9 Use steep low pass filters to limit the upper high frequency range,
partially negating the purpose of wider bandwidth, source material e.g.
SACD, while the resulting filter phase shifts may be audible in the working
band.
10 Have low bandwidth input circuits which are highly susceptible to stray
high frequency input signals, including upper band noise shaper signals and
DAC artefacts. The result is poorer treble sound quality and measurable
distortion.
11 Have power output circuits with poor high frequency resolution resulting
in high levels of intermodulation products at the high frequency end of the
spectrum
12 Have 'sampler' noise-shaped noise floors. The latter vary dynamically
with the level, frequency and complexity of the input signals.
13 Have comparatively small power supply reservoirs, in the light of their
low frequency output current potential and available power.
14 Have thermal dissipation limitations due to the small power module size
which means that dynamic variations are present in the performance with time
and temperature.
15 To protect the fragile output stages all kinds of pre-clip and aggressive
fold back protection regimes are included which are frequency dependant and
are also programmed for duty cycle. Unexpected sound quality variations may
result when operated at higher powers and with more difficult loads.
16 Operate at an equivalent sample rate which is insufficient for good
resolution above 7kHz. DSD, 1 bit pulse-width modulation operates at 2.4MHz,
nearly ten times the usual rate presently used in Class D amplifiers.
16 Deliver high, constant DC voltages (up to 70V) relative to local ground
at the output terminals and hence also the loudspeaker connections and
cable. (not of course between the +,- terminals as both of these are at the
same dc potential)
17 Use a high feedback switch mode power supply which has to react
dynamically to the power draw variations of the power amplifier with the
music programme. Generally these are designed for supplying dc and are
demonstrably imperfect faced with near audio bandwidth loading at a wide
dynamic range. Essentially the supply constitutes a form of audio amplifier
yet it was never intended to be optimised for such duty.
"
I would rate some of these concerns as being valid, and some as being
questionable.