Re: how good are class D amplifiers?

"Don Pearce" <nospam@xxxxxxxxxx> wrote in message
news:4654f936.22358906@xxxxxxxxxxxxx
On Tue, 22 May 2007 16:12:32 +0100, Jim Lesurf
<jcgl@xxxxxxxxxxxxxxxxxx> wrote:

In article <4653a7d1.168858750@xxxxxxxxxxxxx>, Don Pearce
<nospam@xxxxxxxxxx> wrote:
On Tue, 22 May 2007 09:19:09 +0100, "Serge Auckland"
<serge.auckland@xxxxxxxxxxxxx> wrote:

Indeed, and in my previous post of the criteria, it
was stated that THD should be measured at all
frequencies 20-20k and refers to all powers and all
loads for which the amplifier was designed. In
practice, the measurements are actually THD+N as this
is what distortion meters actually measure. Of course
the use of a harmonic analyser for distortion
measurement won't pick up the +N component, but as a
practicing engineer, I found the use of such an
instrument to be tedious in the extreme, and
unnecessary when an overall THD+N figure was so easily
achieved.

S.


The problem becomes more complex when you use an FFT
analyser, as I suspect most are these days. You then
need to consider the number of points in the FFT, and
the way they display noise. Discrete signals are easy -
whatever you do with the FFT, they look the same size,
but the "+noise" bit will change with the number of
points.

Erm. It should be the total noise in the audio range.
This means that however many bins it was divided into
becomes irrelevant as they are then summed. Although I'd
agree that a small fraction of the noise will be in the
input signal bin and would be 'lost'.

In recent years I've tended to use a Stanford
Instruments unit that combines a test waveform generator
and an FFT specan, and 'automates' the process as you
wish. The trick, of course, is to know what process to
specifiy and to understand how to interpret the results
- especially when the spectrum on the screen isn't
simple. :-)

The noise floor problem is more significant when reviews
simply display the floor value in terms of the per-bin
level without having any clue what resolution bandwidth
they are using. In those cases your comment does indeed
apply, and makes the floors shown in some magazines
worthless. Having tried discuss this with one or two
people I fear that this issue whooshes over the head of
some of them. Although there are others who clearly
understand it, but don't use such meaningless plots.


Exactly - although the maths is very easy - just add 10
log (audio bandwidth / (bin bandwidth * windowing ratio))
to the noise level in dB. But as you say, this appears to
be beyond most people.

The problem is that you must do this to the noise, but
not to the discrete signals, and it can get tricky
sometimes separating the one from the other.

Are there many distortion analysers any more that
simply null the fundamental and display the sum of the
rest?

Dunno. The last one I used a lot was the Sound
Technology 1000A about two decades ago. This was very
nice, but took a few seconds to settle into a null, etc,
whenever you altered anything. Worked down to about
0.002% though, IIRC. I think that part of the delay was
for the light bulb in the oscillator to settle when you
changed frequency. ;->

I still have a couple of those tiny bead thermistors in
vacuum tubes that are really good at stabilizing Wien
Bridge oscillators. Better than light bulbs, I think.

I've experimented with all of the common means for stabalizing the output of
Wien Bridge and State Variable oscillators.

Light bulbs, thermistors, and CdS cell approaches all work, but suffer
because they are limited by the response time of their sensitive elements.
In the case of the light bulbs and the thermistors, the response time is set
by the thermal properties of the device.

CdS cells respond far faster. You end up slowing their response down with
electrical circuits, but you can control the response of the electrical
circuit with a lot more flexibility than having to accept the "pig in a
poke" response time characteristics of the light bulb or thermistor. This
allows you to tailor the settling time of the oscillator more ideally.

The other three common means of controlling the response time of the
oscillator are a FET, a VCA, and a analog multiplier, which of course the
VCA is a special case of.

In the end, the distortion of an analog oscillator is dependent on the
sharpness of the frequency selectivity of the filter that is used to make
the oscillator, and the linearity of the means used to control its output.
A state variable filter and a VCA or analog multiplier seemed to be the best
alternatives.

However, the most practical means for producing a sine wave is a table of
numbers that describe a sine wave, read out of storage and convtered to a
signal through a DAC. Now that we have inexpensive ADCs with upwards of 130
dB dynamic range, it is really hard for analog generators to compete.




.

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