how good are class D amplifiers?
 

In article , Serge Auckland
wrote:


"Jim Lesurf" wrote in message
...


Well, I've used both 'traditional' THD+N kit that works by nulling or
filtering the test sinusoid, and a specan/generator that autofinds the
harmonics and works out THD. The second didn't seem at all 'tedious'
to use as it automated the process. It was also faster than the old
distortion kits I used to use that took some seconds to null down for
low distortion.


When I was designing audio equipment, I never had the luxury of an
autofinder, the one I used was a Marconi Instruments meter which in
effects was a highly selective filter and meter, and to use it, one
found each harmnic individually, measured its level, then worked out
the THD by algebra. As I said, tedious in the extreme.

Jings! Yes, I think I'd have thrown it out the window. :-)

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

how good are class D amplifiers?
 

"Jim Lesurf" wrote in message
...
In article , Serge Auckland
wrote:


"Jim Lesurf" wrote in message
...


Well, I've used both 'traditional' THD+N kit that works by nulling or
filtering the test sinusoid, and a specan/generator that autofinds the
harmonics and works out THD. The second didn't seem at all 'tedious'
to use as it automated the process. It was also faster than the old
distortion kits I used to use that took some seconds to null down for
low distortion.


When I was designing audio equipment, I never had the luxury of an
autofinder, the one I used was a Marconi Instruments meter which in
effects was a highly selective filter and meter, and to use it, one
found each harmnic individually, measured its level, then worked out
the THD by algebra. As I said, tedious in the extreme.

Jings! Yes, I think I'd have thrown it out the window. :-)

Slainte,

Jim


It didn't last long! I bought the Radford combination as soon as I could.
However, it did have its uses in development in identifying individual
harmonics, but I always worked on the principle that it the total of all the
harmonics and noise was comfortably below 0.1%, and in the circuits I was
designing at the time, it was around 0.01%, then what the individual level
of any harmonic was mattered not a jot.

S.
--
http://audiopages.googlepages.com

how good are class D amplifiers?
 

"Don Pearce" wrote in message

On Tue, 22 May 2007 16:12:32 +0100, Jim Lesurf
wrote:

In article , Don Pearce
wrote:
On Tue, 22 May 2007 09:19:09 +0100, "Serge Auckland"
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.

how good are class D amplifiers?
 

Jim Lesurf wrote:
In article , Rob
wrote:

I simply don't get this. I've been using 5 SS amps of late (Quad 405,
Rose power amp, Cambridge AV, Behringer A500, and that within a Pure
mini system), as well as others on and off over the years, and I feel
each has 'a sound of its own'.

The problems with the above are as follows:

1) Many people have formed such views as a result of simply using various
amps. I've also repeatedly changed from one amp to another and thought it
made a difference. But then later on I changed my mind when I listened
again. The problem here is partly one of control - e.g. not level matching
- and partly that human hearing varies with time, etc. So each time you
listen your ears/brain may simply respond slightly differently.

2) Yet when people do level-matched comparisons and avoid obvious snags
like clipping *and* have only the sound to rely upon, the result is often
that they can't reliably tell one amp from another.

FWIW A number of tests have also shown that people tend to hear
'differences' even when the same system is used in the same way.

The above does not mean that all amps produce the same results. Nor does it
mean that they all produce different results. But it means that people form
views that may simply be mistaken, and often fail to do comparisons which
help prevent well-known mistakes from occurring.


Yes, I agree. DBT is not a trivial thing though, and surprisingly (to
me) there doesn't seem to have been many rigorous tests to underpin the
'little if any difference' thesis.

how good are class D amplifiers?
 

Don Pearce wrote:
On Tue, 22 May 2007 21:10:47 +0100, Rob
wrote:

Don Pearce wrote:
On Tue, 22 May 2007 20:45:32 +0100, Rob
wrote:

Don Pearce wrote:
On Tue, 22 May 2007 19:11:40 +0100, Rob
wrote:

Dave Plowman (News) wrote:
In article ,
Keith G wrote:
Yep, why waste time *listening* to a bit of kit when you could just
stick a meter on it....??
Indeed. A doctor with a stethoscope beats a MRI scan every time.

Why look at (with your eyes?!) a picture when you can digitally analyse
it? Why go for a walk when there's perfectly good GIS profile of your
route? Why listen to music when ... :-)
But the measurements aren't a substitute for listening - they are
designed to make sure your equipment is not going to mar your
pleasure.
It strikes me that measurement fixation *does* affect listening. For
some, a measured anomaly would take away some of the listening pleasure
even if it was inaudible.

Would it take you that way? Not me, that is for sure. ANd by now I
have a very good idea of what is audible and what is not. Add to that
the fact that it is trivially easy these days to make equipment with
errors many orders of magnitude below audibility, and the situation is
really pretty relaxed.

Well, yes it has affected my listening pleasure for a time. Maybe I'm alone.

The fact that audio design engineers have measured every aspect of
your favourite amp, and slaved assiduously to make sure that it is as
good as it can possibly be has adversely affected your listening
pleasure? That actually sounds a little ungrateful to me.


No. If someone tells me a measurement is awry, it can affect my
enjoyment. I've had all manner of techie types tell me that
such-and-such is off-centre, and while it's highly unlikely to make any
real world difference I think about it. That is until I forget about it :-)

But I appreciate that, fortunately, everyone is not like me.

A bit like making sure the glass on the front of your
picture is nice and clear... Of course there are those who like
cloudy glass with a coloured tint, but not those who want to see the
whole of the picture.

d

Some people like their glass 'distorted' so they can see the bigger
picture.
No, you never see the bigger picture through distorted glass. You
always see less - no choice there, I'm afraid. And of course you have
not the slightest chance of seeing the details.

Try telling my wing mirrors :-)

You have distorting wing mirrors? I don't count gain or attenuation,
of course ;-) Objects seen in this mirror are fatter than they look,
heh?



Ah, lost me again! I'm pretty sure the wing mirrors on my car give me a
wider field of view than a flat mirror might - or at least I was.

how good are class D amplifiers?
 

Jim Lesurf wrote:
In article , Rob
wrote:
Don Pearce wrote:
On Tue, 22 May 2007 19:11:40 +0100, Rob
wrote:

But the measurements aren't a substitute for listening - they are
designed to make sure your equipment is not going to mar your
pleasure.

It strikes me that measurement fixation *does* affect listening.

However not all interest in using measurements and understanding their
meanings would be a 'fixation'.

No I know, just a bit of fun.


For some, a measured anomaly would take away some of the listening
pleasure even if it was inaudible.

So far as I know, there is no law requiring you to read any of the specs or
measurements for any equipment you buy and use. :-)


Surprised it's not in this group's charter :-)

However if you actually *understand* the measurements you can easily make
up your mind if any 'anomaly' matters. Indeed, reading and understanding
'measurements' might save you from wasting time money buying a lemon.

A bit like making sure the glass on the front of your
picture is nice and clear... Of course there are those who like
cloudy glass with a coloured tint, but not those who want to see the
whole of the picture.

d

Some people like their glass 'distorted' so they can see the bigger
picture.

Indeed. However it can be useful for them and others to know the cause and
effect involved. This would then give them info useful when they and others
decide what other 'glass' to choose for specific purposes, or to get
results they would feel are a further improvement.

Interestingly, your analogy also implicitly assumes the 'people' know
that the result *is* being 'distorted' by the 'glass', rather than
assuming that what they see is what they'd get if the glass were absent
and their view was direct. The snag in audio is that many people may
have no such awareness, and indeed, no chance to do the equivalent
of seeing the view directly.

Another snag is that in the case of audio there may be many thousands
of different 'pictures' to 'view' and you might like some 'distorted'
by a specific 'glass', but other 'distorted' in other ways. So perhaps
this is simply another attempt at analogy that falls apart once you
try to use it at more that a trivial level. ;-


May well. My point was to emphasise that 'distortion' is a concept, and
not a single or pejorative (in the context of valves discussions say) fact.

More generally...

The problem with wilful ignorance is that it gives you no guide if you ever
need to change anything. Similarly, it is no help to anyone else who is
interested in the results you got. Nor does it tell anyone if what is
claimed is for the reasons claimed, or is even real rather than delusional.

I suppose I am old-fashioned. I prefer education and understanding to
ignorance, and I prefer views based on reliable evidence. Indeed, I seem to
get a lot of enjoyment and satisfaction out of learning, understanding,
etc. My experience thus far is that this has helped me to design/choose/use
equipment to allow me to enjoy recorded and broadcast music. I have also
repeatedly found that ideas presented in claims by people have no
foundations, so would probably have wasted my time and impeded my being
able to get to where I have in terms of enjoying the results if I hadn't
had the old-fashioned approach of using measurements, understanding, etc,
to try and find my way though the claims. In my experience this has
complimented listening very well.

So, no, I'm afraid I am not personally a great fan of wilful ignorance as a
policy of choice.


That's fine in the main, of course - it's your world and it suits you
(and probably many others). I'm not so happy, though, with lumping
enthusiastic commentary and enquiring minds in with 'wilful ignorance',
which I'm afraid is how I read the essence of what you seem to be saying.


Slainte,

Jim

how good are class D amplifiers?
 

"Rob" wrote in message


Yes, I agree. DBT is not a trivial thing though, and
surprisingly (to me) there doesn't seem to have been many
rigorous tests to underpin the 'little if any difference'
thesis.

What would constitute many?

I've done DBTs of several dozen hi fi power amps, and found near-total
support for the "little if any difference" thesis.

how good are class D amplifiers?
 

"Arny Krueger" wrote in message
. ..
"Rob" wrote in message


Yes, I agree. DBT is not a trivial thing though, and
surprisingly (to me) there doesn't seem to have been many
rigorous tests to underpin the 'little if any difference'
thesis.

What would constitute many?

I've done DBTs of several dozen hi fi power amps, and found near-total
support for the "little if any difference" thesis.


Sure, of course - and it would be stunning if it weren't for the fact
that, in a domestic environment, most people couldn't tell the
difference between two pieces of kit after a couple of goes back and
forth - even if *they* were making the switches fully sighted!!

how good are class D amplifiers?
 

In article ,
Keith G wrote:
Sure, of course - and it would be stunning if it weren't for the fact
that, in a domestic environment, most people couldn't tell the
difference between two pieces of kit after a couple of goes back and
forth - even if *they* were making the switches fully sighted!!

That's a remarkable change of tune from you?

--
*When I'm not in my right mind, my left mind gets pretty crowded *

Dave Plowman London SW
To e-mail, change noise into sound.

how good are class D amplifiers?
 

Arny Krueger wrote:
"Rob" wrote in message


Yes, I agree. DBT is not a trivial thing though, and
surprisingly (to me) there doesn't seem to have been many
rigorous tests to underpin the 'little if any difference'
thesis.

What would constitute many?


Many for me would be any that I come across. I flip through the hifi
press every so often, and I think I'm not far off correct in saying
there's hasn't been a single example in the mainstream media?! Which?
(UK consumer mag) do blind tests, but not with the rigour you require,
and they frequently report differences in amps (and CDPs come to that).


I've done DBTs of several dozen hi fi power amps, and found near-total
support for the "little if any difference" thesis.


Yes, we've done this, and I set out a few problems I had with your
methodology and method. Even so, and FWIW, I found your tests
interesting and valuable, if not entirely persuasive.