[Arny Krueger]

"Ian Bell" wrote in message

Arny Krueger wrote:
"Ian Bell" wrote in message

Arny Krueger wrote:
"Ian Bell" wrote in message


0dB has no meaning.
Actually, it does. In the digital domain 0 dB is
commonly equated with FS.

No, that is 0dBFS.

Yes, so your use of the word "no" must be some kind of
terrible mistake on your part.


No, 0dBFS refers to full scale signal in a digital
system.

Of course!

It is common, especially in live recording to set
'0dB' to -15dBFS so as to ensure sufficient headroom. In other words, 0dB
can be anything you like and therefore
on its own without context is meaningless.

Ian, you seem to be fascinated with telling people that they are wrong, and
then explaining it by reciting either the exact same facts they did, or
paraphrasing them.

While the units are arbitrary and may be confusing to
people who still think 100% analog, it is both
meaningful and commonly used.

No it is not.

Assertion without support, which should be immediately
dismissed.

OK, then we will dismiss your original unsupported
assertion that 'it is both meaningful and commonly used'.

Tit for tat?

Input termination in both cases is unspecified.
If the input is a mic input, then the usual standard of
300 ohms applies.

It is a power amp, it does not have a mic input. 300
ohms is NOT the 'usual standard for a mic input'

Good modern power amps are common, and do not have that
much variation in their noise performance with normal
variations in source impedance.

no response from Ian

If the input is a line level input, then the source
impedance is usually such that system performance is
not affected that much by probable variations.

You miss the point. How is the input terminated when
noise is measured - obviously it is not left open
circuit is it?

I guess you've never measured the actual noise coming
out of a good modern power amp with the normal range of
source impedances. They often don't vary all that much.
Remember, this is not legacy vacuum tube equipment which
was generally far noisier.

A 'good modern one' should not vary much. We have no idea
if the OPs's device falls into this category. However, if
the noise is being minimised then a point will be reached when the source
impedance is relevant because if the amp was extremely
noise free that would be the ONLY source of noise.

Baseless speculation.

OTOH, it will not vary much with source impedance if the
amp is very noisy to start with, like an old tube amp. SO precisdely
the coinverse of what you state is in fact true.

I thank you for substantiating a point I make below.

Output termination is unspecified.
The noise performance of line level outputs is
minimally affected by probable and reasonable
variations in load impedance.

No bandwidth is specified.
Relevant and already mentioned

No weighting or not is specified.
Relevant and already mentioned

No mention of whether the measurements are rms, peak,
quasi-peak
Both SNR and DR are ratios of two measurements. It is
most important that both measurements be done under the
same circumstances.

In fact they generally are not since a signal and noise
have quite different characteristics. The signal will be
measured rms. The noise can be measured in several ways
each giving a different figure.

While people could be stupid and compare a noise level
measured in peak-to-peak volts to a signal measured in
average volts, I know of no actual cases where this
happens, except perhaps in your mind, Ian.

No, but as I keep saying, manufacturers will show their
product in the best light and judicious use of weighting and bandwidths
will alter the 'measured' value considerably. 'A'
weighting, which is pretty commonly used by manufacturers will often
improve and amplifiers
noise spec. by 10dB.

Believe it or not, "A" weighting has a reasonble justification. It weights
the noise in accordance with the response of the human ear at the levels
that noise from a reasonably clean piece of equipment is likely to be heard.
It properly focuses on noise at frequencies where the ear is more sensitive,
and tends to give less weight to noise at frequencies where the ear is less
sensitive.

There
could be equipment noise that has a high crest factor,
but nature does not usually go down that path. What is
then left is a few dB of ambiguity, and most good modern
equipment (which is common) is not so noisy that a few
dB is a deal breaker in actual use.

It is more than 'a few dB'

Baseless assertion.


Variations in the way the above are specified canmake a
large difference to the 'measured' value

Some yes, some no.

and I bet you
can guess which set the marketing department will want
to choose.

One other benefit of the solid state/digital revolution
is that SNR and DR performance is often so good ( 100
dB) that most variations in how they are mentioned
don't matter that much.

Rubbish.

Dismissive, unsupported claim, again itself worthy only
of dismissal.

As I have said before DR and SNR are not the same and
they have little relation to the actual level of noise
heard in the speaker.

Ian, I've shown many ways that just your say so is not relevant or binding.
Come up with some authority other than yourself, or watch your whole
discussion flush down the toilet.

The OP's amp for instance has an
output power of 800W. Suppose this is into a load of 4
ohms then this requires over 56V rms output signal. The
DR is 100dB so the noise at the output is 100dB below 56V
rms which which works out at a mere -65dBu.


The good news is that nobody in their right minds hooks 800 watt power amps
up to lines where 0 dBu is the reference level. IOW Ian, your example is
irrelevant to good, reasonable practice.

The original all tube Leak Point One in 1949 had a
measured output noise and hum of -80dB below 10W into 15
ohms which works out at just over 12V rms. So its output
noise is 80dB below 12V rms which works out to be a mere -58dBu.

And the point of equating 10 watt amps to 800 watt amps is?????????????

So in 60 years of development there's been only 7dB (oh
sorry I should have said 'just a few dB') of improvement due to the
digital revolution.

?????????????????????

A claim that comes about due to the incredible folly of equating a 800 wpc
amp to a 10 wpc amp.

Once again, SNR and DR are different animals.

A truism - therefore something that actually sheds no
significant light. Why do you obsess over these things,
Ian?

no answer

Achieving a 100dB DR is not hard but unless you run your
amp close to clipping all the time you will not achieve
that as a SNR.

Another truism.

Excellent, so then you must agree that SNR is always less
than DR.

It can go either way. The most common current standard for measuring DR
involves the presence of a test signal and the inclusion of nonlinear
distortion. Depending on the piece of equipment, either SNR or DR can be the
larger number.

So to get back to the OP's original problem, having a DR
of 100dB is irrelevant. What matters is what his SNR is.

The fact he can hear hiss from an 800W amp with his ear right next to it
further
demonstrates this because that is not where listeners
will be when the spekaer pumps outn 800W.

The sentence itself is a non-sequitor, but if your point is that judging
amps by putting your ear right next to the speaker is questionable, then I
have to agree with that in general. OTOH, if there is some real world
connection between this test and actual use in the real world, then we have
to give the OP a pass.

The marketing guys are as active as ever in trying to
make their product appear superior to its competitors
and will choose the measurement methods that best do
that.

The myth is that some equipment is head-and-shoulders
better than its competition. Everybody pretty much feeds
from the same trough.

In reality, marketing departments will aim to make crappy
equipment appear much better than it is using specmanship which is
what they have always done.

That's just it, the general quality of audio gear has improved signficantly
since the days of the Leak Point One.