In article , Rob
wrote:
Serge Auckland wrote:
"Rob" wrote in message
...
[snip to bit about measurement]



Yes, I'd realised this - if not strictly taken it into account. Quite
how I clamp the listener's head, and avoid ear twitching, is an issue
for a later day :-)

You don't need to do that. Indeed it would be futile as there will be other
variables which will be likely to change in an unpredictable way and which
you won't be monitoring.

The appropriate response is to make *many* tests and allow your head to
move as you please in each case. This will tend over time to average away
the effect of the uncontrolled variable and systematic effects then show
through the statistical analysis.

Of course, if the uncontrolled/unknown variables are so pervasive and
serious, then it may be daft to waste time trying to see if you can find
any amplifier 'differences' which they have swamped. Indeed, this is often
my experience, and has for years seemed to me to make following this topic
beyond a given point rather a waste of time. However if others wish to
struggle to find evidence for amp 'differences' which seem to vanish in
simply comparison tests I guess I should just allow them to waste their own
time. If they ever find anything I'd be interested. :-)

You should level-match by measuring across the 'speaker terminals
ideally by using a high impedance audio millivoltmeter (once called a
valve-voltmeter). I have found that my normal inexpensive multimeter
is accurate enough at low audio frequencies. Mine actually work fine
up to 20kHz, but if you use a 100Hz tone for level matching you
should be fine. Use your SPL meter to make sure the volume level is
around 85dBC (say 80dBA) at 100Hz and level-match with the multimeter
at that loudness. You should fine it easy to level-match to better
than 0.5dB even with a multimeter.


Ak! I can't have that I'm afraid. I think I understand what you're
saying, but what I'm saying is that loudspeakers are non-linear loads
and something (I know not what) might be happening between the signal
and the sound.

Unfortunately vague feelings you have that something might be happening,
but you've no idea what/how/why/etc are in themselves worthless. The
requirement is quite simple in principle. You form an idea as to a 'cause'
and then devise a test for it, then do the test and evaluate the evidence.

The snag is that a suitable test requires some knowledge and understanding
to devise. And that then doing the test and evaluating the results requires
more understanding and some sweat. The cost of mining understanding from a
flood of formless fears and delusions, I'm afraid.

I suppose my thinking here comes from tests of kit I've
read in magazines, and the suggestion that different amplifiers do not
do the 'wire with gain' thing in a linear way. But I'm afraid I can't
remember the details - I don't even have a hifi magazine in the house,
having given up on them many years ago.

Now, if you're saying to me that any amplifier that has the
specification you've detailed will interact with a given loudspeaker
('normal'/domestic) in exactly the same way within the specification you
detailed then I'll do my best to satisfy myself that is in fact the
case (if I can), and adopt your test. You just have to say 'it is the
case' - I appreciate your patience thus far. In addition, I would
expect the level matching to apply between 20 -20k hz - just my little
quirk :-)

The key point is that the loudspeaker has specifically limited info on
what amplifer you are using. It can't read the name badge, nor the reviews.
It can't look and see if you have valves or transistors. It just finds that
a pattern of voltages is being asserted on its input terminals.

If two amplifiers are able to assert the same pattern and level of voltage,
they will be able to supply the same current and power patterns. Either the
speaker then delivers the same results in both cases or it does not.

If it does not, then any 'difference' is due to the speakers, not the amp.

If it does, then the amps are functionally the same so far as the test
system is concerned.



Thanks again. Really, this doesn't come easy to me. I tried reading a
book on valve amplifier design - quite basic by all accounts. I got
about a third of the way through and got stuck on one paragraph. I
simply couldn't accept on face value what was said. I could make the
effort one day (wilful ignorance,as Jim might say!), and post to one of
the tech groups. I've never got stuck in the same way with some quite
complicated political and social theory. And I would add that i have a
day job ;-)

It is not 'wilful ignorance' if you are trying as and when you can, despite
finding it hard going (or inconvenient).

These matters are *not* either trivial or self-evident. People have spent
many hard years working on them to develop an understanding. The 'wilful
ignorance' is when people can't be bothered to study and understand what
has been done and airly dismiss any evidence-based results which do not
suit their personal fancies.


Just to give you some idea of how difficult it is to assess equipment
subjectively, let me quote from the Hi-Fi Choice reviews for the NAD
3020/3120 and Quad 405:-

NAD. "Bass showed a touch of boom while the mid seemed a little hard
tonally and the treble was mildly grainy."

Quad. " The treble was still showing some mild "feathery muzziness"
while the bass could have offered more extension and impact."

Both these amplifiers are flat to +- 1dB between 20Hz and 20kHz


And I have to take into account I've probably read similar reviews, and
they could affect what I'm hearing. I don't doubt that.

Please bear in mind that a great deal of what appears in magazine reviews
may well be tosh. This can apply to the 'measured results' in some cases as
well as the impressive purple prose about the 'sound'. Treat with great
caution.


I'm archiving about 300 CDs to disc before I get rid of them. I compared
the wav rips to some 192kbs mp3s last night, and while I could hear a
difference (just, I got caught out from time to time) it wasn't
significant, and made *preference* very hard to establish.

Erm... what will you do when the HD fails without warning?

On a tangent, I am interested in learning why my valve amplifier sounds
so good. Put simply, if all of this can be measured, why can't some sort
of (distortion?!) filter be used to recreate the sound? The novelty of
the glow has more or less worn off, and if such a thing existed I'd give
it a go.

1) Don't assume any difference is for the reason you give. :-)

2) Look carefully at reviews of valve amps. Note when you can what they say
about 'output impedance' or 'damping factor'.

3) Note than many reviews and makers specs simply fail to mention various
factors which may well affect the results - particularly with the common
types of valve amp design.

Slainte,

Jim

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