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

1. Listening. The physical swapping out is doable. I would find/have found
level matching incredibly difficult. I tried this some months ago with a
20-20k hz cd, taking levels at various points. I couldn't get close to
level matching across the range - I'd got weird 'spikes' at points in the
frequency range, around 2000hz IIRC. Even using the same amp! Anyway, this
sort-of-obviously means I need some more reliable measuring equipment and
techniques - any pointers here would be appreciated.

This is quite normal:- If you are level-matching by using a SPL meter
in-room, then what you are measuring is the sound pressure at the point at
which you are holding the meter (so much is obvious :-) ) At mid-high
frequencies, the measurement you make will be very influenced by local
reflections, even from your own body. If I put my SPL meter on a
photographic tripod, then move around even only by a few centimetres, the
reading on the meter will vary by a dB or more at mid-high frequencies. At
low frequencies, it is much more stable, being dependent on room
reflections, and hence positioning in the room, much less influenced by body
movements as the wavelength of the sound becomes larger than my body
dimensions (even in my current less than sylph-like state)


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 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. 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 :-)


2. If listening using level matched DBT does reveal difference, the
spotlight then turns on the amps. I would then have to measure the amps
and see what variation there is. Again - a pointer would be useful - even
a book.

To make meaningful measurements on an amplifier you don't need a lot of test
equipment, just a good soundcard that samples at 192kHz and some software. I
prefer to use individual physical instruments, but that's because I'm both
an old fart and happen to have them. If I wasn't the first and didn't have
the second, then I would probably use the PC method totally. I have the
RightMark audio analyser software which is freeware, and from what I can see
works beatifully. http://audio.rightmark.org If you are going to make
useful THD and frequency response measurements, you need a 192k sampling
card to give you some 85 kHz of measuring bandwidth.

You *will* need a good dummy load to run the amps into. I have four 50 watt
4 ohm resistors mounted on a large heatsink, each with a flying lead and
croc clips. I can thus set up 4 x 4 ohms @ 50 watt, 2x8 ohms @ 100 watt, 2x
2 ohms @ 100 watt, 1x4 ohms @ 200 watt and other combinations.


That's very interesting, thanks. I'm sure I have the bits and pieces to
manage that, and it'd be a useful means of checking what I've got.

As for books, can't recommend anything specific, as my training was pretty
much continuous since I was 16, in the days when a 100kHz 'scope was the
best my school had. There are a number of good text books about, but they
tend to be rather expensive. There's usually a bookshop at the pro-audio
exhibitions, so if you get a chance to go to the AES, or IBC in Amsterdam or
NAB in Las Vegas, or know anyone else going, they may be able to find one or
two for you.


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 ;-)


Once differences are identified and quantified, then any audible
differences are soon accounted for. What I am saying is that with modern
SS amplifiers, it is easy except at the very cheapest end for the above
criteria to be met, consequently any but the cheapest amps will all sound
the same when played at the same volume into the same (sensible) load.
I experience four areas of difference - bass, 'soundstage', voices and
'top end'. Bass is easiest (A NAD 3020 was quite 'soft' compared to the
clearly defined bass lines of a Rotel integrated), a Quad 405 is
noticeably sibilant, and a Roksan Kandy I had a while ago was plain
shrill. I'm using a Cambridge AV amp at the moment, and I can't detect a
difference except at very high sound levels between its built in power
amp, and a Rose power amp, and a Behringer power amp. I'm pretty pleased
with the Cambridge for casual listening.

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.

The same applies to CD
players and anything else that meets the criteria. Items that don't meet
the criteria (rarely or never) are transducers and consequently
microphones, pick-up cartridges and loudspeakers will all sound different
for easily identified and measurable reasons.

The *real* point I'd like to explore would be the notion that conventional
measurements are not a reliable guide to sound experienced. Of course
(again) such a statement comes across to some as something between
witchcraft, homoeopathy and astrology, but I set it out here just so you
can categorise my comments properly.

Rob

Yes, witchcraft, homeopathy, astrology *and* subjective hi-fi reviewing are
all characterised by believing things that do not show up under any sort of
scientific scrutiny.

Conventional measurements, *if applied correctly* can characterise
completely the operation of a piece of audio equipment.

That they can is not in doubt. It's possible that some, on learning that
they measure within limits, subsequently hear no difference. And some,
on reading a review that is half-decent, hear difference. It's
bothersome, I know.

What they can't do
is to characterised your reaction to that piece of equipment. What I mean by
this is that we all are conditioned by magazines, friends, received wisdom
etc, and that we characterise the sound we hear according to our prejudices.
When these prejudices are not able to operate, as in unsighted testing, then
many of the previously-held views dissappear.

So far, no-one has been able to come up with realiable evidence that there
are some aspects of audio performance that we have not yet been able to
measure. The closest I suppose is the performance of bit reduced digital
encoders of the psychacoustic type. Conventional test measurements don't
show up the artefacts we all claim to hear, although I have surprised myself
as to just how good MPEG encoders are at low bit rates when listened to
blind. When I was last working professionally with encoders, there was no
standard test signal which would correlate with what we could hear, and as
far as I can recall, every customer had a favourite CD or two which they
used to evaluate audio quality. Possibly someone on this group may have more
recent information on testing audio codecs.


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.

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.

All other audio equipment is now so well understood and characterised by
conventional measurements that when audio differences do show up, they are
easily dealt with. Note however, that some products are deliberately
designed to sound different, for marketing reasons. Linn realised this as
long ago as the 80s with their Kan loudspeaker which was highly coloured and
with an appalling frequency response. I can't believe that a company of
Linn's engineering abilities did this by accident or incompetence, so it
must have been deliberate to stand out in demos as sounding different to the
rest of their competitors.

Speakers really are the sharp end of things IMO. My guess with Linn is
that they had in mind a speaker that could (somehow) sound accurate (or
their version of it) given domestic settings. I'm very happy with
Dynaudio speakers, but I'm intrigued by Keith's various forays -
impressive and disarming at once, and something I mean to try one day.

More recently, popularity of SET amplifiers and
high-efficiency horns points to the desire of listeners to have something
that's different to the prevailing norm.


Agreed - and in the scheme of things does no harm.

http://audiopages.googlepages.com

A good read, and a splendid system by the looks of it. A valve cartridge
pre-amp wouldn't go amiss :-)

Rob