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"Rob" wrote in message
...
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.
Loudspeakers are *not* in any way non-linear loads. The impedance of the
'speaker system varies with frequency, but (except for some electrostatics)
there is no variation with level, consequently they *are* linear. Amplifiers
are also linear to the limits of their distortion characteristics, which is
why amplifiers need to have low distortion, maintained into the minimum load
presented to them. What you may be referring to is that loudspeakers have
impedances that drop well below (and raise well above) their nominal figure.
Some amplifiers may change their sound at the impedance minima. This is
true, but this is why I say that two amplifiers being compared have to be
used within their design limits. If one amp is designed for 4-8 ohms, and
another amp is designed for 3-8 ohms, then both should sound identical into
a nominally 8 ohm 'speaker, but could well sound different into a nominally
4 ohm 'speaker as the minimum load could be 3.2 ohms (the minimum figure to
qualify for a 4 ohm rating).
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.
There are plenty of valve simulators available on the pro market for DAW
plug-ins. They add level-dependent harmonic distortion, some low-level noise
and soft clipping. Some people pass their mix or part of it through a
reel-to-reel recorder to get a similar effect.
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.
I am perhaps a bit more cynical of Linn's motives, as the Kan wasn't in any
sense of the word accurate. Dynamic, aggressive, punchy, loud, but not
accurate...I also like Dynaudio 'speakers, found them very pleasant. Sort of
'speaker you don't tire of.
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 :-)
Thanks. The two turntables both have built-in RIAA pre-amps, which are quiet
and accurate in their EQ, so I have no real incentive to change.
Valve preamps for MC cartridges have great overload figures, but tend to be
a bit noisy, so I prefer SS.
S.
http://audiopages.googlepages.com