"Rob" wrote in message
...
Serge Auckland wrote:
"Rob" wrote in message
[snip]

(1) This was Serge's reply to my question 'what's necessary to ensure
two amps sound the same':

Here is my understanding of the threshold levels:-

Distortions - all types, THD, IMD 0.1%
Important Note: This distortion is measured from 20Hz to 20kHz with a
bandwidth of 100kHz, and must be maesured into the loads declared
suitable
by the designer. For example, the QUAD 405 is rated at 100watts into
any
load 4-8 ohms. Consequently, I would expect it to work with
loudspeakers
rated at 6 ohms upwards. Note that 4 ohm rated loudspeakers can drop
to 3.2
ohms, and would consequently fall outside QUAD's specification for
the 405.
Frequency response +-1dB 20Hz-20kHz
Important note: This frequency response is measured across the
loudspeaker
load, *not* across a dummy load. This requires the amplifier to have
a low
output impdeance as otherwise, the loudspeaker's impedance
characteristic
will modify the frequency response.

Hum and noise 80dB measured on a bandwidth of 20Hz-20kHz, and no worse
than -60dB outside that band. It is important that the amplifier have
no
instabilities at sub or supersonic frequencies.

Crosstalk: 60dB
In practice, crosstalk below -40dB is unlikely to be audible under
programme
conditions. It is important that the distortion of the crosstalk be
below
0.1% as otherwise, the crosstalk distortion could swamp the speaking
channel's distortion. This is a rare condition, but not unknown with
poor
designs with poor power-supply rejection.

Rob,

Are you saying that even with two amplifiers meeting the above criteria
you can hear a difference or that you can hear differences between
amplifiers that may or may not meet the above criteria, you just don't
know as you haven't made the measurements?

I should make it clear that I've never made measurements, beyond crude
level readings using a handheld 10UKP meter. My claim is a 'hunch'
informed by listening, not hard data.

If the former, then we should look into ths
further, as it would be a valuable addition to our knowledge. If the
latter, then of course, amplifiers with different performance
characteristics can sound different. If any of the above criteria are not
met, and the most obvious are level matching and frequency response
differences, then in all probability they will sound different because
they are. Level matching needs to be done carefully, and can only be
valid if the frequency response of the two amplifiers is checked first,
and found to be within +_1dB *of each other* into the loudspeaker load
being used.


I do of course realise that it would be useful (to say the least) to carry
out some measurements. There are two sides to this:

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)

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.


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.

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.


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


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. 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.

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. 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.

S.

http://audiopages.googlepages.com