In article , Serge Auckland
wrote:

"Rob" wrote in message
...

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.

This is another reason why 'casual' listening tests can be so unreliable.
Small changes of the listening position, etc, can affect the sound reaching
the ears. Thus this is one of many factors which can produce 'differences'
even if the amplifiers are functionally identical.



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.

If you can stand the 'noise' it can also be revealing to do some
measurements using the speakers as loads. This may expose effects that
don't show up with resistive loads. However as well as the row, this can
risk speaker damage unless you take care.

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.

I can't off-hand think of a book suitable for 'beginners' that explains how
they can do a sensible set of tests on something like an amplifier. Can
anyone suggest one? I'd agree that these days what would be 'best' would
probably be one that allows the user to make use of their computer with an
audio input as the test instrument, and to use a test CD as the source.

FWIW I've recently been writing a series of short articles for a computing
mag which deals with how people can generate and analyse sound files. It
would be useful to know if there is a decent book dealing with amp testing,
or indeed testing other items of audio gear, which would be suitable for
enthusuasts who don't already have experience and know how such things are
done, and why.



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 problem with such statements in 'reviews' is that we not only have no
idea if they were based on a genuine difference. We also can't tell if they
were due to something like the reviewer moving their head an inch, or
opening the room door, or...

A number of controlled tests have been done where the amps A and B were
played in randomised pairings. i.e. in each case A was played then A, or A
then B, or B then B, or B then A. The listeners were asked in each case if
the sound for each example in a pair was the 'same' or 'different'. This
exposed a significant (statistically) tendency for people to say that AA or
BB were 'different' rather than 'same'. Thus showing that people report
differences when the amp is unchanged and the system has not been altered.


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.

That is perhaps a little unfair as some reports may well be reliable. The
snag is distinguishing them from the unreliable claims. Alas, if you have
no way of telling if an individual claim is reliable or not, and the
authors are well-known for making claims which can't be backed up, then
their claims become worthless. Might as well toss coins to make a decision
as to use the reviews as a basis.

Slainte,

Jim

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