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.

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.

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.

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