My new Cambridge Audio amp and tuner both carry instructions that the
unit should be run continuously (playing music) for 36 hours "to get
it performing at its best".

Does anyone know what such 'running in' is supoposed to do?

Is it running in, or just warming up?

My last two sets of speakers definitely "ran in" - in different ways;
the Rogers LS4as started off bright and thin, and filled out, whereas my
Monitor Audio Silver 8is started off dull and perked up.

Electronic components also improved over (a much shorter) time, but they
also do the same thing if they're unplugged for a few days. So much so
that I've given up unplugging my amplifiers when I go on holiday, and I
used to leave my old CD-player switched on all the time (the new one had
a standby mode).

Quite why things should take longer to warm up than they do to get
physically warm, I'm not sure. No doubt someone will reply in a minute
to say that good equipment doesn't do that, and it's just a figment of
my imagination!

Can't really comment on your imagination. :-)

However my experience is as follows:

That on some occasions the properties of electrolytic capacitors will vary
depending upon the use history and the quality of the caps. However my
experience is that leaving an amp unused for a few days or weeks has no
audible effect in general due to this.

Similar comment to the above about some connections. If the connections
are
not decently ohmic they may be affected by use, or by the unit being left
unused - particularly somewhere cold/damp.

Apart from the above, some components take a while to warm up. This may
affect the bias conditions in gain devices a little. You may also find
that
a mains transformer takes a while to warm up, and this can affect the
amount of mechanical 'buzz' it produces.

In general, though, I'd expect a unit to warm up and not really be
noticably different quite soon after being switched on, and not mind being
left off for many days. There are exceptions, but I'd tend to see them as
signs of questionable design, flaws, or poor storage/conditions of use.

FWIW at one time I used to do things like shove the amp I was working on
into the fridge overnight so I could check out the effect of a 'cold
start'
the next day. This can show up problems. However my experience was that
decently designed amplifiers should not be bothered. A tuner *might*
suffer
some tuning misalignment if the tuning elements are temperature sensitive.
A speaker may well be affected as the suspension stiffness, etc, may be
affected by temperature. The field produced by a permanent magnet is also
affected by temperature - but this should be a small effect for such
modest
temperature changes.

However, generalisations apart, I've no idea why Cambridge Audio would
make
such a statement. Ask them, and see if their explanation makes any sense!
:-)

It is quite possible to perceive changes as time passes for all sorts of
reasons. Our hearing changes during the day, and as we wake up, eat/drink,
are distracted/alarmed/bored, etc, etc. Also the air changes its acoustic
properties a little as it changes temperature and humidity, etc. Also the
level of background 'noise' can also affect perception. Listening to
sounds
also tends to cause our hearing to alter/adapt. Hence we may well hear a
difference if we turn on a system and play the same music a few times at
different times during a day, or from one day to another. But this does
not
have to mean that the equipment is changing its performance.

Slainte,

Jim

Some amps' frequency responses can alter during warm up. Whether this is a
reflection of poor design or the designer's intent is another matter.

One of my favourite amps sounds *very* different when first powered up,
compared with when it's hot, to the degree that a demonstration of it when
cold would not impress me whilst, when warm, I love it.

I have no trouble leaving it on, so this isn't a problem for me. While I may
be curious as to *why* this is the case, I don't really care that it is so -
I prefer it and that's all that's important.