"John" wrote in message
...
"Des" wrote in message
...
Reviewers often comment that it takes about 100 hours before a new amp
settles down and start to sound like it 'should'.

In this case, what constitutes 'burning-in'? Is it sufficient to just
leaving the power on for a week? Or does it have to be driving a load
(the neighbours are going to just love that)?

IMHO yes leaving the power on helps, but it would be better if you
listened
to music 'from cold' to find out if you can hear any differences as things
warm up. If not, then save some electricity and leave it off when not in
use!

I can think of some areas where changes occur in an amplifier over time.
These may be minor and in most amps inaudible, but it depends on the
design
and the listener.

1. Electrolytic caps in PSUs. These 'form' over a very long time (weeks).
Would lead to better performance (ie better smoothing, more DC like
voltages) over time.

2. Temperature effect on electrolytic caps. In switched mode supplies too
much heat dries them out and they lose effectiveness. If you look at
specs.
they quote life of say 10,000 hours. Most supplies last a lot longer than
that!

3. For coupling capacitors between stages (ie in the analogue audio path)
effects 1. & 2. would effect slightly the characteristics of the capacitor
to how well if passes signals of different frequencies. May or may not be
audible.

4. Transistors warm-up. Again for analogue stages the transfer function of
a
transistor will vary with temperature, depends on the design how well this
is compensated for.

HTH, no 'rules', just listen with open ears.

Regards,
John



Capacitors and their effects - apart from long-term drying out and the
inductance of very large values - also fall into snake oil IMHO.

However one thing that can have effect is heat. Most amps have some sort of
temperature sensing in the output stages that are designed to stabilise the
biasing which will inevitably change with temperature. The old way used to
be a small signal transistor either glued into or onto the surface of the
heatsinks - the Japs now build specialist transistors that have this
transistor fitted internally with external access. Leaving the amp on for
perhaps an hour before use will allow the heatsinks - and thus the bias -
to thermally stabilise which could lead to reduced crossover distortion -
the main effect of poor biasing.

Poorly designed amps that run close to instability in the first place (the
exact opposite of inherently very stable designs like Quad - and there are
enough of those around in the esoteric-fi areas) may, when cold, have
relatively significant crossover distortion which can lead to listener
fatigue and perceived 'edginess' to the sound.


--
Woody