Part of the lab equipment where I work is a tone generation
system which needs to make a 1 kHz tone upon demand,
at a predetermined volume (usually 85 dB).

Typically, we use an off-the-shelf frequency generator and
a home stereo amplifier.  We're starting to notice that the
85 dB we calibrate with a soundmeter isn't staying at 85 dB.
We'll come back later and measure it at 90 or 95 or more.

My theory is that when we turn it on and calibrate it, the
amp is cold and behaves as such.  After it's been on awhile,
and especially after it's been in use, it warms up and the
actual transistors used to do the amplification start to
allow a larger current into the load, as a warm transistor
will do (right?  That's why parallel BJTs are bad:  one heats
up, conducts more, heats up more, conducts more, etc.,
until one has 95% of the load and POOF!  Do I have my
facts straight?)

So, here's the Q:  Should we just warm up the amp for a
bit, then calibrate it, or should we be looking for amps
with a different output stage, or should I be thinking about
just building an amplifier?

The problems I see are that it may not stay warm enough
during use for that calibration to necessarily stay valid.
A tone is generated at a random interval of between 15 and
25 seconds, and lasts for 450 ms.  That's a lot of cool
down time, but not enough for it to be the same as the
amp being newly turned on.

Building an amp has its appeal; I've never done it, but
this doesn't exactly have to be Dolby Digital or THX or
anything, and it could allow me to add in a few features.
OTOH, it's a lot of work that might not be worthwhile,
since I probably won't be around to service a custom
device much longer.

Mike H.
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