paddy wrote:
If I want to be able to send the signal from a CD player (or record
deck) to

a) amp (for "pure" listening)

- OR -

b) rough old DJ mixer (for occasional parties)

... is it an acceptable option to use a "Y" splitter cable out of the
CD player (record player)?

I want to avoid doing any replugging when moving from "audiophile
mode" to "party mode". When in "audiophile mode" the mixer would be
turned off. I have tried to look around for an answer to this, and
while people seem to react against Y splitters, I haven't been told
exactly WHY they are such a bad idea.

Is there a good electronics engineering reason to avoid them? Would it
make any detectable difference in the real world (double-blind testing
etc etc)?

It depends on the design of the components you are sending the signals
to. Some devices have low, non-linear, impedances when they are powered
off, which would cause distortion in the signal going to the other
active device that you are listening through. Not too long ago in this
forum, there was a poster noticing this problem when his preamp also
drives a CD recorder in addition to the main amplifier, and when the CD
recorder is powered off, there is noticeable distortion at high levels.
I have also found this problem in a certain receiver, due to the use of
analog switches that basically clamp the input signal to ground when the
receiver is off.