Years ago, in the professional recording and broadcasting industries, there were standards manufacturers adhered to.
One of those standards concerned input and output impedance: low, 600Ω, and matched.
Neither requirement makes sense anymore, but it's instructive to understand their thinking.
Both specs were chosen years ago at the beginning of the telephony industry. Why they chose the low input and output impedance isn't relevant to our story. The matching aspect is.
Matching impedance reduces cable reflections. (Reflections, in turn, reduce high frequencies).
In a reflection, part of the signal gets sent back to the source. This undesirable characteristic can be reduced by matching input and output impedances.
Sounds like a good thing to do, right? Yet audio manufacturers insist on the opposite. Instead of matching input and output impedance, we prefer you to keep the output impedance as low as possible and the input impedance at least one hundred times higher.
Don't we care about reflections?
No. The whole matching standard becomes meaningful when two things occur: long lengths of cable (like miles long) and high frequencies (like MHz).
In home audio, the cables are short and the frequencies not high enough to matter.
Digital audio, however, is a different matter.
