Don Pearce wrote:

The resistors are in series with each other, that combination is in
parallel with the input. Because the resistors are 6.2k, the
combination applies 12.4k as the maximum possible input impedance of a
microphone preamp.


In a professional microphone input you have in parallel:
- Phantom supply (2 x 6.2 k 0.5% resistors to +48V)
- switchable symmetric 20 dB pad (about 2 k input impedance to 200 ohms
output)
- and the microphone amplifier

To get the best possible noise figure from a mic preamp, you must match
the amp input impedance to the microphone. This is 600 ohms in most
cases. but:

if you are going to split one microphone to several mixers (FOH,
monitors, recording), there parallelled impedance of these mic preamps
should be higher than 600 ohms. In practice, many preamps are designed
to 2 k impedance, some monitor amps are as high as 4 k (the loss in
noise figure does not matter much on a noisy stage anyway..)

Hope this helps

P.S. back to the original question: several consumer units are not happy
when loaded with 600 ohms transformers. These output impedance of a CD
player might vary between 30 ohms to several k ohms, usuall having
serial capacitors in the output: this means high impedance at low
frequencies. A 10 k transformer might have better chance not to cut the
bass.