On 2006-04-29, Don Pearce wrote:

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:

Microphones tend to be somewhere around 100 ohms in most cases. And to
get best noise performance, you don't match impedance. You make sure
the impedance of the microphone gives the best balance between current
noise and voltage noise from the amplifier. The amplifier will be
designed to put this point somewhere around the 100 ohms you expect
from a mic.

600 ohms is a figure used for line level connections.

I own two dynamic microphones (consumer grade ones with unbalanced outputs
an 3.5mm plugs) both are 600 ohms.

the few pro or semi-pro mics I've handled were also labeled 600 ohms.

Transformers don't have "an impedance" as such. They transform the
impedance of whatever they are connected to by the square of the turns
ratio. So what impedance is presented as a result of putting in a
transformer depends on what the impedance of the equipment is.

as long as the mutual inductance is above the impedance of the source and sink
yes - a VHF balun won't make a good audio DI unit.

Bye.
Jasen