On Sat, 29 Apr 2006 07:14:53 GMT, Matti Adolfsen
wrote:

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


As far as the DC path is concerned, you are right. The two resistors
are in parallel. But as seen by the microphone signal on the balanced
pair, they are in series.

Once you are in a position to need a 20dB pad, the impedance is no
longer important, because internal noise is no longer an issue.

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.

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..)

If you want to split one mic the best way, run it into a single
preamp, then make the split after that, so that the best noise
performance is maintained.

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.

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.

d

--
Pearce Consulting
http://www.pearce.uk.com