Newbie question - What is CMR?

[Guest]

Hi,

I have built a pair of green preamps, and the circuit has a trim pot. i looked at kev's site and identified them as CMR pots (i think)

what is their function? and what does CMR mean?

thanx !

 

[tmbg]

CMR is common mode rejection, and it has to do with the fact that a differential amplifier rejects signals that are common to both inputs.

In order to do so, it must be a very well balanced amplifier, and the CMR trimmer is likely there to allow you to trim for highest CMRR (common mode rejection ratio), or something along those lines

 

[mkruger]

CMR also stands for Central Machine Room... but that has nothing to do with what your asking... :roll:

 

[Mark Burnley]

Understanding Common Mode Signals PDF 304k

Erm, understanding common mode signals... :?

Mark

 

[PRR]

> CMR pots

Set them in the middle and forget about it.

They don't affect the signal.

They might reduce interference, but if you really need a fussy trim then something is wrong.

People who like to play with meters will feed the same signal into both hot pins and trim CMR for minimum output. This gives best rejection of non-signal interference. However, in real life, if your fixed resistors are 5% or better, you already have lots of CMR. I've done a LOT of good work on unbalanced mikes (zero CMR). Using 5% resistors gives 26dB CMR. If that isn't enough, you need to find the interference source and kill it, or move to a less-nasty location.