CMRR console trimmers

[jrasia]

Hello,

My soundcraft K3 console modules have a number of trimmers(2.2k, 4.7k) on the outputs for my inserts and mix output positions.

My understanding is these trimmers are used to calibrate CMRR.

Can someone explain in simple terms how I might create a testing jig and/or the tools to properly calibrate these trimmers?

I don't have a scope, but I have a decent Fluke 87v DVM and a number of software testing tools on my DAW including tone generator.

Much appreciated!

Jason

 

[RuudNL]

I don't have a schematic of the Soundcraft K3, but usually common mode trimpots at the output are used in a circuit that behaves like a 'simulated transformer'.
If this is the case, correct adjustment is simple.
Connect two resistors of 1 K.ohm, 0.1% to pin 2 and pin 3 of the output.
Connect the other sides of the resistors together.
With a tone (1 KHz or so) at the output at full level, adjust the trimpot for minimal signal, measured between the junction of the two resistors and ground. (XLR pin 1).

 

[jrasia]

Indeed, the outputs have optional positions for transformers.
Thanks for the quick reply!