Simple method for impedance balancing adjustment (R11).

1. Switch-off the mixer.
    Disconnect speakers (if any).
    Connect the mixer housing to the earth contact of the mains supply.
    Adjust the microphone "channel gain" and "level" to minimum.

2.  To protect your ears and prevent feed back, reduce the mixer headphone volume or monitor volume to ZERO (minimum).

3. Connect the microphone under test, and wind the balanced microphone cable several times tightly around a loaded mains supply transformer (for instance the power transformer block of your mixer). Switched power supplies are not suitable.

4. At the mixer, adjust EQ-Lo to "0" (mid pos.). 
    Adjust EQ-Hi and EQ-Mid to minimum.

5. Switch-on the mixer.

6. Adjust the mixers microphone "channel gain" and "level" to maximum.
    Adjust main output slider to "0 dB".

7. Observe the VU meters.
    In each microphone line adapter, adjust R11 for minimum cable-induced-hum.

    Be careful !
    Only if the result becomes to weak to see a minimum on the VU meters:

7. Adjust headphone volume and listen with your headphone while adjusting R11 both to minimum hum. 

8.Lock both R11 with glue.

In a quiet room and if C1,2 and C5,6 are well matched, more than 40dB cable hum and noise suppression can be achieved over the full audio range.