How to measure Common Mode Signals in the input stage?

[Pippi Lindenstroem]

I would like to check if the common mode adjustment in my Hafler P500* is still alright. (*pretty equal to the DH500)
The P500 offers two trim pots (P102 & P102L in the schematic) to null out any
common mode signals on the input lines (see attached schematic).


The Hafler manual stated:

Balanced to Unbalanced Line Circuit
IC-l and IC-2 (both NE5532) are buffers to keep the input impedance
to the summing stages IC-3A and IC-3B (NE5532) independent
of the input. This allows P102 (trim potentiometer) to null out any
common mode signals on the input lines.


How can I go on?
Where to measure (measure points)?
Measure device?
What to measure (How expresses a common mode signal, what will I read)?


I am willing to learn, so many thanks in advance

Pipp

[Jan Dupont]

P102 adjusts the DC-Offset of the Bal to Unbal input circuit.
On J101A short Input terminals 2 and 3 to terminal 1 (Ground) and measure DC-Offset on terminal 5. Adjust P102 to 0mV at terminal 5.

[Pippi Lindenstroem]

Originally Posted by Jan Dupont

P102 adjusts the DC-Offset of the Bal to Unbal input circuit.
On J101A short Input terminals 2 and 3 to terminal 1 (Ground) and measure DC-Offset on terminal 5. Adjust P102 to 0mV at terminal 5.

Thank you very much for your helpful information. I'll try a measurement today.

[andy_c]

Originally Posted by Jan Dupont

P102 adjusts the DC-Offset of the Bal to Unbal input circuit.

Although P102 may have a slight effect on the offset, it appears to be there to optimize the common-mode rejection ratio (CMRR) of the differential amplifier. To do this adjustment, ground terminal 1 and connect terminals 2 and 3 together with a 1 kHz sine wave applied to the terminal 2/3 combination. Monitor the output at terminal 5 on a scope. Adjust P102 to minimize the amplitude of the signal from terminal 5 to ground. You should be able to get a pretty good null. As the signal goes through the null, it will reverse polarity (180 degree phase flip).

[Jan Dupont]

it appears to be there to optimize the common-mode rejection ratio (CMRR) of the differential amplifier.

Yes it does.

This allows P102 (trim potentiometer) to null out any common mode signals on the input lines.

Mine was the easy version. CMMR can be adjusted with shorted inputs, as it is the internal common-mode rejection ("unballance" in Balanced to Unbalanced converter) that is trimmed........