Balanced cable for SE measurements

I’ve been having trouble getting rid of the 50hz component from my SE measurements. As a result of numerous tests, I determined that part of the problem arises in measurement cables that catch 50Hz.

Below is what I’ve come to with the question:

does this make sense?

I built a balanced to SE cable using STAR-QUAD cable (Canare L-4E6S ) in such a way that the ‘-’ input of the QA403 is shorted to ground in the RCA plug. In this way, I hoped to take advantage of the common mode noise rejection properties of the balanced connection when measuring SE.
Below is a photo of the cable:

In the screenshots below are measurements of the DAC output with MUTE enabled in windows.

The measurement with my cable is flat and very stable. I can move the cable around and nothing changes. The measurement using a regular cable has a peak at 50 Hz, which height depends on the position of the cable and is very unstable.

Am I on to something?

I went round and round with this.
A lot of people think the QA ground should be shorted to the DUT (device under test) shield etc. They are wrong. If I understand what you did with yours it’s exactly correct.
But you need this on both sides. I was not able to get rid of my 60Hz until I did this. Now It’s mostly gone.

The lowest noise, and most signal rejection I got was running the QA - center BNC to the shield of the DUT plug and the QA center + to the DUT + pin. The QA “ground” is just a shield over the cable but does not connect or short to anything on the device side.

I had an Ebay vendor make me the cable for 1/4" that works perfectly. I’m sure they can do it in RCA if you wish.

I believe you are right, however, with one exception. If the DUT is an amplifier and testing is done using the generator built into the QA40x, the ground potentials of both devices are already equalized by this generator connection. In this case, the shield of the input cable can be left floating on the DUT side if the DUT ground is probed differentially as in the described case.

On the other hand, however, if DUT is a DAC and there is no other connection to the QA40x than the measurement cable then it is better to connect the cable shield on both sides.

I made two versions of my cable, one with floating shield on DUT side and the other with the shield connected to ground on both sides.