Thank you so very much for your time and excellent explanation I truly appreciate it.
I believe that what you have sent through will really help me to maximize the measurement capabilities of the QA401.
My RCA input or signal input has a 4.7ohm resistor between its signal ground and the speaker negative or speaker ground. This raises the signal grounds node a little reducing noise on the input as I’m sure you are aware.
As I was only using a single ended input to the QA401 I didn’t want to connect the amplifier end of the shield from the coaxial cable directly to the speaker or signal ground as I was afraid that I could make a ground loop. So I wanted to rely on only one ground reference that being the signal ground.
So in my mind I thought that as the QA401 is isolated from my mains or laptop supply ground I can set its ground reference from my signal ground when I connect the signal input, from the output of the QA401.
On your schematic/ diagram, you show the amplifier end of the coaxial cable as not connected to anything. I did not know that you could do that. I thought that it had to be connected to something so it would allow stray noise to have a path. So I connect that end to my signal ground through a 100ohm resistor.
I have ordered some twisted pair cable so I can make a differential cable from the output of my amplifier to the attenuator connected directly to the QA401 as you have shown.
Your attenuation circuit and pcb idea is awesome I really like the pcb mount BNC connectors. I’m going to order some from digikey along with some precision resistors this weekend. I will then make a prototype pcb and then draw up and get made some PCB’s from china once I settled on a circuit.
If you have anything else to add Zoran, I would once again be truly grateful.