1. the resistive attenuator is placed at the input of the phono stage. I need to have c. 60dB of attenuation to remove the hum and the noise from the ADI-2 pro so that the phono stage can be successfully measured without separating the distortion and noise floor measurements.
2. The phono stage noise is quite low- at about -87dBA relative to 1mv input and with 70dB of gain.
• ADI-2’s nominal single ended output level is +19 dBu, 6.9 V.
If you’re heading for 1 mV input your passive attenuator should have 76 dB attenuation.
This would improve hum by 16 dB.
3. I need to use coherent averaging to measure the distortion, but it's barely different from the loop back distortion of the ADI-2 pro.
8. The phono stage has XLR and phono inputs and outputs. The XLR outputs are balanced, the inputs are not. The + and - pins of the input XLR are connected to the + and gnd connection of the phono...
• If you are using ADI-2 Pro‘s XLR Out in single ended mode, pin 3 should NOT be grounded, but left open.
If you short a high signal level to ground, this can degrade signal quality.
ADI-2 Pro‘s XLR Out is NOT servo-balanced.
ADI-2 Pro’s THD specs can be found in the manual, that’s about what you can expect in loopback mode, without the use of a Double-T-Filter for the fundamental, on ADI-2’s input.
7. There is no evident hum in loop back mode- i.e. the phono stage is shorted.
Clear evidence ADI-2 Pro’s in/out is clean, hum origins from external sources.
6. No hum pickup is measured if the input ports are shorted.
So, in the interconnect ADI-2 to Phono the hum source is located.
Try as short as possible cables, add an extra interconnect ground wire of decent gauge.
Screen the (improved, see above) attenuator.
Remember, your signal level is just 1 mV in this stage, EVERY ever so little stray noise current spoils the result there.
Locating the passive attenuator INSIDE the phone stage (from RCA to circuit board) would exclude some interconnect resistances.
The phono stage output impedance is from an opamp with c. 130dB of feedback at 60 Hz. The output impedance is essentially whatever the cable resistance is.
You’re missing an inline output resistor.
Few OP-Amps (mostly some lesser ones) can happily drive an almost pure capacitive load, like a cable, without getting instable.