I have had a few emails and questions regarding this result today
Some general comments, you shouldnt use a RCA to XLR adaptor like this.
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Any noise currents flowing in the RCA screen will be seen by the amp. This is why we dont fit RCA inputs on our amps. You have to use a cable wired as below.
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Fig 3 from Hypex guidence notes
With this configuration the XLR screen is terminated at the far RCA source end. Noise currents flow in the shield wire and not the signal conductors eliminating ground loop based noise issues. Our RCA to XLR cables are configured this way as are the mentioned Benchmark ones.
Secondly it should be noted the Hypex/Purifi speaker outputs are ground referenced. As such from a measurement perspective it is possible to get noise problems and the AP balanced input should have the same cable config as shown above to again make sure ground currents flow in the shield and not the signal conductors.
We often use (as do our customers) RCA single ended sources with our Hypex and Purifi amps and dont hear or measure any noise issues.