Recording noise

[Lttlwing16]

I'm trying to grab some recordings of two different phono preamps (ART DJ PREII vs Darlington Labs MM-6) for comparison. I was setting up my gear last night and was greeted with a 1khz whine when the RCA output of my THX 789 AAA SE pass was plugged into the Mbox. Everything was powered on and ready to go, but the record was not playing and the needle was sitting in it's rest.

Signal Chain: Technics SL-23 with AT 95ML cart>ART DJ PREII>THX 789 AAA SE PASS>Mbox (first gen) Line inputs USB out>HP workstation with Pro Tools 8.1

Some interesting notes:

-This signal also appeared when recording with the below chain and imported into Pro Tools:
Technics SL-23 with AT 95ML cart>ART DJ PREII>THX 789 AAA SE PASS>PC soundcard via RCA to 3.5mm>Audacity

-Interestingly this had to be the inverse sine wave from Audacity because when I imported and played it over the noise from the Pro Tools setup it cancelled out!

- When taking a measurement with the RCA cable from the THX 789 AAA removed from the SE pass jacks and the line level inputs were left plugged into the Mbox, the noise floor dropped to below -120dB with the peaks barely visible at -120dB.

Ground Loop, or CPU hum?

Here is a screen grab of the output on my Bluecat frequency analyzer plugin within pro tools. It looked identical from the Audacity output.

Thanks!

David

 

[Lttlwing16]

Looks like this may be a known issue: https://avid.secure.force.com/pkb/articles/en_US/troubleshooting/en211215

"What is being heared is the inherent noise of the Line/DI circuit which sounds similar to white noise with a prominent 1 kHz fundamental frequency and not as prominent harmonic relative frequencies of 2 kHz, 3 kHz, 4 kHz, 5 khz, etc. This noise should not present a problem unless the input source has an extremely low output requiring the Input knob to be turned up very high. If Pro Tools is not playing back and the "Mix" knob is turned all the way up, you should very little noise (or silence) because you are no longer monitoring input. "