Humming sound on one side of the headset, with a used audio interface

Question

I have purchased a used Apogee Duet 2, and I am testing if everything's fine and I can close the deal. However, when I connect a headset to it, I hear a slight humming sound mostly on the left side.

About the noise :

• The noise is not the typical 50 hz, but at first it was slightly higher than a F#5, now, a C#4
• I thought at first it was only on the left channel, however there's the same humming sound on the right channel, but very very very dim

About the troubleshoot until now

• it does the noise (C#4), whatever the situation:
  • when the interface is connected in USB only, or with the optional power supply in addition to usb
  • when the mac is on battery, or plugged grounded (with the grounded cord between the power block and the outlet)
  • Even when the interface is not plugged to the mac, but only powered by the interface's own power supply (which is not grounded)
• When I play a digital instrument (like an electric piano in garageband, or a software synth), the humming stops when there is sound, and it reappears after the end of the decay! Also, when the release is almost over, there is a sizzle preceding the humming. As if the digital-analog convreters behaved differently when loaded than when not... To me, this could be the symptom of overworked DAC, what do you think? I think such a high-end audio interface should not have this kind of humming.

Also, note that

• I tested with several headsets
  • All were regular ones, 32 ohm impedence I guess
  • (I purchased it to use with a high impedence headset which I don't have yet, but I guess it should still sound nice with regular headphones; and also it does it on only one side, so I think there's a problem)
• I tested with the headset left side on right ear, and right side on left ear.
• It's a rather dim sound (on the left), but I hear very slight sounds, like some USB chargers, USBC multi adapters, and so on. (I once heard a spider walking on the wall, 30cm from my head, in the middle of the night.) On the right side however it's not just dim, it's very, very, very ultra dim.

I read that for humming, it's possible that it's a grounding problem, however I think it's not that given the troubleshooting described above.

Before me, the interface has been used a lot of time mostly to listen to music (so much that the oled screen is overworked: it is dim and there is burn-in). This is not a problem as I knew it, but do you think the DAC inside can be overworked too, due to the long hours of service?

Or is there an option somewhere that I should set up correctly? (I guess not, because otherwise, it should sound the same on both sides.)

Sorry for long question. TLDR: read bold text only.

Edit: About the noise. I tried changing the sample rate: right now, when I change to any sample rate, the humming pitch is C#4, and after I play one note of a digital instrument, the humming pitch goes to A4. Then, if I change the sample rate, it the humming resets to C#4... And it's pretty systematic.

Answer 1

Based on your investigation, it seems very clear that

1. It's not merely a mains hum picked up due to a grounding issue: the frequency is wrong.

But that's academic because

2. It has to be a fault with the item you're purchasing, unrelated to your other equipment: the issue occurs "Even when the interface is not plugged to the mac, but only powered by the interface's own power supply". This is the real killer.

It's always disappointing when new kit misbehaves and we have to countenance delays while returning it, but I'm afraid it looks pretty cut and dried. And your observation about the left/right imbalance in the volume of the noise belies even the already extremely unlikely thought that it might just be some option that needs setting.

Since you are still at the point of testing if everything's fine and whether you can close the deal, you should reject the item without further delay.