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Long time (lurker) fan of StackExchange, first time poster.

I am attempting to connect an Astro A50 bluetooth headset via its own basestation's analog(ue) combined line input/output (4-pole CTIA socket), using an OEP A262A1E audio transformer (inside an A262CAN screening can and also inside a metallised instrument box) to isolate and balance the mic signal for improved CMRR as it is fed into the audio mixer. The headphone signal comes from that same mixer. I am hearing a crazy amount of 'data' noise on the mic signal (that's fed back into the headphones), and I have tried a bunch of different grounding strategies to try to reject/reduce it - see the attached sketch for the wiring diagram:

Bluetooth headset connection circuit

Here are the grounding strategies I've tried:

  1. A+B+C+D ('ground all the things')
  2. A+B separated from C+D
  3. A separated from B, and C+D
  4. A separate from B seperate from C separate from D (none tied to any other)

All four of these strategies sound the same in the earphones - wideband noise across the whole audio spectrum (checked with an 20Hz-20kHz spectrum analyser), which is periodically modulated by regular clicks; it just sounds like data noise rather than hum/hiss etc. The Astro A50 basestation is powered by USB and (annoyingly) has to be powered by a PC as it won't wake up unless it talks to an operating system, so it can't be run off a 5V phone charger or (battery) power bank. So, we have both the PC's ground and the mixing desk's ground in play when the headphones are fed from the mixing desk...

There is a single instance where there is NO such noise on the mic signal, which is when I plugged my (mobile/cell) phone into the headphone socket, running off its own internal battery. The mic signal into the audio desk was clean, and the music played off my phone into the headphones was also clean, and there was only a single electrical ground being used, which was from the PC powering the headset base station. So I guess it could be a ground-loop issue? Any ideas?

[I used to be super hot on this sort of thing, twenty years ago... In fact this transformer was carefully desoldered from a different, unfinished circuit that I made in 1994!]

(Here's the transformer's datasheet:) A262A1E Datasheet

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    \$\begingroup\$ Yes, a ground loop issue is the most likely. \$\endgroup\$
    – Andy aka
    Feb 14 at 10:20
  • \$\begingroup\$ @Andyaka - what would you proposed solution be, to remove the ground loop? \$\endgroup\$
    – AndrewC
    Feb 14 at 10:52
  • \$\begingroup\$ I cannot visualize your full system so I can't make a suggestion. \$\endgroup\$
    – Andy aka
    Feb 14 at 10:54
  • \$\begingroup\$ @andyaka What other information would you need to visualise my full system? I'm happy to provide it, if it allows you or others to offer a potential solution, thanks! \$\endgroup\$
    – AndrewC
    Feb 14 at 10:57
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    \$\begingroup\$ @danmcb "Could be that the output is just noisy and you're stuck with it. Anyway the noise is almost certainly conducted not radiated"... how right you were! Kudos. \$\endgroup\$
    – AndrewC
    Mar 1 at 20:34

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UPDATE: I have finally found that the awful noise is being passed from the PC that powers the base station, via USB, and the base station has abysmal filtering (if any) on the incoming DC power. As soon as I power the base station from a laptop that's running off its internal battery, the wideband, modulated noise goes away. Plug in the laptop's PSU and the noise returns. What a truly awful analogue signal path decoupling from digital and power lines! 🤦‍♂️ /UPDATE

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