I am relatively new to electronics and new member here. I'm designing an interface to enable a passenger in a private aircraft to listen and speak on the aircraft intercom system using her Bose noise-cancelling QC20 headset, while simultaneously listening to her iPad through the same headset. The iPad audio must not leak into the aircraft intercom system. The interface will also provide power to charge the QC-20 battery. The schematic is attached.
I am open to advice and have some specific questions.
- I'm confused about the numerous "grounds" involved. The aircraft interface provides 28v DC relative to "ship's" ground, and the aircraft intercom audio uses, I believe, an independent ground (I have not checked for a voltage between the two). The iPad has its own indeterminate ground level - it's usually on battery power but conceivably could be connected to a charger plugged into a 110v AC source from the aircraft's "inverter" power supply (not sure of the details on this). I've attempted to deal with these as follows:
- Isolate the aircraft 32v DC power ground. It sees the L1 choke and DC-DC voltage converter input (Recom RS-6 2405D) but nothing else.
- Connect the aircraft audio ground and the power converter's ground.
- Put a high-pass filter on the intercom audio in lines (COMM L&R) to block any DC offset.
- Put an optional high-pass filter on the connection between the iPad ground and the audio ground/power converter ground described above. Using JP4, this filter can be bypassed and the iPad ground connected to the interface's ground, or the iPad ground can be left floating.
- The 100nF/10K high pass filters have a -3dB freq of about 160Hz. But if what I'm trying to do is block DC, is there any reason not to go much lower (like I tried to do with the circuit off JP4)?