Here's my latest design for the aviation mic based on the feedback.
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Things I wanted to fix:
- Actually be able to control gain properly
- Flat(ish) frequency response despite being a diff
- Prevent motor-boating / noise from bias power supply
Major changes are:
- Lowered working voltage to 5V supplied through a low current vreg to try to eliminate motor-boating problem
- Using an Instrumentation Amplifier (AD8220) instead of dual op-amps
- Using "normal" FET electrets (CMI-5247TF-K) to gain better RFI screening
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AD8220 data sheet is here:
C3/C4/C5 are AD8220 input RFI suppression, as advised in the AD8220 datasheet.
C6/C7/C9/C10 are AD8220 output RFI suppression, as advised in the AD8220 datasheet.
C8 prevents ringing, as advised in the AD8220 datasheet
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R5 sets the gain. At 2k2 this is approx 30x. The usage environment is the electret capsules will be in a mouthpiece a few cm away from the person speaking, so I am assuming an effective SPL of 85-100dB, so an electret capsule output of ~3-10mV RMS. The comm radio accepts 70mV-1000mV RMS, so 30x seemed sensible (90-300mV RMS). There's a pot that can trim it if someone has set the comm radios to max input sensitivity.
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As before the left side Mic Board sits inside brass mesh screening, mounted onto the end of a flexible gooseneck arm. The power/control board sits in a screened PCB enclosure mounted onto the headphones headband. I'm using a thin CNC coolant hose for the flexible gooseneck arm (works really well). Shielded 2-core Canford audio cable connects between the two boards.
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I'm aware that this is probably rather over-engineered for a simple active noise cancelling mic scenario, but I'm enjoying myself!
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Are there any glaring flaws with this design?
