ESP32 LyraT v4.3 Aux input circuit is highly confusing and surely wrong

[danak6jq]

The Aux input from the 3.5mm Aux input jack has a 0.1 uF capacitor from left channel to right channel (C62), as well as 0.1u caps shunting each of the left and right inputs to Au_ground (C64, C65). This seems really wrong for audio input processing. Anyone have any idea why? I'm about to look at the Everest docs and make sure then remove those parts. C62 destroys left-right isolation and C64, C65 destroy frequency response. Possibly these are smaller value capacitors actually on the PCB? I am seeing insanely high cross-over from the left to right channel, though.

Espressif - I'd like a response on this, I believe it is a bug in the hardware.

Cheers,
Dana

[ESP_Sprite]

Agree that these components look out-of-place - I've asked someone on the hw team to figure out if these components have a use.

[danak6jq]

Thank you for looking into this. I also do not understand the purpose of C66 and expect it would also reduce the audio quality of the right input. I understand that R78 / R132 is for Aux Detect (detect when a connector is plugged-in to the jack), R107 is intended to load the right input the same as the left (R78).

Thanks again. I believe that C62, C64, C65 and C66 perhaps should all be removed for good stereo capture. It is possible that C62, C64 and C65 are intended to reduce RFI but the values are far too large for that purpose.

[danak6jq]

C66 is effectively from a DC bias slightly lower than Au_VCC33, coupled to ground via 10uF. It looks completely wrong for capturing audio.

Cheers,
Dana K6JQ

[danak6jq]

To confirm, I removed C62, C64, C65 and C66 from the board, and now I see good separation between left and right aux in, and equal amplitude (within < 0.01dB) between the two channels. Before, right channel amplitude was substantially reduced by the additional loading by C66.

I do not understand the purpose of those capacitors; I have not tested the Aux Detect code yet, I can only imagine the caps were loaded in to prevent noisy aux detect or something? They certainly don't appear to serve any good purpose for audio capture.

Thanks for follow-up on this!

Dana

[danak6jq]

Agree that these components look out-of-place - I've asked someone on the hw team to figure out if these components have a use.

Any update? I removed C62, C64, C65 and C66 and now I've got a very decent line-in sound-card

Cheers,
Dana K6JQ

[danak6jq]

Anything? I suppose no one uses that line-in circuit, eh? It's quite broken with those caps.

[ESP_Sprite]

Nothing yet, still waiting for a hardware engineer to look into this.

[danak6jq]

Thank you - I'm looking forward to more information.

Cheers,
Dana

[ESP_Sprite]

FWIW, they do agree that just knocking the caps off is probably the easiest solution for existing LyraT boards, seems the discussion is mostly if we can/need to find a better way for newly produced boards.