09-25-2020, 04:02 AM
I don't interpret the schematic that way - R802 and R803 have a value NC/0R. Typically this is for a part that may only be populated for testing or as an alternate component. I haven't checked the board but my guess is they're not populated, and would be used to connect headphones if U801 wasn't populated. If R802 and R803 were populated the dip switch wouldn't be able to disconnect the headphones. I think it's being used as a pure switch.
The BCT4717ETB-TR datasheet general description calls out its well matched and flat on resistance for audio use, although the subsequent plots fail to cover that frequency range as you point out. Have you found the resistance of the series inductor? While the high output impedance isn't ideal it's also unfortunately not unusual, even with 'proper' headphone amps. It shouldn't introduce harmonics (unless the claim about linearity in the datasheet is wrong) but will alter frequency response if the headphones have a low enough impedance that varies with frequency. Typically it's a bit of a bass boost which I'm getting with CX300-IIs (18R). The Porta Pros always sound bass-heavy to me, and at 58R won't see so much effect anyway. The biggest issue I have is whatever causes the peaky CPU use leading to buffer underruns - much more intrusive than frequency response problems.
The possible DC offset is more of a concern - time to dig out the test gear...
If you want to do some proper audio testing have a look at https://www.roomeqwizard.com/ - it's capable of a lot more than just room acoustics measurement. I think you could use the offline measurement method with the PinePhone to get frequency response and probably a distortion profile.
https://www.roomeqwizard.com/betahelp/he...ments.html
You'll need an audio interface with a half-decent line level input.
The BCT4717ETB-TR datasheet general description calls out its well matched and flat on resistance for audio use, although the subsequent plots fail to cover that frequency range as you point out. Have you found the resistance of the series inductor? While the high output impedance isn't ideal it's also unfortunately not unusual, even with 'proper' headphone amps. It shouldn't introduce harmonics (unless the claim about linearity in the datasheet is wrong) but will alter frequency response if the headphones have a low enough impedance that varies with frequency. Typically it's a bit of a bass boost which I'm getting with CX300-IIs (18R). The Porta Pros always sound bass-heavy to me, and at 58R won't see so much effect anyway. The biggest issue I have is whatever causes the peaky CPU use leading to buffer underruns - much more intrusive than frequency response problems.
The possible DC offset is more of a concern - time to dig out the test gear...
If you want to do some proper audio testing have a look at https://www.roomeqwizard.com/ - it's capable of a lot more than just room acoustics measurement. I think you could use the offline measurement method with the PinePhone to get frequency response and probably a distortion profile.
https://www.roomeqwizard.com/betahelp/he...ments.html
You'll need an audio interface with a half-decent line level input.
