(09-25-2020, 04:02 AM)wibble Wrote: 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.
From what I can tell populating R802 and R803 would bypass the analog switch and put the headphone jack into audio mode permanently. Since they are listed as "NC" I assume they were to be used for testing after removing U801 (the BCT4717ETB-TR). Do note that populating those resistor footprints on the board with 0R might damage U801 since it's going to put all it's available sink current into making COM1/COM2 follow either NO1/NO2 or NC1/NC2 using its internal reference(s).
The DC offset should be measured under load and across L/R just to be sure. We don't know if the A64 HP-FB pin is high or low impedance but I think it's pretty safe to assume the BCT4717ETB-TR is low impedance as long as it's powered on. I measured the DC offset with my scope and the transient isn't sharp enough to be heard as a pop, it takes around a second for it to settle so I'd say it's well below the audible range.
As for the datasheet mentioning audio switching I think they meant at the preamp stage (that said I still disagree with the application when they're specified for strictly positive signals). Switching amplified signals (which is what we're doing) can be done but it's a can of worms and in applications like these a discrete OpAmp would be more suitable. But please don't do that, use a complete headphone amplifier IC instead. Using an off the shelf IC knocks down the board space requirements, has huge advantages in external noise suppression and usually comes with click/pop rejection.
To test things out I connected an external headphone amplifier with high input impedance (>1k) and the bass remained boomy but slightly attenuated compared to a direct connection. I might try connecting it straight from the R802/R803 pads later if I can find them.
The rule of thumb with amplifier impedance is to have at least 1/8th of the headphone impedance, which would require us to have <1 Ohm to sound good with anything we can reasonably expect users to plug in. This is because any harmonics inherent of the headphone will be attenuated by current induced back into the amplifier through the membrane driver coils. If the amplifier output has too much impedance the attenuation here will suffer because it cannot drive the amplifier output hard enough to match the amplifier input through the induced harmonics from the headphones. This isn't linear along any dimensions so you need a good chunk of luck or a catch-all like low impedance to make it sound good on dynamic headphones. Planar headphones do not suffer from this in the same way but I think those are rare enough among PinePhone users to be treated as an edge case.
BTW, the impedance of the series inductor looks to be specified as 600 Ohms at 100 MHz but no other details. That number doesn't mean anything in the audible range since 100 MHz is very far off the audible range and from the graphs I've seen they should more or less turn into oddly behaving resistors in analog audio circuitry. They were probably meant for digital signaling.
This is one of my test tracks which turns out to sound particularly bad on my device and headphones https://www.youtube.com/watch?v=GAdn1ny1JmA (although I have purchased FLAC versions I use for testing). It has plenty of detail and a wide range of effects. Listen for distortion in the mids and you'll know what I mean.
For reference I use a Scarlett Solo 3rd Gen fed into a Loxjie P20 in fully balanced mode with Ericsson 5670 tubes and a set of balanced modified ATH-M50x headphones. I'm not expecting the PinePhone to match that performance (even though my desktop combination has some impedance issues), but it has got to do better than what it does right now by the time we reach a wider market than developers and early adopters.
Thanks for reminding me that REW can do offline measurements, I'll have to get a hold of some adapters first but I'll definitely give it a try.
