I have not yet figured out the best USB-C option for my phone, but I wanted to share some thoughts based on what I have researched.
First, a lot of the adapters with a built in DAC (which is what you need with a FP4, while some phones can send an analog signal over their USB-C port, in which case a simple passive adapter is all you need, but won’t work here with FP), and thankfully so many of the USB-C adapters have decent to great sounding DACs built into them. People usually throw around the Apple and also Samsung adapters as the best sounding cheap dongles.
However, there are a bunch of technical quirks and details that make it a lot harder to find a perfect adapter. First, I don’t remember all the technical details and terminology, and some of it probably is already covered here on this thread (because I remember reading through this thread at some point when researching). Basically, there was something about the Apple and Samsung adapters that didn’t completely follow the procedure and standards of how sound delivery over USB-C is supposed to be done, which results in a total max volume drop on many phones (maybe a 20% reduction or so?), and I think FP4 is one of these phones. Second, if you live in Europe, these adapters are further affected by some regulations in EU, which means that the European versions of the adapter have a reduced power draw that results in an even larger reduction in volume compared to the adapter sold in the US.
I used the Samsung adapter for about 3-4 months, because I read it was a bit less affected by these factors, while the European version of the Apple dongle would have as much as 50% drop in volume it could produce. Still, I noticed that when with my older phone with a headphone jack I found comfortable listening level to be just around the mid point of the volume scale, with the Samsung adapter on FP4 the same level was around 3/4th of the scale.
I largely stopped using the adapter because it became so sensitive to any sort of touch or nudges that it would constantly cut the music playback, and I think it’s due to something in the adapter itself being wonky or starting to break, because I still don’t get any disconnects with the phone when charging or moving data via USB cable and heavily shaking the cable (meaning, it’s probably not due to the USB port in the phone, nor is it due to the plug tightness of the adapter as I did tighten the grip of the plug with some pliers).
Otherwise I thought the adapter sounded wonderful and pretty neutral (no strong coloration in the voice, clear bass, mid range and treble), it also supported the mic/handsfree button of my headphones if I wanted to pause a song or skip it. The volume level wasn’t an issue for the most part, but some music with much wider dynamic range was just barely at comfortable volume levels when maxing the volume scale. Unfortunately, if I wanted to touch up the sound even a little bit with EQ, there just wasn’t enough headroom in the volume scale.
I have understood that you can get the best results weirdly enough with some Chinese brand adapters that use a specific type of DAC chip that’s very widely used by many generic adapters. Get one that seems to have a sturdy build, and since they are made by some random small brand, they apparently do skirt around some of the regulatory factors that have affected these big brand adapters.
Don’t know about the policies on this forum about linking stuff, so I’ll just say that some of the more technical information about DAC chips I found on audiosciencereview forums.
I’m not currently using any USB-C adapters since I’m still trying to figure out what adapter to get and where (in Europe) I should buy one. I’m just using some old Nokia handsfree Bluetooth receiver to connect my IEMs for casual listening.
)