You can easily find a cheap old phone with an SD slot. If you have music downloaded then you don't need good specs, just put it in flight mode so the battery lasts longer.
Note that you might want to get something supported by the Rockbox open-source firmware, just in case the player's own software is less than ideal.
I recently got a HIFI Walker and I really like it. They also have full android versions if you want more functionality.
While researching this very thing, I came across this site hifioasisand used it to guide my first purchase.
The reviews are pretty good and they account for a lot of variables.
If you don't want the cost of a walkman, old LG phones like the V30 were renowned for having high quality DACs and 3.5 mm headphone jacks.
You can pick used ones up on eBay. They don't have 5g or a ton of ram so I wouldn't use it to replace my phone, but it has great audio quality and an SD card for not much money.
Can always go with a modded iPod. The great thing about digital audio is it doesn't really degrade. If you have the money to spend though the fiio's are in a league of their own at the high end.
I use a $50 anbernic handheld that runs a nice mp3 player, has a 3.5 jack, and has physical buttons. It is a great little ipod. This one and the ones you should consider generally have dual microSD slots and that should offer the storage you are looking for.
How is the audio quality on those? I've got the sp and was considering setting it up as an mp3 player for when I'm off grid but I don't have the spoons to do if it's gonna suck.
You could try rummaging around at 2nd-hand stores or pawn shops.
Fiio makes some good ones. I have an old X1 that has always been flawless.
The audiophile community keeps them kicking around! Whathifi usually have good reviews and recs. Let me find a link
Unfortunately, the audiophile community is entirely composed of rubes
HIFI WALKER H2-128GB (or H2 Touch): This is perhaps the best match for your request. It features a, 2.4-inch screen, physical buttons for navigation, Bluetooth 5.2 for wireless headphones, and supports up to 512GB (or sometimes 1TB) MicroSD cards. It is known for high-resolution audio support (DSD, FLAC).
Surfans F20 HiFi MP3 Player: Similar to the HiFi Walker, the Surfans F20 offers a sturdy design, scroll wheel, and physical buttons. It supports up to 512GB SD cards and features Bluetooth 5.2.
Sony NW-ZX707 Walkman (64GB Internal + SD Card): While pricier, this Android-based DAP features a 5-inch touch screen, but crucially retains physical side buttons for volume and playback control. It supports MicroSDXC cards for massive storage expansion. Modded Apple iPod Classic (7th Gen): These modified classics, often sold on eBay, feature SSD storage upgrades up to 512GB, 1TB, or 2TB, and have been updated to include Bluetooth functionality and a USB-C port.
MECHEN H1 Pro / MECHEN M30: These budget-friendly options offer Bluetooth, Wi-Fi, and support for up to 512GB MicroSD cards.
Additionally, for the H2 and F20, there are rockbox CFW if you don't like the firmware it came with.
Rockbix is a custom firmware dating back decades giving relatively easy and logical way to browse and play your own music.
as an owner and regular user of the H2, i feel it important to point out that it also works as a USB DAC.
it also displays lyrics for anything you have .lrc files for - the collection of mp3s on my 512GB microsd card took me about a month to get lyric files for. I created a set of scripts that would either look online and download them, or ran them through some speech-to-text on linux to create them from scratch. This last part was critical as I have a huge number of old-time radio shows and drama which obviously wouldn't ever have lyrics for, but is a lot of fun to read along.
those two features (the USB DAC and the lyrics file support) are what sold me on the utility of this player. Overall, it's a fantastic device and I enjoy the snot out of it.
Looked up prices for these and they're all around $120, modded iPods ranging between $150-$900 and Sony Walkman one is around $900.
I don't have a specific recommendation, but I believe the key words to search for these days are "Digital Audio Player".
I've been following the open hardware Tangara for a while, but they're between production runs right now so you can't buy from them (you might be able to build your own though, the design is all there)
As a heads up, like so many other technologies the middle has fallen out of the market thanks to the proliferation of smart phones. You'll be paying a lot for anything decent from what I've seen
If you don't mind a bit of DIY tinkering you can get an older music player and upgrade it with a new battery, new drive, and rockbox. If Bluetooth is a must you could also get a 3.5mm adapter. Here is an example upgrading an iRiver player.
A kid i know has one from fiio, which he likes
Its small, not 512gb by itself, but features bt and sd card reader
Has physical buttons and a small display
Fiio echo mini
They probably has other models too
I've got one of these. Aka Snowsky echo mini, fiio's brand aimed at accessible/fun audio. It still has the pedigree of a higher end digital audioplayers, including a nice DAC and 4.4mm balanced output. Its also got a firmware update to be used as an inline DAC.
Qualms: lower end Bluetooth codec and lack of gapless playback.
I wouldn't use it if Bluetooth was a must, but I've enjoyed using it with a cheap pair of IEMs.
I kinda want one of those, they're cute. No bluetooth support though, and only supports up to 256GB on the MicroSD slot. Fine if you just have MP3s but you'll eat that up pretty quick with FLAC.
I went through a handful of devices, and none of them ticked off all the boxes for me.
Now I use a Unihertz Jelly Star. It's tiny, it has Bluetooth, Wifi, expandable memory, headphone jack, and in a pinch it's also a phone with a camera and flashlight and so forth. The battery charges very fast, and it's got a halfway decent external speaker. More dedicated audiophiles than me would have to weigh in on if it's pumping out audio signal in all the right bass and treble frequencies at appropriate levels, but it does everything I wanted from a digital audio player.
The Jelly Star will even get an Android 16 update this month, after staying on Android 13 for ages. Unihertz got pressure from the competition. :)
If you want to self-host and store the files on a home server instead of your phone, you could try out Navidrome
Edit: I guess you specified that you know there are other solutions, sorry if this was unsolicited. I have a love for physical tech too so if that’s the reason you want an mp3 player, I get it. I’m collecting records myself :)
Just yesterday I saw a post about using retro game handhelds as a music players. That might interest you.
From the sbcgaming subreddit, the one I see constantly popping up is the TrimUI Brick (Hammer for metal case).
Of course any retro games handheld with Android will work just as well as any other phones with Android. And you can just put it on airplane mode or just don't install any apps that'll give notifications.
Additionally, there's a rockbox app for portmaster.
I used to use my PSP as a music player because my subwoofers would cause my CDs to skip. Good times.
Rockbox has a 3DS port in progress.
I hate .mp3 as an audio codec. However, if you must use .mp3 that why not just get a used Android phone with a McroSD slot? You could even get a USB-C DAC to go with it for better sound.
The Tangara team are taking a break right now, but it's a cool project that is worth being aware of
Try Dankpods' YouTube channel, I believe he is into them and has some favourite, though I can't remember exact model numbers from the top of my head.
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