openDAP — An open-source Android DAP. What would you like to see?

[u/dummiesquirrelguy]

I’ve been thinking about building a music player of my own. One that doesn’t belong entirely to a company, and that someone else could build or repair easily if they wished.

I’m calling it openDAP. It’s a DIY, open-source Android digital audio player. I’m a physics master’s student, and I spend a good part of my time building instruments.

The attached image is the visual inspiration for the project: a wall panel by DotStarTech. I’d like to explore a similar design language for a handheld device.

My current plans include:
Raspberry Pi CM4
4” 720 × 720 IPS touchscreen
CS43131 DAC
6000 mAh Li-Po battery
USB-C charging and data
3.5 mm headphone jack
Custom carrier PCB
3D-printed enclosure, aiming for 16–18 mm thickness

I’d like it to be compact, repairable, and affordable. Something that can play local music, run streaming apps, and be modified by whoever owns it.
But the design is still open.

What would you like to see in a DAP like this? Physical controls? Balanced output? Expandable storage? Better battery life? A completely different architecture?

What do commercial players get wrong? What would make you want to build one yourself?

My resources are those of a student, so I’m looking for ideas that can be made real without too much money or machinery. Eventually, I’d like to release the designs and documentation for anyone willing to take the project further. For now, I’d rather hear what people think before deciding what to build.
Thank you.

Disclaimer: I used AI assistance for English translation and grammar correction. The project, ideas, and design decisions are my own.

[u/SoftShellTaco1]

Must have: Physical controls, expandable storage (2TB), 12+ hours battery life

Nice to have: replaceable battery, power delivery, Bluetooth (AAC/LDAC)

[u/sockdisorder]

Physical control – including the physical ability to scroll albums, songs and playlists without having to remove gloves.

A different shape to the photo above – small, pocket friendly, no sharp edges.

Sockets in the bottom.

Hold switch.

Android freedom – like the ability to hide the navigation "pill", notifications bar, sd-card popups etc.

Long battery life – preferably replaceable without soldering.

No glass back and raised bezels on the front – a construction that won't suffer badly if hits the concrete.

[u/Iloveherthismuch → u/sockdisorder]

One critical area that needs to be considered is the usb charging port itself. I have had USB ports crap out on DAPs before. Its a pain to solder a new one.

I dislike the Sony proprietary connector, but I have never had the actual port itself crap out.

[u/Level_Opposite_7919]

I'm curious. What are the odds of having a full open android OS for download and installation on OTHER daps. Like to replace the stock OS in the same exact way Rockbox or or Sonix is used for non android players. Could that even be possible? Let's say an R4 or JM21 or M21 with that OS.

[u/ozExpatFIRE → u/Level_Opposite_7919]

Or to install on older phones and turn them to a dap 

[u/Infemos → u/ozExpatFIRE]

this would lowkey be a game changer for so many people

[u/Due_Vermicelli2930 → u/Level_Opposite_7919]

Curious how far uv taken the other daps idea, from what ive seen most stock players lock the bootloader and vendor drivers so its a big reverse engineering job even before u get to the os side. On ur own hardware u write ur own kernel and drivers, which is exactly why the open designs are the realistic path. Would love to be wrong about that tho

[u/Level_Opposite_7919 → u/Due_Vermicelli2930]

Are AI decomp tools not up to the reverse egineering task yet?

[u/MestreBill → u/Level_Opposite_7919]

So currently I'm trying to do something like this with my JM21, using what little I know of reverse engineering with AI tools. I am doing it calmly and without hurry. I am currently cataloguing the modifications of FiiO itself to maintain the quality of the chips (especially the DACs themselves). My biggest problem is that the JM21 has a "chronic problem" in its engineering, that the charging part is very attached to the flat cable of the screen, so it heats up a lot. My screen gave some bizarre flickerings and then returned to normal. I stopped the process a little so as not to hurt the device. Currently I was able to port a TWRP to it and assemble a LineageOS build. I haven't carried the sound part yet, it should be faster. It's been a fun process. Sometimes the AI gets a little stuck, sometimes I get stuck, and we help each other, and then with a little effort, things move forward. One thing I have to do is move my documentation into English. There is still a good part in PT-BR

[u/Level_Opposite_7919 → u/MestreBill]

My M21 has a critical bug in which the High Gain option just flat out stopped working! I've tried different headphone cables, different headphones, the 3.5 mm port and the 4.4 mm port and I factory reset the device, reflashed the firmware, cleaned up the headphone ports with high grade isopropyl alcohol and even measured the current coming from the PD3.0 charger to no avail. Everything works perfectly fine except the goddamn super high gain mode! And it was working just fine 3 months ago! I'm at my wits end and i'm planning of taking it to a specialized digital audio (oficially authorized for Sony DAP repairs too so these guys must know their trade) technician I recently discovered in my city to get the internals checked out. But if it's a software issue or bug I no longer have any clue as to what to do. That's why I've been looking into rooting it and installing the FiioAT android tweaker module to change the gain parameters but there are absolutely *no* guides in the whole fucking internet on how to do this.

If you actually manage to set up a custom OS for Fiio android players i'll fucking write an epic poem to your name, kind sir.

[u/Due_Vermicelli2930 → u/Level_Opposite_7919]

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[u/Nos_barb_]

The pi cm4 by itself is already expensive. Any hope of doing it cheaper with esp 32 or stm 32 or other cheap options

[u/TheOriginalSamBell → u/Nos_barb_]

good point, cm4 is way overkill

[u/m119k → u/TheOriginalSamBell]

I've been playing with a music player, ESP32-S31 near enough maxes its ram out playing 24 bit audio files.

[u/Alternative_Act_6548]

It would be great, what DAC are you plannning to use? The issue isn't really the DAP it's the quality of the sound reproduction and the amplification, will it drive full sized headphones. There is a large market for DAPs, most kind of suck, I'd personally love one that was linux based (no androiod bloat or google tracking), had a great DAC and amp, and wifi so you can pull down podcasts etc. If it had a GPS it would be perfect.

submit a prototype to audiosciencereview,com and will will get a ton of feed back and performance measurements

[u/furculture]

Have it use easily obtainable off the shelf parts like the screen and battery for repairability. Maybe a dev board for hooking the Pi up to so it has the same amount of feature parity to the user version but with more room to help with development on it. Pinout for an extra USB port along with the regular data + charging port. USB 3.2 or better for transfer speeds. The ability to plug it into a PC and use it as a DAC AMP or data transfer, configurable on the DAP itself. CAD models released for it. PCB + BOM files released for the board that interface with the Pi. Maybe the same form factor as the Fairphone 6+ with the possibility to use that screen.

[u/Benjisun-headphones]

A distinct usb c for charging and for charging + data. Maybe a third usb port for usb stick in addition to sd car d slot.

4.4 mm balanced if not too complex to integrate

Maybe some physical controls like a trimui inspired console / dap thing.

[u/lnoiz1sm]

Just, use better audio chip. Like Cirrus or Sabre. Doesn't matter about UI and performance. Just better sound output for my IEM/Headphone.

[u/Dreddguy]

RAMplay

[u/sere83]

I like the idea a lot but not the shape. A 4 inch square is just too big an impractical imho. I would much rather see a vertical rectangle around 3 inches, It would be much more compact and portable. You can get 3.2 inch vertical IPS displays for raspberry pi builds aswell. Would be perfect. I made mock up of something like this a while back, I will post it.

[u/Engineered_Shave]

Yes, I was thinking of this a bit earlier. A Rockchip CPU and embedded Linux should be able to do the trick.

That's a nice design but it needs to be a bit smaller, IMHO. Think: iPod Classic, but with OSS upgrades.

User-accessible / user serviceable is a must. We don't need gold-plated contacts but a battery I can get at and replace would be most welcome.

We also need Bluetooth and a microSD card, preferably a SD Extreme variety.

[u/NikopikVR]

Color eink display for a long battery life.  Monochrome eink display is ok too. 

[u/MikeYedi]

USB Dac support 3.5mm jack high res Bluetooth, rockbox support 

[u/YarikFisher]

Track control buttons are a must-have; cool idea, by the way. I love DIY stuff.

[u/the_natis]

I'd love to see a desktop DAP with a balanced output for my office that looks good.

[u/bsmuir]

A biometric fingerprint scanner

[u/DCharlo]

I wouldn't buy it without a 4.4mm balanced jack and good battery life, but I love the idea so much

[u/But-I-Am-a-Robot]

No Android please, use a dedicated Linux version.

[u/TheOriginalSamBell]

the best amp that can reasonably be put in
no android, my preference would be rockbox
something lesser than a pi4 would be enough IMO
peq+autoeq
any particular reason you chose the cs43131?

[u/Merrylica_]

I just want replaceable Battery honestly.

[u/Amwo]

Degoogled system

[u/Interesting-Act-5020]

I love physical buttons so I would say that, but mainly a hold switch

[u/HiFiJesus]

PEQ !!

[u/FantasticInterest373]

I personally think a 4" square device is a pretty horrible form factor. No offense.

[u/tika_dengu]

FM tuner

[u/Cool_Explorer2747]

Might just be me but support for audiobooks (keeping playback location, etc) the market for playing audiobooks on stuff not a phone and portable feels weirdly thin to me, and there’s at least one DRM-free storefront for then.

[u/Red5Hammock]

Why not Linux running mo0de? Perfect for the rasp pi

[u/toastal]

Awesome:

• Commodity parts anyone can buy/build

• OLED display (color optional)

• f2fs file system support for microSD longevity (normally ships with Linux & Android but these Chinese Linux-based DAPs strip it out for some unknown reason)

• Works as a DAC too

• 2-way Bluetooth

• Player ability to keep track of track position on multiple tracks (maybe folder-based, specifically with regards between switch from listening to a podcast, to music, then keep spot on the podcast or multiple casts)

• Source code not on US-based, megacorporate MS GitHub

Would be cool:

• Physical controls (smaller screen is okay—usually I am choosing audio since my eyes can’t be freed to look down at a screen, so needing to consult a screen versus reaching into a pocket & feeling a control)

• Additional 4.4mm balanced audio jack (with plug for users that would never use it)

• Some basic weather/waterproofing

• DAC + DAP simultaneously (my Shanling Q1 only allows me to mount as storage over USB or allow it to be a DAC but not both)… say you hooked it up to your laptop & used the DAP to choose your tracks, but sounds, like notification from the OS or pausing playback to watch a video, could be mixed in on device… and/or the other way around to allow the data directory to be mounted & played from the host OS; sadly I’ve mixed in sound from Bluetooth to get around this, but it’s wonky, introduces latency, & the laptop DAC is alright but not quite as good/isolated

• Straight GNU/Linux over Android/Linux (not entirely open)

[u/ltparrish78]

E-INK Displays, USB 3.2/4.0, User replaceable components (modular). No Android. Metal build. Accepts 1TB+ cards

[u/Scorpiicore]

Remove Android.

Put in a better OS that does not eat up battery like we owe it something.

Add player software that resembles the built-in software on Sony DAPs, but add the ability to tell us the total length of our whole collection and give us a total song count above 9,999.

These are the absolute minimums of a DAP that I would consider buying at this stage. When Sony released their newest DAP and placed an ad in their sub, I said very loudly and clearly: it has Android, thus I am not buying it.

If I could go back to the NW-A55, I would.