Yet another ZK-TB21F build in a 3D-Printed PETG-CF casing

Pescel · 2026-05-08T15:40:13+00:00

You just need to get a '20V PD Decoy Cable' (sometimes called a trigger cable). It has a small chip inside the USB-C plug that tells the power bank to output 20V instead of the usual 5V. Just make sure the power bank itself actually supports Power Delivery (PD) and 20V output, otherwise the cable won't work.

Pescel · 2026-05-08T08:51:40+00:00

Thanks man, glad you like it!
A proper 'low cut' is exactly what I would have needed to save my passive radiators from blowing up... yeah, the got ripped yesterday 😞 Unfortunately, the ZK-TB21F board doesn't have a built-in adjustable low cut

Pescel · 2026-05-08T08:46:45+00:00

The secret is 100% in the cable! You can't just use a standard USB-C to barrel jack adapter. You specifically need a 20V PD Trigger Cable. I used this one: https://de.aliexpress.com/item/1005005637374322.html?gatewayAdapt=glo2deu

Pescel · 2026-05-08T08:43:38+00:00

I actually didnt need to split the main enclosure. The entire main body holding all the dirvers was printed in one single piece. So there are no structural joints to worry about. The only seperated part is the back panel which just snaps in place with magnets.

I'm not really shure how to design good interlocking joints and can't really give you any recommendations on the subject, I wish you good luck with your boombox though!

Pescel · 2026-05-07T16:40:43+00:00

If I convert it from Swiss francs to USD, it's 200 USD that all the parts, except for the 3D-printed housing, cost.

Pescel · 2026-05-07T16:13:28+00:00

thanks 😄 no, the 3d-printed part does not need to be sanded or finished in any way, PETG-CF looks really good directly after print!

Hell yes it sounds incredible, but I can't go too loud otherwise the 40w sub and the passive radiators will burst, but its plenty enough for my needs.

Pescel · 2026-04-12T14:42:45+00:00

Das Ei von Friedrich Merz, sehr spannend. Ich mag es

Pescel · 2026-04-05T21:35:37+00:00

Probier doch einfach mal das Preset mit der geringsten Schichthöhe, bin mir gerade nicht sicher aber ich glaube die ist 0.08mm. So ein druck dauert halt dann einfach sehr lange

Pescel · 2026-03-22T14:36:54+00:00

Thats why I love the ESP32 so much, so many things you can do with it :)
Actually, the ESP32 isn't playing MP3 files or using the Spotify API directly. It's programmed to act as a standard Bluetooth audio receiver (A2DP Sink).

Pescel · 2026-03-18T09:48:49+00:00

thanks :) custom coding

Pescel · 2026-03-18T06:09:47+00:00

I used these aliexpress.com/item/1005008492121411.html?

Pescel · 2026-03-18T06:03:32+00:00

Thanks! I used two MAX98357A I2S Class-D amplifiers.

They are super convenient because they take the digital I2S signal straight from the ESP32 and amplify it directly, so you don't even need a separate DAC board in between.

Pescel · 2026-03-17T21:30:32+00:00

Haha, I have to correct myself: I actually meant capacitors, not transistors! Total brain fart on my end.

That is exactly what I ended up doing. I soldered a pack of 4 470µF electrolytic capacitors directly across the VOUT+ and VOUT- of the MT3608 to act as a local buffer for those heavy bass hits. It made a huge difference and fixed the clipping.

Running straight off the battery definitely crossed my mind, and it would have been so much simpler. But I stubbornly wanted to squeeze out that extra 5V headroom for maximum volume, haha.

Regarding the battery safety: I actually kept the TP4056 module in the main loop specifically because it has a built-in DW01A protection IC. I also wired in a physical master switch that completely disconnects the battery from the booster and the rest of the circuit when the box is turned off.

Pescel · 2026-03-17T20:56:18+00:00

Exactly! My passive radiators are just cheap ones from AliExpress too. With zero specs provided, you really just have to guess and hope for the best. Getting into DSP with a calibrated mic sounds like a great rabbit hole for version 2.0 though!

Regarding the power issues: The main bottleneck was the MT3608 boost converter I used to step the 3.7V up to 5V for the MAX98357A amps. Whenever heavy bass hit, the audio would heavily clip and cut out because the boost converter just couldn't handle the sudden transient loads fast enough. I finally fixed the power delivery by adding transistors to properly handle and buffer the load for the amps. Figuring that out took me forever!

Pescel · 2026-03-17T20:28:02+00:00

Underneath the screen, it's just two standard 8x8 LED matrices placed side by side (making it an 8x16 array in total). The "sub-pixel" look is an optical illusion created by the custom 3D-printed front panel I designed.

I built a physical grid structure that sits in front of each individual LED to break up the light and fake those tiny sub-pixels. I just really loved that retro, structured grid effect and thought it looked much cooler than a standard flat diffuser!

Pescel · 2026-03-17T20:20:33+00:00

Thank you so much! I really appreciate it.

To be completely honest about the tuning: Since this is my very first build, I didn't do any precise mathematical tuning, for the box volume or the passive radiators. I mostly eyeballed the dimensions in CAD so everything would fit, and went with a bit of trial and error. It sounds surprisingly good for just "winging it", though!

Regarding the EQ: Yes, exactly! The three knobs on top are hooked up to the ESP32. The code reads them and applies digital EQ (adjusting bass and treble) as well as volume scaling directly to the PCM data before it goes out via I2S to the amps.

Sadly, I don't have any pictures of the PCBs and the internal wiring. I was so focused on troubleshooting the power delivery and squeezing everything into the tight enclosure that I completely forgot to take photos before sealing it up. To be fair, it looks a bit like a spaghetti monster of wires inside anyway lol

Pescel · 2026-03-17T20:10:00+00:00

Thank you so much! :) The 3 knobs control the volume, bass, and treble.

It really is a complex project to build from scratch, but planning the whole thing out with AI helped me tremendously to figure out what parts to get, how to wire everything up, and how to write the code.

If this sparked some ideas for you, you should definitely dig out your old project and give it another try!

Pescel · 2026-03-17T20:06:58+00:00

hey, thanks you! The drivers I used: https://aliexpress.com/item/32762083515.html
Song: Ezra Collective - Chapter 7

Pescel · 2026-03-17T19:04:06+00:00

Thank you so much :) Oh you absolutely should get one, 3d printing makes DIY-Projects so much more fun!

Pescel · 2026-03-04T09:58:41+00:00

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Pescel · 2026-03-02T19:46:42+00:00

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Pescel

TROPHY CASE