2.1 Bluetooth Sound System - Fully Printable

Strictly speaking, fully printable is a slight exaggeration. To be honest, I am still struggling with printing PCBs (although this would be reeeeally nice), drivers, and a few electronic components... ;D No, seriously, all you need in addition to the printed enclosures is a small amplifier board (and power adapter), the speaker drivers, cables to connect all this to the amplifier, and a few screws. All in all this should be a very economical project. All parts print on an 18x18 cm printer bed, assembly is super easy and (virtually) does not require any soldering or electronics knowledge.

First a little background. So what is a 2.1 sound system? In essence this is a stereo system with an added subwoofer. Stereo, or stereophonic sound is a “method of sound reproduction that creates an illusion of multi-directional audible perspective” (Wikipedia). Stereo sound is recorded by positioning two microphones relative to the source of sound (e.g. music) so that they simulate the experience of listening. They record simultaneously but not he same sound. E.g. the sound of the instrument that is closest to microphone A will reach A before it reaches microphone B and vice versa. Together this creates a stereo record that is similar to the listening experience created by your ears (which are “mounted” to your head, not unlike microphones, so they can distinguish the direction which the sound comes from). When reproducing stereo sound the speakers are positioned in a way that allows these two channels to give you the best feeling of the room the sounds were recorded in.

Now why would you want to add a subwoofer to a stereo system and why only one? The problem is that small drivers / speaker enclosures are not very good at reproducing lower frequencies. So you would need two pretty large stereo speakers (e.g. in a 2-way or 3-way design, which means each speaker has a woofer, a mid range speaker and a tweeter) to reproduce the full range of frequencies in stereo (this would be called a 2.0 system). However, while our ears/brain are pretty good in distinguishing directions in mid and higher frequency ranges, this does not apply to lower frequencies. The good thing is that this means that you don’t actually need stereophonic reproduction of lower frequencies. In fact you only need a single subwoofer for the lower frequencies, which can be placed pretty much anywhere in the room, and two small satellite speakers for the mid and upper ranges placed so that they give the best stereophonic experience (typically front right and left). The separation of the lower frequencies is done by a crossover (in essence a simple filter) e.g. integrated into a 2.1 amplifier that filters out the lower frequencies from both channels and pools them in a single subwoofer channel (this is what the #.1 in 2.1 stands for).

A little background. TPA3116D2 is a Texas Instruments chip that has been around for a number of years and has turned into one of the most popular chips in compact class D amplifier boards. It is extremely power efficient and hence only requires small heatsinks (if any). These boards are also very popular among manufacturers of budget amps (the majority of those 50-100$ amps on the market are TPA3116 and some of these are really good). However, sound quality very much depend on board design. This board is definitely my favorite, good sound, no noise... Another advantage is that this board has separate bass / treble controls for the satellite speaker plus frequency and bass volume for the subwoofer, which is particularly helpful with printed designs.

So here is what you will need for the amplifier:

- The actual amplifier board: ZK-TB21 Bluetooth 5.0 2x50W+100W Amplifier Board (around $ 25) (from Amazon: ZK-TB21 Bluetooth 5.0 2x50W+100W Amplifier Board). Comes with a basic top/bottom plate (which I am not using), connector, and shiny knobs (also not used in this project, but you can).

- Plus a 12-24V switching power supply (from Amazon: 24V 3A external power supply. The TPA3116 operates on a wide range of voltage. However, I would definitely recommend a 24V/3A+ (72W) power supply to get the full output.

Filament. Use your favorite color / material. I use this super cheap and easy to print carbon fiber PLA for the main body (on Amazon Sunlu carbon fiber PLA) , for the knobs silk silver PLA (or gold/bronze, if you like them a little flashier), and the top panel in sparkly black/vertigo black PLA, which is a slightly metallic looking dark grey.

Printing and assembly. Printing should be very straightforward. No supports needed. I print everything in 0.2 mm resolution and with 30% infill (lines). Print one each of 1.) the main body 2.) the top panel 3.) the larger volume control knob 4.) four of the smaller bass/treble control knobs (you could of course use the shiny knobs that come with the amp instead) and finally 5.) four of the feet. Slide the board into the enclosure and fix with the 5 nuts of the volume controls. You can use the two small screws (they come with the board) to further secure the board (I have added screw holes to the enclosure) - but it works well without. Attach the feet (either with short M3 screws or a drop of glue). The top is mounted with 4 M3 screws. Insert 4 M3 nuts into the respective openings (see picture above) and heat fix or glue them in. Connect the speakers to the terminals (see below for how to connect everything) and the 12-24V power to the corresponding connector, mount the top panel with the M3 screws and you are ready to go.

Background. Here a little theory for all those interested: What is a back loaded horn speaker? Think of it as a reversed megaphone or gramophone. A (front) horn loudspeaker uses an acoustic horn to increase the overall efficiency of the driving element (i.e. the sound coming out at the front). A back loaded horn speaker does pretty much the same. However, here the horn is mounted at the back of the loudspeaker. So you get the direct output from the front of the driver PLUS the one from the horn at the back. Which frequencies are amplified and how well depends on the air chamber, throat (the narrow part), and on the length, design, and diameter of the horn. Particularly with small (and cheap) drivers this can very much improve the sound quality. Although strictly speaking V4.0 is not a back-loaded horn (at least not like V2.0 - see my earlier instructable). This one comes with a relatively (for a printed design) large main chamber (without corners or flat faces) and with a small horn attached to the back and opening up to the front. However, the concept remains the same. It is like kissing the frog. You kiss that 3" loudspeaker and hope it turns into a prince. And it does... (particularly in a 2.1 system, which helps overcome some of the inherent limitations of printed speakers with small full range drivers).

And here is what you will need:

- Since this is a passive system, basically all you need is a driver. Mounting holes are 84 mm diagonally spaced (i.e. on a circle of 42 mm radius) and the opening for the driver is 78 mm in diameter. I use Visaton FRS 8 M, which may be a little hard to find outside of Europe, but pretty much any of these 3” drivers (with square frame) should fit or if you are willing to spend a little more, e.g. Aiyima has a few really nice 3" drivers (that should fit) to choose from.

If the diameter of the mounting holes (see diameters in the image above) of your driver is (slightly) different you can simply scale the entire model (all parts) in your favorite slicing app to make it fit.

Printing and assembly. This should once again be a simple print. No supports needed. Print in PLA (or ABS, PETG...) at 0.28 mm resolution and (around) 20% infill (I use lines). I once again printed the main body in this cheap Carbon PLA (- not so much because it is stronger, rather because carbon gives this beautiful soft touch to the surface) (on Amazon: Sunlu Carbon Fiber PLA), and the speaker ring in silk bronze (and a few other silk colors). No gluing (well almost) needed, the body is comprised of a single part. Mount 4 M3 nuts with a drop of (hot) glue to the inside of the mounting holes inside the enclosure (I have made a recess for these on the inside, so they should be easy to mount - this makes the driver easy to remove), add some damping material to the main chamber (optional), insert the driver with the attached cable, lead the cable out through the corresponding hole, and attach the speaker ring and speaker with M3 (with nut) - M4 (without nut) screws. The feet (2 front, one back) are glued into the respective holes. For connecting them to the amplifier please see below.

This is a small, fully printable subwoofer, in essence a 3 liter enclosure, which, thanks to the isobaric design, performs like a 6 liter enclosure.

So what is an "isobaric" subwoofer and why would you want to print one? Isobaric speaker design is basically a simple trick to reduce the volume requirements for a speaker enclosure (by approx 50%, so helps overcome the no. 1 limitation of printed speaker designs). What you do is you line up 2 drivers one behind the other to double their ability to compress the air in the enclosure behind. This way the outer driver "thinks" that it has a much greater volume to work with. Isobaric only applies to the space between the two speakers, in which the pressure remains moderately (but not 100%) constant, very much unlike the air in the chamber behind the second driver. If the drivers both point in the same direction they are simply connected in parallel (which reduces their impedance to half), which makes their membranes move in parallel. However, you can also have the membranes point at each other (as I did in this design). This may look a little awkward but works great and helps save additional space. However, in this case you have to invert the phase of one of the drivers, which is simply done by connecting the "+" connector of one of the drivers with the "-" of the second (see wiring diagram). This way you can print a 3 liter enclosure, which to the driver "looks" like a 6 liter enclosure.

Material. Again an ultra economical design (I know that's a little strange but I just love to see what can be done with such basic components and there is always plenty of room for improving by simply upgrading components). The drivers I used were € 5.5/piece for the 4" option and €6.5/piece for the 5" variety! However, even the 5" variety I used is only 25W RMS (which isn't exactly much for a woofer - plus a Qts of 1.15), so they sound good at lower volumes but won't be any good for your next rave party. As usual, the more you are willing to spend on your drivers the more sound you are likely to get. And remember, you will need two of these!

Here is what you will need:

- The drivers I used are ultra cheap Dynavox 50W, 8 Ohm, 13 cm (5") woofers (around € 6.5/piece in EU), which once again may be a little hard to find outside of Europe but pretty much any of these Amazon 5” woofers should fit (compare the mounting holes with the ones in the attached image). I have included a step file for you to adapt the mounting plate to your own drivers if needed. Or if you are willing to spend a little more, e.g. Aiyima has a few 5.25" woofers (that should fit) to choose from.

- I use these speaker terminal connectors (around € 0.5/5 pieces) but they are basically optional.

- Plus a few M3 screws

Remember, you can use pretty much any woofers as long as they have roughly the same diameter (up to 5.25" should be OK - see image above for diameters and I have included the .step file for you to adapt to your needs). The better the driver, the better the sound you will get.

Printing and Assembly. This should be a simple (although slightly longish) print. No supports or attachments needed. Print in PLA (or ABS, PETG...) at 0.28 mm resolution and 20%+ infill.

- Once again I print the main body some ultra cheap, easy to print carbon fiber PLA (on Amazon Sunlu Carbon Fiber PLA), which gives this beautiful surface, but of course any PLA, ABS, PETG/color will do. - The upper section is printed in wood PLA (on Amazon Sunlu Wood PLA) looks great (much better than in the pictures) together with the carbon and silk copper,

- used for the connecting ring: silk copper/bronze PLA.

Print one part each of the main body, the speaker mount, the 10 mm connecting ring (which is mainly there to make it easy to adjust to the size of the driver - see below) and the 55 mm top section, plus the grille as well as 4 pieces of the feet (these preferably in TPU but PLA will do). I have included the speaker mount plate for 100 mm, 130 mm, and for 5.25". I have also included a step file for you to be able to adjust to the exact size of your drivers. In case your driver is higher than the 60 mm (10+55-5 mm) top section (which is not unlikely), simply scale the z-axis (only) of the 10 mm ring (this is the golden section between the lower and upper parts) in the images) to whatever size you need (e.g. in Cura).

Wiring and Assembly. Wiring is really simple. However, - this is important - you have to invert the phase of one of the drivers, which is simply done by connecting the "+" connector of one of the drivers with the "-" of the second (see wiring diagram). Mount the 2 drivers (membranes pointing at each other) with 4 M3 screws/nuts to the speaker mount (make sure they are not touching). Connect the lower driver to the terminal connector. Add damping material to the main chamber (optional but suggested). Insert the mounting plate with the two drivers. Insert the grille and connect to the amplifier (see below).

Connecting is really simple. Make sure that you have properly connected the wires inside the subwoofer (see above) - this is the only tricky / unconventional part.

See the diagram above for how to connect the speakers to the amplifier. Make sure to always connect + to + (usually the red wire) and - to - (usually black) - the only exception being the internal wiring of the subwoofer. Connect the speaker cable from the left satellite speaker to the L+/L- screw terminals on the amplifier, the cable from the right satellite speaker to R+/R-, and finally the cable from the subwoofer to the S+/S- connectors. Check that everything is wired correctly before connecting the power supply. Turn on the amplifier by turning the volume button, either connect your sound source through the aux in jack (e.g. from your computer or smartphone) or by connecting to the Bluetooth receiver. Adjust bass volume, frequency, and satellite bass and treble and...

ENJOY THE MUSIC!