It's been about a year since I made this and I just can't get over how good it sounds. I have made some minor changes and thought I would post them. I'll leave the original text and add the updates. If you have built a speaker based on this instructable, I would love to see it so I know my efforts were not in vain.
A few months ago, I came across numerous sets of jumping water speakers that had leaked all or most of the liquid. I was able to refill and seal a few sets but many were damaged beyond repair. I thought these speakers had a nice sound so I decided to recycle a few. They even have 3W +3W amplifiers I could use.
Step 1: What You Will Need
- Speakers. I used 4 of the 1.5 inch, 4 ohm, 3 watt drivers from 2 sets of water speakers.
- Amplifiers. I used the amplifiers from 2 sets of water speakers. You can also use PAM8403 modules
- LED's from water speakers
- Bluetooth receiver. I used an XS3868 Bluetooth Stereo Audio module but would recommend using a USB Bluetooth receiver because it's much easier.
- A switch for power
- Battery charge module. You can use mini or micro USB.
- 5V step-up converter. Needed for most USB receivers. Could also be used for amplifiers for higher output at the expense of battery life.
- Lithium battery and holder. You can use multiple cells or LiPo batteries if you want. I thought of using a power bank to get the battery, charger, and converter.
- Unfinished wood box or other suitable rigid enclosure. They were sold separately at my local Hobby Lobby so I bought the small size.
- Passive Radiator or port.
- 470µf or other large value capacitors.
- Paint, hookup wire, rubber feet, sealing material, hot glue
Step 2: Plan Your Build
Hopefully, step 1 left you with a good idea of parts you will need. You may have some of the parts already or have a different idea than what I did. That's the way to think. Read over this project and a few others and decide what you like and what is practical to build with the skills and tools you have. Be creative and share your ideas with others.
If you decide to build one like mine, I'll start by telling you some things could be better. Using 4 speakers definitely makes it louder but not without some issues. Placing 4 drivers with the same frequency range on the same surface can make them sound terrible because of a lobing or combing affect. I'll leave you to google this for yourself but I believe having 2 front drivers and putting the other 2 on the rear or sides would be better. If you want to follow my build and have the same amplifier boards, there is a way around it so read on.
One of the key features of my Bluetooth speaker is the passive radiator. It can't perform magic tricks but it can help make the best of a small speaker design. I used 60x90 mm passive radiators and modified them. The plates were too heavy and had a resonant frequency too low for these speakers. I replaced the metal plates with thin wood and was able to tune them so follow along if you have these. There are many choices for passive radiators, including using an old speaker or even a piece of wood or cardboard mounted on a replacement surround. The key is being able to tune it by adding weight. Mine are tuned around 100 Hz. I'll have a whole section on box tuning later.
On the electrical side of things, I should have kept the electronics closer together. I placed the boards around the sides because I wasn't sure whether I might add more speakers at the time. I did add the second passive radiator but I don't think it was needed. I also had to upgrade the battery. A single 18650 lithium ion cell should power it for a few hours but I made the mistake of buying a $1 battery. You can read about these elsewhere, but if it has "fire" in the name, it's probably worthless and possibly dangerous. Stick with brand name cells unless you have another way of verifying the quality. I ended up using 3 cells from a laptop battery.
Once you know what parts you will be using, arrange them in a way that they will fit. Take note that the bottom of this box, which becomes the back, is thinner than the other panels. That makes it ideal for holes that are harder to cut like the one for the charge port. It will also need to be reinforced with cross-bracing to make it acoustically solid. I used 3/8 inch square stock that you can get from your hobby store.
Step 3: Cut the Openings and Paint Your Box
The reason I chose the pre-made box is I'm not much into woodworking and don't have the tools, skills, or time to make my own. I just had to make a few holes, paint it, and figure out a way to seal it. The suitcase design makes it easy to open and change things.
- Start by cutting the holes for the speakers. As I said earlier, putting only 2 speakers on the front and the other 2 on the sides or back can save you some trouble. I used a 35mm hole saw in a drill to make them. The inner edges needed a little sanding but were otherwise very neat.
- Cut the hole for the passive radiator. I don't have a jigsaw at the moment so I used a small hand saw and Dremel router bit for these.
- Drill the holes for the power switch and LED. Mine are on the top near the handle.
- Drill and cut holes for the charger and LED's. The LED's are perpendicular to the port so mounting the charger board on its side would allow a direct view of the LED's through a hole in the side. Other people have used hot glue to channel the light to a hole beside the port. I put a dead LED next to the port that acts like a lens.
- Glue bracing strips to thin back panel. I used 3/8 inch square stock but it's not critical. The shorter pieces are just butted against the sides of the long ones. Wood glue will work but epoxy is quicker. As long as it sounds solid instead of hollow when you knock on it, you have achieved your goal.
- Paint or finish your box. I used 2 coats of acrylic craft paint. If you get paint on the bronze trim, wipe it off while it's wet.
Step 4: Electronic Modification and Preparation.
The amplifier boards in the water speakers have 3 circuits. Two of them are audio amplifiers based on LM4871 chips. The third one is used for the water pumps and is not needed. The layout of the board makes it easy to cut away the unnecessary circuits. Desolder the USB port and smaller capacitor and cut along the red line in the picture. A Dremel cutoff wheel or even a hacksaw will work. Just be careful not to damage the small components on the board.
If you put all 4 speakers on one side of the case and you have amplifiers like the ones here, a small modification will greatly improve the sound quality. What this does is filter the high frequencies out of one speaker and boost them in the other. This will minimize the acoustic comb filtering effect.
- Solder a .0033µf capacitor across R19 on U1. This will be across pins 4 and 5 of U1.
- Solder a .0022µf capacitor across R5 on U2. This will be between pin 4 of U2 and C5
Update: The modification above was causing excessive high frequency gain. I changed the capacitor on U1 and added a resistor on U2.
Solder a .0047µf capacitor across R19 on U1. This will be across pins 4 and 5 of U1.
Solder a .0022µf capacitor in series with a 2.2kΩ resistor across R5 on U2. This will be between pin 4 of U2 and C5
The output of U1 should go to the lower speaker while the output of U2 goes to the upper speaker.
Step 5: Wire It All Together
Start by soldering wires to the Bluetooth module. If you use the XS3868, try using the wires from the water speaker cable. You should see now why I recommend the USB Bluetooth receiver. When you are convinced that all wires are correct, secure them with hot glue.
Continue wiring it together. You will want heavier wire for everything else. The battery should always be the last thing connected. It's also good to have some way to quickly disconnect the battery later. Placing the cells in a battery holder is a good way to do that.
Recheck all of your wiring. If everything looks good, flip the power switch. You should hear tones from the Bluetooth module. That's good enough for now.
Step 6: Mount the Components and Test
- Mount the speakers with a generous amount of hot glue. Make sure the left and right orientation is correct
- Mount the circuit boards by tacking with hot glue in each corner.
- Mount the power switch and LED
- Mount the battery holder with double-sided tape or the battery pack with velcro
- Plug in the USB charge cable and verify that the charge light works.
- Turn on the power switch and pair it with a device.
- Verify that there is audio and that noise is not excessive. Most people report a little bit of noise from the Bluetooth module. If it is too loud, try moving or adding ground wires or another capacitor across the Bluetooth module power wires.
- Seal the box by placing the seal strip around the top and bottom. The seal strip separates down the middle making 2 strips. Place the seals so they extend above the edges of the box. 1mm should be enough to ensure the seals meet.
Step 7: Passive Radiator Tuning
There is more than one way to design speakers. If we had Thiele/Small parameters on the drivers, it would be possible to calculate the enclosure size and port dimensions with any of the free speaker box calculators. Since that information is not available, I'm using the TLAR (that looks about right) method. I picked an enclosure that seemed like a reasonable size and it works.
I mentioned earlier that the passive radiators I chose were too heavy. The metal plate weighs about 32g. That results in the resonant frequency being too low. I removed the metal plates and replaced them with light plywood that weighed 5g. That brought the resonance of the PR up to a usable frequency. I used 5 minute epoxy to attach the plates and mount the PR's in the box.
Before the tuning, I would like to explain some speaker design theory in simple terms.
- All conventional speakers have a resonant frequency. If you tap on it like a drum, that is the frequency that will be produced. The sound output is usually highest at resonance and then drops off below the resonance. Bigger speakers usually have a lower resonance.
- Putting a speaker in a box raises its resonance. A smaller box raises it more than a large one.
- A passive radiator has its own resonance, also affected by the box. Adding weight will lower that resonance.
- Tuning a passive radiator below the speaker's resonance can extend or boost the low frequency response.
Now for the tuning. You'll need several tools to generate and measure test frequencies. I use PA Tone, Bass Tester, and Function Generator for Android. You also need a real time analyzer like Visual Analyzer and a microphone for your computer, unless you really know what you're listening for. I'll go over that later.
Now that you have all of the apps installed and working, open Visual Analyzer. You'll need to select the microphone as the "input device" at the top and check the "hold" box in the "main" tab at the lower right corner.
Next, bring up Function Generator on your phone. In the "SWEEP" box set "START" to 20Hz, "STOP" to 500Hz, and "TIME" to 30sec.
To take the measurement, hold the microphone about 1cm from the speaker. Click the "ON" box on Visual Analyzer and then the "OUT" box on Function Generator. You should hear a low to high sweep and have a graph on your computer screen. Now click "OFF" on Visual Analyzer and "OUT" on Function Generator. Right-click on the graph to save it. Now do the same for the passive radiator.
The highest spot on the graph indicates the resonance. This should be around 200Hz for these speakers and 100Hz for the PR. If the speakers resonate lower, the PR can be tuned lower. Since the PR is driven indirectly by the speakers, the output at resonance or "boost" will decrease with frequency.
Hopefully, you'll find that the PR resonance is where you want it or higher. If it is too high, add weight to the PR to lower it. I used coins secured with tacky putty. I have 2 dimes on each passive radiator. If I only had one PR, one dime would result in the same resonant frequency.
There is one more check that is helpful in PR tuning. If you have a good ear, it might be the only method you need. Open Bass Tester on your phone that is connected to the speaker. Play a tone of 50Hz at medium volume. Slowly bring up the frequency until you can hear it well. That's probably around 70Hz. Place a finger on the passive radiator so that it can't vibrate. Note whether the volume increases or decreases when it is held still. It will probably sound louder when held. Continue increasing the frequency until it starts to sound louder with the PR free. It will also vibrate against your finger if you hold it lightly against it. This might be around 100Hz. Continue increasing the frequency until holding the PR makes no difference. That might be around 140 Hz. Congratulations, you just found the range of frequencies that are being helped by your passive radiator. If you don't like it, you know how to change it.
Step 8: Possible Modifications
The sound is wonderful like it is but I would like to try a couple of things. These may or may not improve the overall quality but I may try them in the future.
Operating the speaker directly from a battery decreases the power output from the 3W amplifiers. It might even be less than 1W when the battery charge is low. I would like to try step-up converters to ensure 5V to the amps until the battery is discharged. This won't make it much louder but it will be noticeable.
There is a faint digital noise caused by the Bluetooth module. I believe using a USB Bluetooth receiver would eliminate this. I might also power the receiver with a separate step-up converter.
I would like to add a little more bass to the speaker. This thing has much better bass than you would think from these tiny speakers. It sounds natural and balanced but there are ways to boost it a little. I could use a 100 Hz low pass on the lower speakers and run the upper ones straight. That would mean there is twice as much power given to bass but everything else would be lower in volume. The other option is adding more speakers and amplifiers for bass only. I could probably tune the passive radiators a little lower with more power driving them.
If you have tried any of these, drop me a comment and let me know how it worked out.