How to make a loud portable speaker?

Well, the first thing you need to be concerned about is the amplification side. We’re gonna cover a DIY Bluetooth speaker amplifier that will definitely put a smile on your face. Why is this important? Because you see all these plug and play solutions which are not extensively thought out. Yes, they are simple to install, but let me tell you what’s the problem. Why do you rate an amplifier board at 24V and include a 12V battery board to go along with it? Yes, it will play sound, but as soon as the battery loses a little juice the amp starts distorting because the amp is not meant to be played at such low voltage. Not to mention that the rated power is never achieved with 12V.

In conclusion, what are we going to do? We are going to make everything from scratch. So no plug and pluy stuff. Only some head scratching, patience, tinkering and hopefully, if we don’t screw anything up, a great result. We want a DIY Bluetooth speaker amplifier which will be : high power, capable of driving decent speakers, and enough batteries for a usable playtime.

Parts list and schematic for the portable amplifier

Because this is no plug and play setup, make sure you have your soldering iron ready and prepare to get your hands dirty. Following up will be the circuit schematic of the whole thing. This should be self explanatory by itself, but I take each piece one by one and explain what it does and with what you might replace it with.

DIY Bluetooth speaker amplifier

This is how everything fits together. Now it’s time for the components list :

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I added some links to the various items. Remember, you can choose whatever combinations you want in making your DIY Bluetooth speaker amplifier. Next, I will explain what each item does. As a result, you will understand better and maybe use different items to suit your needs better.

Battery pack

The battery pack is the most difficult part to pull off. You also need a spot welder and some nickel strips to make the connections between cells. If you don’t have any of those, you can buy a 29.4V, ready made, battery pack, and that’s the end of it. Here are some pics with how I did it, to give you an idea :

modular 18650 battery pack Parallel connection nickel strip 7s2p battery pack sense wires BMS

Panasonic cells were my choice for this battery pack, but those are hard to find right now. You can use any 18650 quality cells with good mAh rating. The BMS is a universal one. This means you can connect any battery combination from 3 to 13. In our case : 7. However, you can use a normal 7S BMS as well (which you should probably do anyway). I used this universal one because I had one handy. In this case snip the wires corresponding to batteries 8-13. Please make sure it’s an universal BMS. Usually a 13S BMS works for exactly 13 cells, and no less.

If you still have some questions regarding this side, check out a previous article : another BMS setup. What I want to point out that some BMS’s don’t have “C-” solder point, where you connect the negative side of the charger. If that is the case, solder the charger to the “P-” point.

Connectors

There are few connectors here to make our life easier :

  • The 5.5×2.5 mm DC jack male. The purpose of this connector is to give power the the amp. You can also skip this connector and solder 2 wires directly to the board. You can find the solder points right next to the DC socket. However, since the 2 points are really close to one another I prefer to use the plug. Safer and more convenient. If you do want to solder, make sure the wires are not connected to the battery or you will most likely burn something up.
  • The 5.5×2.1 mm DC female socket. This is to connect the charger. However, make sure you check your exact charger dimensions. You might need other dimensions.
  • The Jack male – RCA male audio cable. You need this to connect the Bluetooth module to the amplifier. Again you can solder directly to the boards, but I find this much too convenient. Make sure the cable is short enough so you don’t clutter the inside of the speaker box (0.3m / 0.5m / 1 ft cables are fine).

Buttons and battery indicator

You can see in the first section of the DIY Bluetooth speaker amplifier diagram, a button in series with the battery indicator. The battery indicator is always on and has a back light. This might be upsetting in the dark. That’s why I placed a switch to turn on the indicator when I wish. If this does not bother you, you can skip the switch.

Furthermore, there are battery indicators with a button on the front panel. That will make the switch redundant. To set up the switch for 29.4V do the following : There is a button on the back. While holding the button down, connect the indicator to power. Now you are in the configuration menu. Press the button until something with the number “7” appears. Then disconnect from power. After you connect to power again (without holding the button), you can use the button to switch from voltage to percent. Percent is much more familiar, so I will leave it at that.

The other switch is much more important. It’s 24V and 5A rated on DC. The 24V seems low, but that will not burn the switch. However, high current will. So you need something with at least 5A rating. Since I wanted these aluminium anti-vandal switches this is the only one I could find. It’s an ON-ON switch.

Anti vandal button

It has 3 pairs of connectors. Use the ones which are to the exterior. The middle ones leave un-connected. So now it’s basically an ON-OFF switch. The 3rd pair of connectors (to the middle) are for the 12V led (the button is illuminated). But I’m not going to bother converting 29V to 12V just for this led I don’t even want.

The step-down converter and isolator

There are 2 odd components in this schematic. One is step-down converted which converts the 29V from the battery to 5V. This is because the Bluetooth module works with 5V. If you connect it directly to the battery you will have one dead BT module. After this voltage conversion, you can directly connect it to the Bluetooth board. However, by doing so, you will get an annoying hum, because of ground-loop issues. To solve this problem, you use the the 5V/5V converter. And the hum goes away.

The Bluetooth module is a straightforward mini-board. You can power it up using micro-USB connection. However, this time, I chosen to solder directly to the board. micro-USB connections are poor, and you don’t want to find that your Bluetooth module is un-powered after the speaker box is finished.

For the charger make sure you get an exactly 29.4V one with at least 2A (for faster charging).

DIY Bluetooth speaker amplifier

Now that we have cleared everything up, let’s focus on the amplifier. And we shall talk some specs. This amp is rated to be 3 ohm stable and will put out 2 x 100 W at 4 Ohms. Now this sounds all too good on paper, but in reality we know that is not always the case. I connected some beefy resistors to the amplifier and made some measurements.

I used an iPhone to connect to the Bluetooth module and used a 500 Hz sine wave to make the measurements. The volume on the phone (at least on this iOS 12 version) has 16 notches. Conveniently enough, the amplifier has a red LED that shows clipping. Here are the measurements.

Power ratings at 8.2 Ohms :

  • 100% volume – 2 x 54 W – severe clipping
  • 94% volume – 2 x 35 W – noticeable clipping
  • 87% volume – 2 x 20 W – no clipping
  • 81% volume – 2 x 11 W – no clipping

Power ratings at 4.1 Ohms :

  • 100% volume – 2 x 90 W – severe clipping
  • 94% volume – 2 x 66 W – noticeable clipping
  • 87% volume – 2 x 37 W – no clipping
  • 81% volume – 2 x 21 W – no clipping

What is safe to say, judging by there numbers, is that you can get 2 x 40 W of clean power into 4 Ohm speakers.

Battery life

I made some tests for the battery life as well. For this I used some speakers, but used the resistors as well. Problem is that the resistors don’t mimic reality. In the sense that the battery will drain much faster on a resistor, rather than a true speaker. The resistor has a fixed impedance, while the speaker will have very high impedance at high frequencies, which will draw significantly less current. Anyway, I did measure the play time on some 6 Ohm speaker. Imagine that this is a problem. To play some speakers full blast for half a day. You will get someone annoyed, including yourself. Since all battery life testings are made at 80%, that is what I did also. And played some random long house mix on youtube, with good audio quality.

Measured battery life :

  • 4.1 Ohm resistor – 6:10 hours of play time
  • 6 Ohm speakers – 11:20 hours of play time
  • 8.2 Ohm resistor – 10:20 hours of play time

You can clearly see that an 8 Ohm resistor has a worst battery life than a 6 Ohm speaker. This will not be the case with an actual 8 Ohm speaker.

Battery life through speakers :

  • Estimated 4 Ohm speakers – 8:30 hours of play time
  • Measured 6 Ohm speakers – 11:20 hours of play time
  • Estimated 8 Ohm speakers – 14:10 hours of play time

What is also an important metric is the charging time. This will vary depending on your charger. Using a 2A charger I managed to get the battery from flat to full in approx 3 hours.

Conclusion

When you are making a DIY Bluetooth speaker amplifier you have to know how to read between the lines, or to take the numbers in the specification sheet with a grain of salt. Since I made all the measurements beforehand, you know something for a fact. This amp will deliver 2 x 40 W watts of clean power into 4 Ohm speakers, and it will play for 8 hours straight. Now you can choose your speakers and enclosure however you want.