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Last issue, we built a rear rack-mounted sound system by using some easy to assemble off-the-shelf parts. I’ve been thinking for a while about all the different places an efficient battery- or solar-powered sound system would be useful. Building one for the last issue gave me many more ideas about how to do it, so I ended up making a bunch more mobile sound systems almost as soon as I finished the first one! As was mentioned in my previous column, the main objective is efficiency – if your equipment makes more sound with less power, you can reduce both your system weight (good for a bike setup) and your power (good for any off-grid setup, mobile or otherwise). Perhaps this is getting a bit removed from bikes for you. If so, I promise something more bikey next issue.

This project is a battery-powered Active speaker, i.e., a speaker with an integrated amplifier inside the cabinet. Active speakers that plug into wall adapters are common and really convenient – you just plug your mp3 player (or guitar) right in. Building a 12-volt active speaker is a bit more technical than our last project, but it lets us make a more convenient and compact system; it also provides additional efficiency improvements that weren’t possible in the simpler setup I outlined last time, which had an external amplifier.

In a normal un-powered PA speaker box there are two drivers: the woofer and the tweeter. Each driver is designed to produce low frequency or high frequency sounds. Also inside the speaker is a crossover – a device that takes the “full range” input signal and splits it up – sending the low frequencies to the woofer and the highs to the tweeter. In an active speaker, we add the amp inside the box and reconfigure the crossover.

To build the active 12-volt speaker you’ll need:

* Speakers: I tested a couple of different PA speakers in the $100 to $200 range. Of the four models I tried, the most efficient for its weight was the Peavey PR12N; the best sound quality was from the Behringer B212XL. Check my online notes for more details.

* Amplifier: I tested several super-efficient T-amps. For a higher power system I used the 2x100 watt TK2050 amp from eBay vendor “sureelectronics,” a steal at $40 shipped. These run on anything from eight volts to 36 volts – super convenient for off-grid! To get full-rated power you need 24 volts, but they do work fine on 12 volts if that’s what you have. Again, check online for more.

* Crossover: There isn’t anything off-the-shelf here. I designed a small custom circuit and I’ve provided the plans for it free online. I have some kits available so you can make one much more easily. The kits include all the parts and a printed circuit board; you just need to solder it together. Send me an email if you’re interested.

* Power source: 12V or 24V Gel Cell batteries made for computer backup systems are very inexpensive and come in lots of sizes smaller than a car battery. You can find any size at batterywholesale.com

Final assembly:

* Open up your speaker either by removing the back panel or the woofer;

* Clip out the original crossover; it isn’t compatible with the upgraded system.

* Mount the amp and new crossover inside the speaker box. I mounted mine to the back panel.

* Wire one channel of the amp directly to the woofer, and the other to the tweeter.

You only need to wire an audio input and power input to the back of the speaker.

Lots more details are available online at instructables.com/group/momentum

To get in touch with Dan, email

dan(at)monkeylectric.com