Project Time: 1-8 hours
Project Complexity: Hobbyist
Project Cost: Under $100

Driver Selection
I chose the Dayton ND65-4 2 ½ and a 3in passive radiator. I have used the ND-91’s in past projects and new I would get adequate bass, good mids and decent highs (Plus they look cool).

Enclosure Design
Since I can’t use a table saw in my apartment I chose to go with a small project box. The box measures 7 ¾ x 4 ½ x 2. The plastic was very easy to work with. Two 2 ¼ in holes for the ND-65’s and One 2 ¾ in cutout for the passive radiator on the back. The hardest part is getting the micro and standard USB cutouts done to the correct size. A small emery board cut into strips, tiny file and a lot of patience is what I can recommend. A good amount of Permatex around both USB ports is a must. If they are not sealed well you will get an annoying rattle. I also used a moderate amount of Acustastuff once everything was in place.

Amplifier/Crossover Configuration
The amplifier was nothing fancy. My main concern was battery life so I went with a small 5V 3W per channel stereo amp. The battery is a protected 3100mAh 18650 lithium Ion battery. It charges via small lithium ion single cell charger that has a standard female mini USB port. A 3v to 5V boost circuit rated at 2 amps provides the amp with a consistent 5V. I also added a micro female USB port to power the amp. It’s a nice option since most people have a wall plug for their cell phone. Anything rated a 5V (500mAh-1amp) works great.

Conclusion
The end result is a very portable, versatile and decent sounding system. It’s not going to rattle your windows but impressive for the size. The passive radiator really adds a punch. After a full charge I used it for 5 days straight (~6hrs straight each day) with no loss of power. Total cost was approximately $100. Parts List: Amplifier, project box, 3V to 5v boost circuit, 18650 Lithium Battery and holder, Single cell Li-Ion charging circuit, female Micro USB, Rocker Switch, 3.5mm jack, ND65-4 2 ½ and a 3in passive radiator.