Portable Suitcase Boombox

I picked up a nice suitcase that someone had thrown out and was wondering what to do with it when I realized that it wasn't actually a suitcase, but rather an ornamental box made of fiberboard.

This made it great for a speaker enclosure, most of which are made of MDF anyway. Since it had already been painstakingly fitted with leather straps and metal clasps and handles, I decided to make it portable so I could bring it out for picnics.

I had a lot of help from these other projects, especially with the lithium battery power pack:

https://www.instructables.com/id/DIY-HiFi-Bookshel...

https://www.instructables.com/id/Make-an-inexpensi...

https://www.instructables.com/id/Li-ion-battery-ch...

After removing the inner cloth lining, I took measurements of the internal and external dimensions, as well as any important fixtures, and drew them out in Fusion 360. This really helped me with the placement of the components and save time before starting any actual work. Alternatively, using paper cutouts to visualize the layout works too, but is kind of tedious.

I was able to fit 4 speaker drivers into the box front. I used full-range speaker drivers since I was more concerned with volume (intending to use it outdoors) and not audio quality. I also didn't want to deal with crossovers.

The drivers were salvaged and it was important that they all had the same input impedances so that they could be driven from the same channel. The impedance should be written somewhere on the speaker along with the power - each of mine were 4Ω, 20W RMS per channel.

I bought an inexpensive Lepai amplifier (LP-A68) - severely underpowered at 15W RMS per channel, but packed a lot of functionality into a compact package. I've also owned several variations of the better spec-ed Lepai 2020 and those would work great here if you wanted better audio quality, but the box dimensions aren't optimized so quality won't be great in any case.

I opted to mount the drivers onto a separate MDF board, rather than onto the suitcase directly, because I wasn't confident of cutting nice holes into the suitcase. By using a separate board, I could use my desktop CNC to precisely cut the holes, and also if I messed up I didn't destroy the suitcase itself. It was important to get as good a fit of the drivers in the mounting holes as possible to prevent sound leakage that would sound muffled.

After cutting the interior MDF board to spec, it was easy to cut the external suitcase holes since they didn't need to be precise. I used steel L-brackets to secure the mounting board to the suitcase front.

The amplifier sits on a small MDF shelf stuck to the front panel with hot glue.

To mount the amplifier front control panel to the suitcase, I had to cut 1cm off its aluminum enclosure. This had the unintended effect of un-grounding the front panel, resulting in an unwelcome speaker buzz everytime I touched the dials. My fix was to run a wire to the back panel, but the lesson here is not to mess with the amplifier grounding/shielding unless absolutely necessary!

The amplifier takes input power between 9 - 14.4V, but the manufacturer recommends 13.5V 2A. Taking nominal voltage of 3.6V for NCR18650B Li-ion batteries, this means either 3 or 4 cells in series. I decided to use 3 cells in series, which let me put 4 cells in each parallel circuit for total capacity of 12.8 Ah (rated cell capacity 3200 mAh). The configuration is 3S4P (3 sets of 4 cells parallel), and I used a 12.6V 8A battery management protection board.

Parts used:

NCR18650B Li-ion battery: http://www.gearbest.com/batteries/pp_383600.html

NCR18650B spec sheet: https://www.math.ubc.ca/~wetton/papers/NCR18650B.p...

Battery management system: https://www.banggood.com/3-Packs-12_6V-8A-wBalance...

I put oldish looking melamine on-off switches between the amplifier and each set of drivers so that I could use either set of drivers individually. A three-way switch selects between charging the battery, running the amplifier on battery power, and running it on dc-mains power. The three way switch is a selector handle from an old dryer, so 'WOOL' turns off the speaker, 'SILK' is battery power, 'RAYON' is mains and 'COTTON' charges the battery.

I added a digital voltage meter that connects directly to the battery as a rough capacity check.

It was important to cut the openings so that the components sit flush, to prevent sound leakage. I sealed any gaps with hot glue.

To finish, I used thin corkboard, available at art supply shops, to make backings for the switch panels and also for the speaker coamings to cover up the rough edges of the holes.