Boombox
I’ve recently got a boombox in exchange for development of a Li-Ion charger (the same charger is inside the boombox).
The charger is a very simple one based on the L200 linear regulator and an Atmel Mega168 microcontroller. The L200 is configured to charge with a constant current/constant voltage scheme limited to 2A and 16.75V, the microcontroller measures the current going into the battery with an INA139 and cuts of the power to the regulator with a P-type mosfet when the current is below 200mA. A charge is initiated only when the battery voltage is below 16V. This is done to ensure that there’s no risk of continuously topping up the battery. When the unit is powered by an external laptop-style power supply (19V) the audio amplifier is switched by a relay to be driven by the external power, so there’s no power draw on the battery when powered from an external source. The microcontroller also displays the current battery level and charge status on some LEDs.
The audio amplifier is a AMP-9 Basic amplifier from http://www.41hz.com which is based on a Tripath TAA4100A monolithic switching amplifier, which has a very high efficiency. There’s also a minimal amount of electronics before the amplifier (basic ESD protection with diodes, mono mixing, volume adjustment). With the batteries placed inside the boombox currently (14V nominal, 4AH Li-Ion) I get around 15 hours of low volume play time (limited mostly by the quiescent current of the amplifier) and about 6-7 hours of painful-at-less-than-3-meters volume, which is very acceptable for me.
The enclosure is designed as a base reflex enclosure (not made by myself) and considering that it’s made from 2mm aluminum it works very well. There’s certainly a noticeable tonal difference compared to heavy MDF/fiberglass damped speakers, but considering the total weight of 5kg for the entire system (speakers, amplifier, enclosure, battery, power supply) it is very acceptable and certainly better than other “boomboxes” I’ve seen.
This is essentially everything you need when going to the park, beach, or anything else!
