When I set out to make a 2nd one I wanted it to be cheap compared with the MKI speaker case. About half of that cost in the battery and charging, so using a lower power amplifier was going to be the first thing on the chopping block, out with the old 15w power guzzler, and in with the cheapest amp I could find.
a generic PAM8403 3w amp
It’s runs off of 5v which meant I could use an inexpensive mobile phone battery pack to run it, being lithium ion it has substantially better power to weight ration than SLA batteries, meaning it’s lighter in weight. Being a completely unit I didn’t need to worry about charging circuitry, over discharge protection or balancing the cells. A good idea in theory, the battery once again proved to be the trickiest part, because if you don’t draw enough current from a this particular battery it turns off :/. And this amp is very low power.
My original solution was 57Ω resistor, to bring the power usage up to a high enough level to prevent shut off, this was around 120ma including the led in the switch. But that was just burning off wasting energy, thankfully I found this neat little circuit from teensy creator Paul Stoffregen, It simulates a higher load by pulsing a high load every 20 ms and that’s enough to keep the battery on even at low volume. This lowered power consumtion to around 20mh idle. I made it up on some perf boad, but you can also buy a pre-made board from OSH park.
This is where the magic happens.
Another place I could save money was to use a switch that integrated the switch and status light, the amplifier actually has a build in switch, but pressing buttons is fun!It’s pre set up to run at 12v so it’s a little dim on 5v, but it’s still fairly noticeable even under moderate light.The under side of the switch is the main hub of the device
The inside is even more spartan the previous model, given the low power of the amplifier the acoustics aren’t too bad, the bass tube is mostly for show, once again cheap pillows provide filler to smooth out the sound.
charging is also simple, a 3.5mm to USB cable allows you to charge the unit from a phone charger, I would have liked to have a micro USB jack but I was unable to find them in panel mount style, at an affordable price.
The only thing I wasn’t able to easily solve was how to externally display battery level with out completely dismantling the battery pack.
By cutting a lot of corners and making design compromises I was able to bring the cost down by more than half, and while I haven’t weighed it’s noticeable lighter than the MKI. Is it as good? No, it’s not nearly is loud, it doesn’t have as good bass response, but it’s quite serviceable.
EDIT: On September 2016, sadly the MKII was stolen from The person I gave it too, if anyone comes across MKII please let me know.