In November 2012, I switched jobs. Due to a change in IT environment, I was forced to find a new amp I could drive from a wall source. I thought it would be cool if the amp was portable, and it would be even better if it was a DIY kit. A quick google search later, I found what looked like the best contender: the O2 amp, as designed by NwAvGuy. It satisfied all three requirements, and I will spoil the ending right away by saying it's an absolutely outstanding product that exceeds every one of my expectations.
I started by ordering a parts kit from Switzerland, from Head 'n' HiFi. The unlucky store owner had just been flooded, and customization options were limited at the time; however, it seems there are quite a few new panel/case colours and power supply options in stock at the time of this writing (January 2013). I wound up with a useless European power supply, but luckily I had an old Intellivision power supply on hand which fit the bill exactly. I'm really not quite sure how that wound up in my parts bin, but I'm glad it was there... it kind of lends a retro feel to the whole thing. :P
The Head 'n' HiFi kit is great; it comes with all sets of gain resistors, plus "revised" versions of component values as outlined on NwAvGuy's blog. This initially caused a bit of confusion for me because NwAvGuy's bill of materials still listed the old component values when I downloaded it. However, once you scour the blog deeply enough, things become clear. Specifically:
- R9 and R25 have the new values to avoid power cycling when batteries are low, which is to say 33Kohm and 1.5Mohm, respectively.
- C6 and C7 are 1.0uF instead of 0.22uF.
|The kit's parts.|
|I began by sorting and labelling the parts.|
|My cat insisted on helping me sort them properly. Clearly much better.|
|Putting in a few resistors.|
|A few capacitors, an LED...|
|A few more random parts - sockets, battery holders.|
|Jacks are in - getting close.|
|Circuit completed, time for some testing.|
|After much debugging - ready to mount in the case!|
One thing these pictures don't show is that I spent a lot of time finding three microscopic solder bridges on the board. One of these was 100% my fault, just shoddy work, but I will only assume partial karmic blame for the other two. :P On the PCB I received, the solder mask is ever-so-slightly offset, which exposes the ground plane in a few places close to R1 and R2, the large battery-charging resistors:
|Exposed ground plane trace, highlighted in purple. Little alien solder blob was carrying 0.3 wayward amps.|
As you might have guessed, both this crescent-shaped sliver of copper and its cousin a few centimeters over were thirsty for illicitly-acquired solder.