Modding the Grado RA-1This project started as a joke. I had a Grado RA-1 that I traded some cables for. I never really liked the amp, even with RS-1's, and thus never really treated it very well. The Grado got dragged around a lot, and eventually broke due, I assume, to power being hooked up backwards (I was running it off of two wall warts that connected to the 9V tabs, but didn't have any inherent direction.) Having heard that the Grado was very simple, I thought I'd open it up and fix it myself.
Grado, however, had gone to great lengths to keep anyone from opening their amp. The wood screws holding the bottom on have non-standard hex heads that require a special tool. Not having a special tool, I used a wrench and scratched up the bottom pretty badly. Further, once the amp is open, one finds that Grado has stuffed all of the parts into wells in the wood, and dumped epoxy on top. It took me hours to get through the epoxy.
Once inside, I found what Grado was hiding -- a $0.40 opamp, 6 $0.10 resistors, 2 tiny capacitors, and 2 (unnecessary, it turns out) input caps. These parts, combined with the switch, potentiometer, and jacks, make this a $30 knockoff of Chu Moy's super simple headphone design. (Grado did change how the voltage divider works, but not by much.)
I always knew that the Grado sounded like crap, but after opening it I knew that it was crap. Thus, I decided that rather than trying to modify a poor circuit, I would build a decent amp and put it inside the Grado case. The amp I decided to build is the Pimeta. It is a pretty common DIY design, and there are tons of threads on it at Head-Fi.
When I first built the amp, I hooked it up with some batteries, and it lasted about 2 hours. I then went out and got some rechargeable batteries, but, having biased the amp into class A, they took longer to recharge that they lasted. Thus, a dedicated power supply was in order.
The cheapest thing I could find that would provide a decent 24V was a Velleman kit. It is a pretty simple design with 4 diodes to rectify the AC, a LM317 to regulate the voltage, and a couple of caps for smoothing. While the Velleman design isn't so bad, it is very cheap. The electrolytic caps are spec'd at 35V even though the manufacturer claims that the power supply can supply 35V. This turned out to be a problem for me. I was using a transformer with 30V secondaries, and the caps exploded. I replaced the caps with 63V ones, and everything seemed to be just fine. Well, everything was fine until the regulator went bad causing the resistor and trimmer to burn up which then sent about 50V into the amp which blew the first set of opamps. Furtunatly, nothing else was damaged in the episode, but there was quite a lot of smelly smoke.
The next part of the project was to build a source. I decided to use one of Guzzler's USB DACs as they are small, cheap, sound good, and are pretty simple to build.
Once all of the circuits were built, I bought some blocks of mahogany, cut them up on a table saw, and ended up with a "Grado" stereo.