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.)

The former Grado insides The thing that amazed me the most was not the low quality of the parts in the amp (everyone knows about the cheap parts in the amp, but no one ever remarks at how terrible the wire is), but was instead the terrible build quality. Solder joints where ready to come apart in places (particularly the wires soldered to the RCA jacks). The opamp pins were bent like it was crushed at some point, the small caps on the board were bent and cracked, and there were burns in the case around the RCA jacks where someone had trouble with a soldering iron.

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.

Front view with a new blue LED

Back view with upgraded switch and jacks.

Carving out the inside of the wood block. The shiny stuff is a metallic paint that is used to shield the amp. It is conductive, so I had to scrape it away in circles around all of the panel components to keep them isolated.

View of the Pimeta sitting in the carved out case. The wood is extremely hard, and my dremel didn't cut so much as grind. Using one of the dremel cutting disks, the friction started a fire in about 6 seconds. Instead, I used the cutting thing that looks lik a drill bit. When I was done, I didn't have any big chunks that were removed, just a big pile of dust. You can see on the battery door where I slipped. The Pimeta has stacked buffers, but the second one is surface mounted under the board. It is biased into class A, but is otherwise stock. The gain is 4.

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.

Inside the power supply.

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.

DAC board mounted in the case. I took a cue from Grado and epoxied the board in to keep it from moving.

Outside of the DAC.

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.

Completed project, front view.

Completed project, back view. Note the addition of a DC jack.