The Original Bijou
One other design goal, was to build the amplifier as compactly as possible. I decided to use the Bottlehead style for the layout with all of the controls and power sockets on the top plate because I wanted to have the plate made by Front Panel Express. I used their software to design the plate. I also used a Hammond 270AX power transformer that I had in my parts box. The transformer is actually slightly overloaded, because its 2.5A heater winding is running at 2.8A and its 50mA high voltage winding is running at about 50mA. This is pushing the transformer a little, and it does get warm, but these currents are not destructive to a 105C device.
After a few iterations designing the top plate, I settled on this 10" x 8" layout (click to enlarge):
Along with the Hammond transformer, I decided to use some of the Neutrik RCA and Headphone jacks that I had collected.
There was nothing special about building this amp. The wiring is straightforward. Heaters are AC so they are tightly wound and kept close to the chassis. There is a star ground in the center of the chassis plate. I built the mosfet regulator on a perf board which sits underneath the vent holes at the upper left and is attached using one of the screws that holds the power connector socket.
Here's a photo of the underside with PS, heater and ground wiring. The regulator is at the top right. Yes, I painted the perfboard black.
Here is the completed wiring. Notice the very large 330n Orange Drop film capacitors. The latest design has reduced these to 100n so they don't take up so much space.
Turning It On
When I first fired up the amp, there was a very small, but detectable buzz. I spent a lot of time trying to track this down, including rewiring some of the ground connections and fooling with the B+ wiring. Nothing, however, seemed to relieve the problem.
At one point during debugging, I connected a clip lead to one of the rectifier plates and let it drape onto the wiring. The buzz disappeared. There was clearly some switching transient radiation getting into the amplifier. And there was no way to rearrange the layout to move the rectifier farther from the amplifier.
After many more hours of debugging (and blowing up the mosfets by shorting the B+), I decided to take the easy way out. I connected a piece of wire to the rectifier plate where I had attached the clip lead and arranged this wire on the chassis as an antenna to cancel the buzz. In process of debugging I made a few other small changes before eventually restoring the amp to its original design. The amplifier is dead quiet now for any setting of the feedback pot and exhibits a tiny bit of noise only at full volume. And I'm not the first person to cancel noise by positioning wires.
If I were to build this amp again on this compact chassis, I would swap the location of the rectifier and the filter cap at the back of the chassis, moving the rectifier farther away from the body of the amp. I suspect that this would solve the buzz problem. It might also help to make a slightly bigger chassis to further separate the PS from the amplifier. This very small design was, again, an experiment to see what was possible.
Performance of the P2P Bijou
The only amplifier that I have to compare the Bijou to is my experimental Fender Tone Stack amplifier. To my ears, the Bijou sounds more open, with a better defined soundstage both left-right and front to back. There are no tone controls in the Bijou, so I can't crank up the bass like I can with the Fender amp, but the Bijou still delivers excellent bass. Its midrange and treble are crystal clear. Voices and instruments on good recordings stand out on the soundstage as clear as a bell.
This is an excellent headphone amplifier that should make most headphone addicts happy with what it can do. And it has enough power to drive my Sony's (45R) beyond earsplitting levels for me.