One Channel Schematic
After introducing the ECC99s into the design and building my own Bijou, I made a few other changes. One change is to reduce the size of the coupling caps from 330n to 100n, making them smaller. This is compenated for by increasing the size of the grid leak resistors from 100k to 470k. A few other front end biasing changes were also made.
Several members of the Headwize DIY community participated in a prototype evaluation of the amp and boards. As a result of this evaluation, it was decided to standardize the Bijou at 250V instead of the original 220V. Changes to the original Bijou are negligible. See the Tweaks section for 220V operation.
Here is the current Bijou schematic.
The output is coupled to the headphones through both a large electrolytic capacitor (C5) and a smaller, 1μf film capacitor (C6) to get the best frequency response through the coupling capacitors.
The Power Supply
The power supply uses a single EZ80 rectifier and a mosfet series pass regulator. The standard 250V PS uses a Hammond 270DAX transformer. Using the 270DAX solves another problem with the original Bijou transformer the 270AX. With at total of 2.8A heater requirements and only a 2.5A secondary, the 270AX was slightly overloaded. The 270DAX has a 3.5A winding, easily supplying the full heater current.
Here is the PS schematic.
Bijou 250V Regulated Power Supply
WARNING: This power supply uses potentially lethal HIGH VOLTAGES, more than 300V in certain places. Construction and testing of this PS should only be done by someone who is thoroughly familiar with safety precautions around high voltages.
WARNING: If you short the output of the regulator while it is under power, both mosfets will be destroyed and you will have to replace them.
If handled properly the PS will operate with stability for a long time. The one in my Bijou has been operating for almost two years without problems.