Wiring and Ground
Two boards, a 30V split supply, a 6.3V heater supply, volume pots, etc. are required to make a complete amplifier. Each board is designed to have a single star ground connections. If the sigma22 power supply is used, it is reccomended to use it as the star ground location.
Two power switches are recommended for the EHHA, one for the heaters and one for the +/- 30V rails, because the tubes needs to warm up (for about a minute) before powering up the amp. Otherwise, the EHHA may be prone to misbehavior while the tubes reach their stable operating point.
2-Channel Standard Wiring
Two channel wiring is as shown below (Click to enlarge).
Because the heaters use AC, the heater wiring has been kept off-board to eliminate hum that might be caused by high heater currents running through board traces.
The star grounding arrangement shown here should minimize or eliminate hum and buzz.
Screw type board-mounted connectors have been used to make board-to-board wiring as simple and flexible as possible. However, it is not necessary to use the connectors. They can be left off the boards and the wires soldered directly to the through holes.
Here is a schematic for wiring the heaters using a 12.6V transformer.
EHHA Board with 12.6V Heater Supply
Three channel wiring is similar to two channel, passive ground wiring, except that the headphone jack's ground connection should be connected to the output of the ground channel board. It should not be connected to the star ground or the power supply ground. The input of the ground channel board, along with input ground and star ground should be connected to the star ground.
Headphones will need to be re-terminated (to dual 3-pin male XLR connectors) and may also need to be re-cabled (to separate the left and right ground returns) in order to be used with a fully-balanced amplifier.
A four-gang potentiometer (or stepped attenuator) is used to control the volume of all channels simultaneously.
Please note that a balanced headphone "sees" the difference voltage across the positive and negative amplifiers, thus the effective voltage gain is twice that of each amplifier alone. For example, if each EHHA amplifier is configured for a gain of 8, then the effective gain will be 16.