Wiring & groundWARNING: You will be working with wiring that carries AC mains voltage, which is a shock hazard and potentially lethal. If you're not sure what you're doing, do not plug in the AC power cord until you resolve all your questions. When in doubt, ask for help in the forums or get someone more experienced to help you.After you finish assembling the circuit board, it's time to work on the chassis case. When ready, mount the σ22 circuit board into the case with standoffs or spacers. Make sure none of the leads and traces on the bottom of the board touches the metal chassis. If your standoffs or spacers are short, you should glue an insulating plastic sheet beneath the board to prevent board flexture from causing short circuits. 
Wire the power indicator LED to the σ22 board, pay attention to the polarity. The LED is usually installed on the chassis front panel. Mount the power transformer and connect its secondary leads to σ22's "Transformer" terminal block. Use the transformer's supplied documentation or label to identify the color and orientation of the leads. If your audio circuit being powered is located in the same chassis as the power transformer, keep the transformer as far away from the audio circuit as possible. This minimizes magnetic leakage from the transformer from inducing hum and noise into the audio circuitry. If you are using a dual secondary transformer (4 wires), take care not to reverse the phase of the two windings. If you are using a center-tapped transformer (3 wires), then the tap should be connected to one of the two "G" pads at the transformer input block. Next, wire up the transformer's primary leads to the power switch and fuse (if the IEC power entry receptacle doesn't have these built-in) and then to the IEC receptacle. For the power switch, you could use a single-pole switch on the "hot" side only, or use a double-pole unit to switch both the "hot" and the "neutral" lines. Some transformers have dual primaries, when they are connected in parallel, it supports 120V AC mains; and when connected in series, it supports 240V. Use the transformer's documentation as your guide. The σ22 PSU is purposely designed without any current limiting in order to allow high current peaks, therefore, an AC line fuse is mandatory not only for safety, but because it is the only defense against severe damage when something goes wrong. Therefore, if your IEC receptacle does not have a built-in fuse, install a fuse holder (either panel-mount or chassis-mount) and wire it inline with the "hot" tab of the IEC receptacle. An appropriately rated "slow-blow" fuse should be selected, based on the expected load. For most headphone systems, a 2A fuse should be sufficient. However, you may need to use a bigger fuse for higher-powered applications. Use insulated FastOn connectors on the IEC receptacle, do not allow any terminals carrying live AC mains voltages to be exposed. You may also use heatshrink tubing to enhance your wiring. If your chassis is metal, the AC earth ground connection on your IEC receptacle should be tied securely to the chassis. This is a safety measure, to prevent electric shock if any AC wiring comes loose. If the device being powered by the σ22 also resides in the same chassis, you should isolate the device's signal ground from the chassis (making sure that all input and output jacks are isolated). Then, connect the signal ground to the chassis through a "ground loop breaker". This prevents a potential ground loop which would cause hum and noise. A ground loop breaker is a 10Ω 5W resistor in parallel with a 0.1µF capacitor rated at more than 250VAC. For safety this capacitor should be rated for class X or Y with flame retardant casing. See the β22 amplifier wiring instructions for details. Do not connect the load (i.e., the amplifier or other device to be powered by the σ22) at this time. That will be done in the next section, after some power-up sanity checks. Inspect all your wiring to make sure there are no errors. Use a multimeter to check for short circuits. Do not plug in the AC power cord until you're absolutely sure everything is correct. Proceed to the Initial check section for power-up sanity checks. Main: σ22 Main | Prev: Instructions | Next: Initial check |
