The EB-600/312 is a delayed-turn-on and muting circuit, based on the NE555 timer. Depending on the time constant of R1/C2 the timer switches the relays on after the predefined delay. The relay contacts are called C for Closed, O for Open and G for Ground. For normal operation the G-contacts are connected to ground and the C-contacts to the outputs. This means that the outputs are shorted to ground when the amp is switched on and remain shorted until the relays operate (normally 10 seconds). This prevents DC-thumps to be sent to the power amplifier during switch-on. When the power is removed the relays close again, shorting the output.

If the preamp outputs cannot be shorted, then the relay contacts have to be connected in series with the outputs. In this case connect the G-contacts to the outputs of the amplifier and the O-contacts to the output connectors. After the predefined time delay the relays operate and connect the amp outputs to the connectors.

The muting circuit also has a Manual Mute function, which can be used under listening. Put an appropriate switch on the front of the amplifier and connect it to the pins called Manual Mute on the board. When the switch is open the amp is muted. When it is closed the amp is operating normally. If this function is not needed then the two pins have to be jumpered on the board, otherwise the relays do not get supply voltage.

The muting circuit requires 12-15V to operate. Connecting 9-12VAC or 12VDC to the board can provide this. Total current consumption is approx. 100mA.

There is no set-up procedure for the board. Connect 12V to the board and after approx. 10 seconds the relays will operate and the LED will light up. The LED can be mounted on the front panel to indicate proper operation. Changing C2 can change the time delay. Increase C2 for increased delay or reduce it for shorter delay. The PCB size is 100x45mm.