To prevent inadvertent setting, the bridged-mono mode switch is located inside of the 420. To set or reset the switch, use the following procedure:

1. Disconnect the 420 from the AC power source.

2. Remove the top cover.

3. Locate the switch, which is situated near the input connectors.

4. Depress the switch plunger to select mono-bridge mode so that the plunger remains IN. For stereo mode, the switch plunger should be OUT.

5. Replace the top cover and all fasteners previously removed.

In mono-bridge mode, use the CHANNEL 1 INPUT, and connect the speaker across the two plus (+) output terminals. The channel 1 plus output terminal is the in-phase or non-inverting output connection. Connect it to the plus side of the speaker. Only the CHANNEL 1 GAIN control is functional. If you connect a pair of headphones to the front panel STEREO HEADPHONES jack, they will receive out-of-phase signals not harmful to anything (except phase or polarity fanatics).

In mono-bridge mode, the 420 delivers 40 watts into an 8 Ohm load. The minimum load impedance is 8 Ohms.