Processing and Control for Professional AV Installations
Legacy Hardware

420 Stereo Power Amplifier

The Symetrix 420 is a two-channel power amplifier intended for use in professional and commercial audio systems. The 420 may be operated as a two channel amplifier with 20-watts per channel, or as a single channel amplifier capable of 40-watts output (mono-bridged mode). The amplifier is intended for use with near-field monitors, small speakers used for “radio reference” audio-for-video, and with headphones. In stereo mode, the minimum load impedance is 4 ohms, in mono-bridged mode the minimum load impedance is 8 ohms.


Maximum Output (stereo/2-channel) 20 watts RMS, per channel, into a 4 or 8 Ohm load. Maximum Output (mono bridged) 40 watts RMS into an 8 Ohm load.

An under powered speaker can be just as damaged as an over powered speaker. When an amplifier is driven into clipping or square wave it is essentially asking the speaker cone to go full out (+) then full in (-) in zero time.

Are the speaker connections shorted? Disconnect speakers, disconnect inputs, replace fuse, turn unit on. If the fuse blows immediately, this usually indicates a power supply fault or possibly shorted output transistor(s).

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.

The minimum load impedance of the 420 is 4 Ohms in stereo or dual-mono mode and 8 Ohms in mono-bridged mode.

The 420 is ideally suited to headphone distribution systems. There are two basic strategies involved in making a loud headphone system:

1. If you have low impedance phones, then you need high current capability to make them loud. This translates to a modest 8 Ohm power rating, like 20 watts.

2. If you have high impedance phones, then you need high voltage capability to make them loud. This translates to a moderate 8 Ohm power rating, like 100 watts

Check input signal levels, and level control settings. Check gain settings on upstream equipment. Is the input signal already noisy?