Processing and Control for Professional AV Installations
Legacy Hardware

303 Interface Amplifier (Bi-directional)

The Symetrix 303 Interface Amplifier creates a proper interface between consumer and professional audio gear. This tool provides bi-directional level-matching and balanced/unbalanced conversion between professional (+4 dBu nominal level, balanced) audio equipment and consumer (-10 dBV nominal level, unbalanced) audio components.


There are two separate AC supplies coming out of the PS-3: 8.5 VAC @ 1A and 34.5 VAC CT @ 250mA. Pinout is as follows: 8.5 VAC on pins 6 and 1, 17 VAC on pins 7 and 3, center tap @ pins 4 and 5 are connected to earth ground. This is when viewed at the end of the PS-3 7-pin DIN plug.

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The unbalanced cables (those connected to the -10dBV equipment or RCA connectors) should be kept at 6 feet or less to minimize the noise and hum that may be induced. The balanced (+4dBu) cables can be ran hundreds of feet with very little induced hum and noise.

Check the input signal. The 303’s unbalanced inputs are designed to run at -10dBV nominal, and the 303’s balanced inputs are designed to run at +4dBu nominal. Make sure that you are feeding a line level (not mic level) signal to the 303 at the appropriate nominal level. Check gain settings on downstream equipment. The system gain structure should be such that the 303 operates at or near unity gain. Is the input signal already noisy?

No, the only option would be to use a 12VDC to 117VAC inverter with the supplied PS-3 inline power supply.

Check the input signal. Is it too hot, or is it already distorted? Check the output loading. It should be above or equal to 600 ohms. Is the input of the device following the 303 clipping? Is something else clipping?