The HD-PA35U is a single input audio amplifier for systems demanding the highest energy efficiency. It may be used as a single zone amplifier fed from a variety of RDL or OEM mixers. It is ideal for providing one or more additional zones to a mix originating from another RDL HD series mixer amplifier. The 35 watt power amplifier output is driven from a high-efficiency Class D amplifier stage. The amplifier normally shuts down (standby “sleep” mode) when audio is absent and automatically turns on when needed. The amplifier output level may be VCA controlled using various optional RDL remote controls, or by industry standard 0 to 10 Vdc or 0 to 10k ohm controls. The output drives 4 ohm or 8 ohm speakers. The output impedance is switch-selectable on the rear panel. The HD-PA35U is engineered, tested and manufactured in the U.S.A. to strict energy conservation standards. The unit consumes less than 1 watt of mains power in standby mode, and meets power amplifier efficiency specifications with an analog-filtered Class-D digital output stage. The HD-PA35U provides a front-panel control to set the input gain. An optional remote control connected to the rear-panel RJ45 jack provides for remote adjustment of the output level. The tone controls adjust the equalization at the amplifier output. The presence of a signal at the input causes the HD-PA35U to fully turn on, activating the power amplifier and all associated equalization and compressor circuitry. When an input signal is absent, a delay timer is initiated. Rear-panel DIP switches are provided to set the power-down delay timer to a value between 10 minutes and 2 hours in 10 minute increments. After the time-out period, the HD-PA35U enters the sleep mode until an input signal is received. The switches may be set to disable the sleep mode for systems specifying continuous amplifier operation. The amplifier includes an analog compressor/limiter for audio fidelity noticeably superior to conventional class D amplifiers with digital limiting. Increasing the input gain can substantially increase the average output power beyond that of a standard 35 watt amplifier. A red front-panel LED flashes when the compressor is preventing output clipping. Normal audio level signals remain unaffected by the compressor thereby preserving audio dynamics. The audio is compressed according to three dynamic time constants providing aural transparency while maintaining clean, unclipped amplified audio for input overloads of up to 20 dB. The HD-PA35U, with compression, is capable of producing average audio output levels and clarity normally expected from amplifiers with a much higher output power rating. During power-up and power-down, the amplifier output is protected against clicks, pops and thumping by internal soft-start solid-state switches. A blue POWER LED illuminates when the HD-PA35U is powered from its external 24 Vdc power supply (included). The power LED is dim when the amplifier is in the standby mode, and is bright when the amplifier is fully active. A front-panel pushbutton is provided to manually cycle between standby and active modes. The power amplifier is equipped with both thermal and output short-circuit protection. The high-efficiency Class D output stage produces minimal heat for all levels of expected voice or music modulation.