Radio Design Labs HD-PA35
35 Watt 4/8 Ohm Power Amplifier


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The HD-PA35 is a single input audio amplifier for systems demanding high 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 W power amplifier output is driven from a high-efficiency Class D amplifier stage. 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 10 kOhm controls. The output drives 4 Ohm or 8 Ohm speakers. The output impedance is switch-selectable on the rear panel. The HD-PA35A has all the features of the HD-PA35 except its 35 W amplifier provides a constant voltage output (25 V, 70 V or 100 V) instead of a low impedance speaker output. The HD-PA35 is engineered and manufactured in the U.S.A.

The HD-PA35 provides a front-panel control to set the input gain. An optional remote control connected to the rear-panel detachable terminal block provides for remote adjustment of the output level. The tone controls adjust the equalization at the amplifier output.

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 W 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-PA35, with compression, is capable of producing average audio output levels and clarity normally expected from amplifiers with a much higher output power rating.

A blue POWER LED illuminates when the HD-PA35 is powered from its external 24 Vdc power supply (included). The power LED is dim when the mixer amplifier is in the standby mode, and is bright when the amplifiers are fully active. A front-panel pushbutton is provided to manually switch between standby and active modes. The power amplifiers are 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.


  • One input with front-panel volume control
  • Bass and Treble tone controls
  • Designed as a single zone amplifier
  • Master volume VCA controlled with remote adjustment capability
  • Amplifier output on detachable terminal block
  • Unbalanced summing inputs on RCA jacks
  • Includes power supply
  • High-efficiency Class D operation


  • Rated Power: 35 W RMS
  • Audio Input: Unbalanced stereo summing input on RCA jacks
  • Maximum Input Level: + 9 dBV (+15.5 dBV, one input); (for 1% THD+N at output)
  • Input level: -15 dBV (180 mV, to cross compressor threshold equaling 23 W amplified output, both input jacks driven)
  • Noise: <-75 dB (max gain)
  • THD+N: < 0.5% (1 kHz at compressor threshold)
  • Compressor: Threshold 2 dB below rated output, automatically adjusting attack and release times
  • Frequency Response: Constant voltage: +/- 1 dB (50 Hz to 20 kHz)
  • Tone Controls: ±10 dB @ 10 kHz, ±9 dB @ 100 Hz (front-panel) with center detent
  • Power Amplifier Output: 35 W RMS (4 Ohms), 32 W (8 Ohms) on detachable terminal block (switch-selectable 4 or 8 Ohm impedance); <1% THD+N
  • Indicators (3): Front-panel: Signal present (green), Compressor activity (red), Power (bright=on, dim=standby, blue)
  • VCA: Output level, 0 to 10 Vdc control, detachable terminal block (compatible with RDL VCA wall controls)
  • Front-panel controls (4): Input, EQ (2, bass and treble), Power (latching pushbutton, on/standby)
  • Ambient Operating Environment: 0° C to 50° C Maximum; 20° C Recommended
  • Power Supply (included): 100 to 240 Vac, 50-60 Hz, 50 W; 24 Vdc output to amplifier chassis
  • Dimensions and Weight: Height: 3.5 in.(8.89 cm); Width: 8.5 in.(21.6 cm); Depth: 6.75 in.(17.1 cm); Weight: 4.5 lb.(2 kg)
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