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The HD-RA35UA is a four input audio mixer amplifier for systems demanding the broadest range of features and the highest energy efficiency. Two output power amplifier zones, 35 watts (Zone 1) and 4 watts (Zone 2), are driven from high-efficiency Class D amplifier stages. The mixer and amplifier sections all normally shut down (standby “sleep” mode) when audio is absent and automatically turn on when needed. A paging source is normally connected to Input 1. The Zone 1 amplifier output level for inputs 2, 3 and 4 is VCA controlled using various optional RDL remote controls. This allows the user controls to be conveniently wall mounted without running audio to and from the remote control location. The Zone 1 output provides a constant voltage output (25 V, 70 V or 100 V). The Zone 2 amplifier output drives an 8 ohm speaker and/or 600 ohm telephone equipment.

The HD-RA35UA is engineered, tested and manufactured in the U.S.A. to strict energy conservation standards. The unit consumes less than 1W of mains power in standby mode, and meets power amplifier efficiency specifications with analog-filtered Class-D digital output stages for both power amplifier output zones and for the 600 ohm output.

The HD-RA35UA features two mixers. The Zone 1 mixer is split between the front-panel (Paging Input 1 and tone controls) and a remote control (Inputs 2, 3 and 4) providing user level controls for the 35 watt amplifier output. The tone controls adjust the equalization at the Zone 1 amplifier output and can be switch selected on the rear panel to also adjust the equalization at the line-level output. The Zone 2 mixer on the rear panel provides level trimmers normally set by the installer to provide music-on-hold and/or for background music in a small zone equipped with an 8 ohm speaker. The Zone 2 section includes a 4 watt amplifier, a 600 ohm transformer-balanced MOH output, and an active balanced line-level output that may be used to feed another audio power amplifier.

The HD-RA35UA has four inputs that are common to both mixers. Inputs 1 and 2 accept balanced mic or line level sources. Inputs 3 and 4 accept unbalanced mono or stereo line-level sources.


  • Input 1 is switch-selectable MIC or LINE. Phantom voltage (IEC standard 24 Vdc) is switch-selectable for the microphone input. The line input is transformer isolated and accepts balanced or unbalanced, high or low impedance audio sources. A gain trimmer is provided to set the input preamplifier to the optimum gain for the installation.
  • A paging source is normally connected to Input 1. This input is equipped with push-to-talk terminals and with an adjustable threshold VOX circuit with LED threshold indicator, either (or both) of which may be set to duck (“fade down” or “mute”) inputs 2, 3 and/or 4 by 25 dB when a paging signal is active on Input 1. The attenuated inputs fade up to normal volume when the paging message is finished. A rear-panel trimmer allows the start of the fade-up to be delayed from 2 to 6 seconds.
  • Input 2 is switch-selectable MIC or LINE. Phantom voltage (IEC standard 24 Vdc) is switch-selectable for the microphone input. The line input is active balanced and accepts balanced or unbalanced, high or low impedance sources. A gain trimmer is provided to set the input preamplifier to the optimum gain for the installation.
  • An RDL Dual-LED VU meter is provided on the rear panel to indicate proper gain adjustment for Inputs 1 and 2. Correct input gain adjustment insures adequate mixer stage headroom.
  • Inputs 3 and 4 each provide unbalanced stereo RCA input jacks, summed to mono.


The presence of a signal on any of the inputs causes the HD-RA35UA to fully turn on, activating both Zone 1 and 2 power amplifiers and all associated mixing, equalization and compressor circuitry. When input signals are 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-RA35UA 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 HD-RA35UA is equipped with an RJ45 jack for direct connection of an RDL triple remote level control (sold separately). Remote controls have three user volume controls and are available with pushbutton muting with LED indicators for each input. Remote control model examples: D-RC3 and D-RC3M. A rear-panel switch is provided to defeat the front-panel tone controls for Input 1, normally used when a paging source is connected to Input 1.

A dual-mono line-level output is provided on RCA jacks to feed the mono or stereo inputs of other audio equipment or power amplifiers. A rear-panel switch enables or disables the front-panel tone controls from adjusting the LINE OUT equalization. The Zone 1 mixer preamplifier output feeds the associated power amplifier input through a send/return effects loop on RCA jacks, normally bypassed by a rear-panel switch. This switch is turned off if an external module is connected to the loop. Ground-referenced 24 Vdc power is available on a rear-panel terminal block to power an optional external RDL module which may be mounted on the panel space provided. External module type examples: audio filtering or processing, mixing, isolation, twisted-pair sender/receiver.

The Zone 1 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-RA35UA, 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 and preamplifier outputs are protected against clicks, pops and thumping by internal soft-start solid-state switches.

A blue POWER LED illuminates when the HD-RA35UA 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 cycle between standby and active modes. The power amplifiers are equipped with both thermal and output short-circuit protection. The high-efficiency Class D output stages produce minimal heat for all levels of expected voice or music modulation.


  • Four audio inputs
  • Paging input with front-panel volume control
  • Three inputs with VCA volume control adjusted using RDL remote control
  • Bass and Treble tone controls
  • Designed for background music with paging and music-on-hold
  • Manual or automatic (VOX) priority ducking
  • Balanced inputs and amplified outputs on detachable terminal blocks
  • Unbalanced inputs and outputs on RCA jacks
  • Includes power supply
  • Automatic sleep mode meets strictest energy saving standards


Amplifier Zones (2):
Zone 1 (Main): 35 W RMS, remote-controlled mixer (3 user controls) with paging input level control on front panel; Zone 2: 4 W RMS (8 Ohms), rear-panel mixer (trimmers)

Audio Inputs (5):
2, balanced, mic or line switch selectable on terminal blocks; 2, unbalanced stereo summing inputs on RCA jacks; 1, unbalanced amp in on RCA jack

Gain Adjustments (2):
Inputs 1 and 2: Rear-panel single-turn trimmer

Maximum Input Level:
Mic Inputs: -9 dBu; Balanced line inputs: + 20 dBu; Unbalanced Inputs: + 5 dBV (+11 dBV, one input); (for 1% THD+N at output)

Phantom Voltage:
24 Vdc (IEC 61938: 2013), switch-selectable for each mic input

Ducking/Muting Actuation:
Automatic (Input 1 VOX: rear-panel adjustable signal threshold with LED indicator) or Manual (contact closure to ground)

Ducking/Muting Release Delay:
Rear-panel single-turn trimmer, adjustable 2 to 6 seconds

Switch-selectable for inputs 2, 3 and/or 4, 25 dB ducking attenuation, nominal

Effects Loop:
Unbalanced RCA jacks (preamp output from main mixer, input to power amplifier)

External RDL Module Power Output:
24 Vdc, 100 mA maximum, ground-referenced

Input level (to cross compressor threshold equaling 23 W amplified output):
Mic Inputs: -50 dBu (2.5 mV), (front panel and rear trimmer at max.)
Balanced line inputs: -21 dBu (70 mV), (front panel and rear trimmer at max.)
Unbalanced Inputs: -15 dBV (180 mV), (front panel and rear trimmer at max., both inputs driven)

Noise (line or amplified outputs):
Mic or Balanced line inputs: -70 dB (below compressor threshold at max gain); Unbalanced Inputs: <-75 dB (max gain)

< 0.5% (1 kHz at compressor threshold, main amp output); < 0.1% (line output, main)

Mic inputs: > 50 dB (50 Hz to 120 Hz); Balanced line inputs: > 80 dB (50 Hz to 120 Hz, transformer-coupled input)

Threshold 2 dB below rated output, automatically adjusting attack and release times

Frequency Response (mixer):
Mic to line output: ±1.5 dB (40 Hz to 25 kHz); Balanced line input to line output: ±1 dB (20 Hz to 20 kHz); Unbalanced line input to line output: ± 0.5 dB (20 Hz to 20 kHz)

Frequency Response (amplifiers):
Constant voltage: ±3 dB (50 Hz to 18 kHz); 4/8 Ohm: ±1 dB (50 Hz to 20 kHz)

Tone Controls:
±10 dB @ 10 kHz, ±9 dB @ 100 Hz (front-panel) with center detent

Audio Outputs (2):
-10 dBV nominal (316 mV), dual mono unbalanced RCA (main mixer; FLAT or EQUALIZED, switch-selectable on rear panel); 0 dBu nominal balanced, terminal block (zone 2 mixer)

MOH Output:
0 dBu nominal (775 mV), 600 Ohm transformer-isolated, 16 dB headroom

Power Amplifier Outputs (2):
Zone 1 (main): 35 W RMS (25, 70 or 100 V); Zone 2: 4 W (8 Ohms, on detachable terminal block)

Indicators (9):
Front-panel: Signal present (4; 1 per input, green), Compressor activity (red), Power (bright=on, dim=standby, blue); Rear-panel: dual-LED VU meter (for inputs 1 and 2, green/red), Ducking/muting threshold (yellow)

VCA (3) :
Zone 1 (main) inputs 2, 3 and 4, 0 to 10 Vdc control, RJ45 (compatible with RDL VCA wall controls)

Front-panel controls (4):
Zone 1 (main) paging level control (input 1), EQ (2, bass and treble), Power (momentary pushbutton, toggle on/standby)

Rear-panel controls (22):
Gain trimmers (2, input 1 and 2), Mic/Line switch (2, input 1 and 2), Zone 1 line output pre/post-EQ switch, Effects loop bypass switch, Sleep (standby) mode delay (4 binary switches), Input 1 ducking/muting detector sensitivity trimmer, Phantom voltage switches (2, input 1 and 2), Ducking control switches (3, to duck inputs 2, 3, and/or 4), Tone control paging bypass (includes or excludes input 1 from tone controls), Zone 2 mixer level controls (4, inputs 1 through 4)

Efficiency Standard:
“Energy Star® Program Requirements for Audio/Video” Version 2.0, applicable for passive remote control models (without muting)

Sleep Mode Power Consumption:
< 1 W (amplifier and included power supply)

Sleep Mode Delay:
10 minutes to 2 hours (switch-selectable in 10 minute increments)

Sleep Mode Disable:

Active Mode Delay:
2 seconds (nominal) after input audio detected

Audio Detector Threshold:
-80 dBu mic (at max input gain), -55 dBu balanced line (at max input gain), -42 dBV unbalanced

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: 10 in.(25.4 cm); Weight: 7.3 lb.(3.3 kg)

Package Type:
Cardboard Box

Package Dimensions:
17.75 x 9.125 x 3.9375 in.

Shipping Weight:
9.82 lbs.

WEEE weight:
8.15 lbs.

Tariff code:


RDL Three-Year Limited Warranty Installation of these products by the Purchaser shall constitute Purchaser’s acknowledgement and acceptance of the terms and conditions for sale and of this warranty, and shall constitute the Purchaser’s determination of the suitability of this product for the Purchaser’s intended application. Radio Design Labs, Inc., also referred to as RDL®, warrants to the Purchaser that products manufactured by RDL are free from defects in material and workmanship. This warranty applies to the period of three years from the date of shipment, except for component parts or products purchased from other sources and supplied by RDL. Such component parts or products bear only the warranty of the manufacturer thereof in effect at the time of shipment to the Purchaser. RDL will, without charge, and after written notice has been received and acknowledged by RDL, repair an RDL product if proved to be defective according to the usage of the trade, when such equipment is received by RDL at the location it designates with shipment costs prepaid by the Purchaser. RDL shall not be liable for any expense whether for repairs, replacements, material, service, or otherwise, incurred by the Purchaser or modifications made by the Purchaser to the product. No equipment shall be deemed defective if it shall fail to operate in a normal or proper manner due to exposure to excessive moisture in the atmosphere, excessive temperature extremes, improper environmental cleanliness, or any other environment not consistent with the principles of good engineering practice. In no event shall RDL have any liability for consequential damage or expense directly or indirectly arising from the use of the products, or any inability to use it either separately or in combination with other equipment or materials, or from any other cause, whether used in accordance with instructions or not. This warranty is void if equipment is altered in any way by other than RDL. RDL products are NOT intended for use in life support appliances, devices, or systems. Use of RDL products in such applications is done solely at the liability of the reseller/installer and/or end user. RDL reserves the right to change design parameters and specifications at any time without notice. The performance measurements reflect the products at the time of printing. This warranty is in lieu of all others, either expressed or implied. No representative is authorized to assume for RDL any other liability in connection with RDL products.