Radio Design Labs HD-MA35UA
35 Watt Mixer Amplifier, 25/70/100V with Power Supply


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Radio Design Labs' HD-MA35UA is a 4-input audio mixer amplifier for systems demanding the broadest range of features and the highest energy efficiency.  2 output power amplifier zones, 35 watts (Zone 1) and 4 watts (Zone 2), are driven from high-efficiency Class D amplifier stages.  The mixer & amplifier sections all normally shut down (standby "sleep" mode) when audio is absent and automatically turn on when needed.  The Zone 1 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.  This 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-MA35UA 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 analog-filtered Class-D digital output stages for both power amplifier output zones and for the 600 ohm output.

The HD-MA35UA features 2 mixers.  The Zone 1 mixer is located on the front-panel providing user level and tone 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-MA35UA has 4 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-MA35UA 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-MA35UA 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-MA35UA is equipped with a master VCA for remote adjustment of the Zone 1 amplifier output.  An RJ45 jack is provided for direct connection of an optional RDL remote level control.  Certain installations require the paging source to be audible even if the remote control is turned down.  A rear-panel switch is provided to cause the paging input to bypass the master VCA and the tone controls, allowing pages to be heard at the level set by the Level 1 input control on the front panel regardless of the VCA remote control volume setting.  If this bypass switch is not enabled, the paging source level is controlled by the master VCA.

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 3 dynamic time constants providing aural transparency while maintaining clean, unclipped amplified audio for input overloads of up to 20 dB.  The HD-MA35UA, 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-MA35UA 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.


  • 4 inputs with individual front-panel volume controls
  • Bass and Treble tone controls
  • Designed for background music with paging and music-on-hold
  • Master volume VCA controlled with remote adjustment capability
  • 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

Typical Performance:

  • Amplifier Zones (2):
    - Zone 1 (Main): 35 W RMS, front-panel mixer (user controls)
    - 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 1938: 1996-12), 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
  • Ducking/Muting: 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)
  • THD+N: < 0.5% (1 kHz at compressor threshold, main amp output); < 0.1% (line output, main)
  • CMRR:
    - Mic inputs: > 50 dB (50 Hz to 120 Hz)
    - Balanced line inputs: > 80 dB (50 Hz to 120 Hz, transformer-coupled input)
  • Compressor: 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: Zone 1 (main) master level (with switch-selectable input 1 bypass), 0 to 10 Vdc control, RJ45 (compatible with RDL VCA wall controls)
  • Front-panel controls (7): Zone 1 (main) mixer level controls (4, inputs 1 through 4), 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), VCA paging bypass (includes or excludes input 1 from master VCA control), Zone 2 mixer level controls (4, inputs 1 through 4)
  • Efficiency Standard: "Energy Star® Program Requirements for Audio/Video" Version 2.0
  • 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: Selectable
  • 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: 3.5" H x 8.5" W x 10" D (8.89 x 21.6 x 25.4 cm)
  • Weight: 7.3 lbs (3.3 kg)
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