FM Plus Large-area Dual FM and Wi-Fi ALS with 4 FM R39

Ideal Use Cases

The FM+ 557 PRO D is built for large-area assistive listening installations that need both FM and Wi-Fi delivery from one system. Its Dante-enabled transmitter and four FM R39 receivers support the creation of ADA-compliant assistive listening spaces, while the selectable 72.1–75.9 MHz FM band gives an installer 17 wideband channels to work with. It is especially relevant when a project calls for a fixed transmitter, receiver support for four users, Ethernet-based administration, and integration with balanced or unbalanced microphone or line-level sources.

Why Choose This Model

This model combines a Dante-enabled FM and Wi-Fi transmitter with four receivers in one defined system. The transmitter offers a high-resolution 3.12-inch OLED interface, button-based setup, a hosted web page, and remote control and monitoring. Audio processing is DSP-based, with selectable 1:1, 1.5:1, or 2:1 compression plus high-pass and low-pass filter frequency control. A 10-LED audio meter spans -18 to +9 dB in 3 dB intervals, using green, amber, and red indicators to identify normal levels, near-overload peaks, and overload peaks during setup.

Installation and Integration

For audio connection, the transmitter provides one combination 3-pin XLR and 1/4-inch TRS input. It accepts balanced or unbalanced line-level and balanced or unbalanced microphone signals, with selectable 14.4 VDC phantom power. Input gain is adjustable from 0 to -50 dB in 1 dB steps. Monitoring and downstream connections include a 1/4-inch TRS headphone output carrying a mono signal and a -10 dBV RCA line output. Network integration is handled by a rear RJ-45 Ethernet port supporting 10/100 Base-T IEEE 802.3 and CAT 5e runs up to 328 feet or 100 meters.

The half-rack-width chassis occupies one EIA rack space in height. One or two units can be installed in a standard 19-inch rack with the optional RPK 005 single-unit or RPK 006 dual-unit kits. RF connections include a center #10-32 stud and a rear F-connector. Compatible antennas are ANT 005, ANT 021, ANT 024, and ANT 025; antenna selection should follow Williams AV antenna installation guidance for FCC-compliant configurations.

Performance Highlights

With an ANT 005 coaxial antenna mounted to the rear chassis connector, FM transmission range is specified at up to 1,000 feet or 304 meters. Frequency accuracy is ±2 ppm from 0°C to 50°C, maximum deviation is ±75 kHz, and the transmitter’s typical RF signal-to-noise ratio is 67 dB. In the Music audio preset, frequency response is 31 Hz to 16 kHz at ±3 dB, while total harmonic distortion is less than 0.25% at 1 kHz at the RF output. Common-mode rejection is greater than 57 dB at 1 kHz for microphone or line input.

RF output can be set to Full, Medium, Low, or Off. The transmitter also supports a selectable RF timeout of 30 minutes, four hours, or Always On; the timer begins when audio remains below -18 dB and no button is pressed for the chosen duration. Each receiver uses two AA alkaline or NiMH batteries. With two BAT 001 alkaline batteries, listed life is approximately 50 hours, and the receiver enters sleep mode after about six minutes without an RF signal.

What Makes It Different

The system’s distinction is its combination of dual FM and Wi-Fi assistive listening, Dante capability, four FM R39 receivers, and installation-oriented controls in one named configuration. The receiver covers the 17 listed frequencies from 72.1 through 75.9 MHz and selects channels through a seek button in the battery compartment. Its earphone or headphone cord serves as the receive antenna, and the 3.5 mm stereo/mono output can deliver up to 35 mW peak into 16 ohms. A green LED indicates power and flashes green for low battery, giving users a direct status cue.

On the transmitter, a green On Air LED confirms RF transmission, while the backlit latching power button distinguishes on, off, and sleep states. These indicators complement web-based management and the front-panel OLED, providing multiple ways to check and manage system status.

Final Assessment

The FM+ 557 PRO D is a focused choice for a large-area, ADA-compliant assistive listening system when Dante, FM, Wi-Fi, four FM R39 receivers, and rack-oriented installation are required. Its selectable RF power, detailed audio metering, configurable DSP, wired network control, specified FM reach under the stated antenna condition, and receiver battery management provide a documented foundation for system design. The listed warranty is two years for parts and labor, with FCC, RoHS3, WEEE, and Industry Canada approvals.