STM-2X Switched Microphone Preamplifier
The switched microphone preamplifier shall be an RDL STM-2X. The unit shall provide one balanced mic-level input, one balanced line-level output, one balanced mic-level output, one external switching terminal, a multi-turn gain trimmer and a provision to enable phantom voltage. 

The microphone input shall be electronic balanced with an input impedance no less than 500 O and shall be fully compatible with either an unbalanced or balanced audio source. A multi-turn continuously adjustable trimming control shall be provided to set the microphone gain to a fixed value from 35 dB to 65 dB.

A phantom terminal shall be provided to connect external 24 Vdc which shall be filtered by the preamplifier to provide 24 Vdc phantom voltage compatible with IEC 61938: 2013.

A switching terminal shall be provided to activate the preamplifier outputs.  The outputs shall be active when the control terminal is pulled to ground by a dry switch or relay contact or by an open collector capable of switching 0.5 mA.  The unit shall switch the audio using a solid-state soft switch with a transition time not to exceed 5 ms.  When the outputs are inactive, the OFF attenuation of the preamplified input signal shall be at least 100 dB at 1 kHz and greater than 80 dB at any frequency from 50 Hz to 20 kHz.

The unit shall provide audio frequency response of 50 Hz to 30 kHz (± 1.5 dB) with total harmonic distortion and noise <0.05% and a noise floor exceeding 70 dB below an output level of +4 dBu with the input adjusted to 50 dB gain and a source impedance of 150 O. 

The input shall provide common mode rejection greater than 50 dB from 50 Hz to 20 kHz. The unit shall provide output headroom of no less than 20 dB above an output level of +4 dBu with the unit powered by 24 Vdc.

The unit shall provide one line-level balanced audio output at a nominal rated output level of +4 dBu with an output impedance of 150 O. The unit shall provide one mic-level balanced audio output at a nominal rated output level of -45 dBu with an output impedance of 150 O. Each output circuit shall be configured to produce an unbalanced output of proper phase between the positive output leg and ground. The two outputs shall be sufficiently isolated that a short-circuit on one output does not materially reduce the amplitude of the remaining output. 

The unit shall be equipped with an LED power indicator.

The unit shall be constructed in an electrically non-conductive, thermally conductive enclosure. Internal circuitry shall be protected from moisture and vibration using a thermally conductive compound that permanently secures the circuitry within the enclosure.

The face of the unit shall provide graphical labels identifying wiring connections and operating controls. Indicators and the gain control trimmer shall be oriented toward the front edge of the unit. 

Connection to external wiring shall be provided using a fixed terminal block on the edge of the unit. The terminal block shall accept wiring gauges from 14 to 22 and shall include a positive electrically conductive mechanism to prevent wire abrasion by the terminal screws.

The unit shall measure 3” (7.6 cm) x 1.6” (4 cm) x 0.7” (1.7 cm) and be compatible with all RDL STICK-ON mounting accessories. The unit shall be supplied with a two part re-closable polypropylene fastener system equipped with a rubber adhesive that bonds to low surface energy materials.

Power requirements shall be 24 Vdc ground-referenced with a maximum current not exceeding 25 mA. Phantom power requirements shall be 24 Vdc ground-referenced with a maximum current not exceeding 10 mA. The unit shall utilize an external power supply. The unit shall be engineered, assembled and tested in the U.S.A. and be specified for continuous duty in professional installed systems. The unit shall be specified for operation in an ambient temperature between 0 and 55 degrees C. The unit shall comply with relevant EMC requirements and shall bear the CE marking.

The unit shall be an RDL STM-2X.