The TX‑PA40D is the ideal choice in many applications where a high quality, high‑efficiency utility amplifier is needed to drive a pair of 4 Ω or 8 Ω speakers. It is specifically suited to fixed‑gain amplification applications where gain is controlled by other equipment. Examples include projectors with variable audio outputs used in classrooms, boardrooms and video conferencing.
The TX‑PA40D features two unbalanced line level inputs on high‑reliability gold plated phono (RCA) jacks. A front‑panel recessed gain control trimmer permits setting the desired amplifier output level for each channel. The gain range accommodates the range of typical unbalanced audio signal levels. Each of the two TX‑PA40D outputs will drive a 4 Ω or 8 Ω speaker or multiple speakers connected to present a 4 Ω or 8 Ω load to the amplifier. The output impedance is switch‑selectable on the front panel of the module.
A front‑panel switch is provided to sum the audio inputs to mono. In the STEREO mode, the TX‑PA40D independently amplifies each audio channel, left (L) and right (R). If speaker placement in the specified installation is incompatible with stereophonic amplification, the INPUT MODE switch may be set to SUMMED MONO.
The TX‑PA40D includes a stereo analog compressor/limiter for audio fidelity noticeably superior to conventional class D amplifiers with digital limiting. The input GAIN settings determine whether the limiter alone is used for clipping suppression, or if the full compressor/limiter will be used to substantially increase the average output power beyond that of a standard 40 W amplifier. A red front‑panel LED flashes when the limiter is preventing output clipping. Normal audio level signals remain unaffected by the compressor thereby preserving audio dynamics. If the input level is increased so the compressor is active, the LED remains dimly lit between peak flashes. 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 TX‑PA40D, with compression, is capable of producing average audio output levels and clarity normally expected from stereo amplifiers with a much higher output power rating.
A blue POWER LED illuminates when the TX‑PA40D is powered from its external 24 Vdc power supply. The module is 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. Continuous full‑power operation with audio tones will not damage the amplifier, but is not recommended.
Wherever an ultra‑compact, high quality, high efficiency audio power amplifier is needed to provide reliability and unsurpassed versatility, the TX‑PA40D is the ideal choice. Use the TX‑PA40D individually, or combine it with other RDL products as part of a complete audio/video system.