Dynacord Stereo System manual When To Specify Mixer/Amplifiers

originate, and from what type of interface? The answers to questions such as these will help to determine the input configurations of the unit.

Similarly, take a look at the expected load requirements. For example, how many loudspeakers will be needed to adequately cover the space and assure good intelligibility? What type of loudspeakers will best serve the application: ceiling speakers, wall speakers, paging projectors, or a combination of different elements? Will 25V or 70.7V distribution be necessary, or is a direct voice coil connection sufficient? Must the system handle multiple independent zones? Such factors will determine the power output required of the unit.

Additional elements to consider include the level of ambient noise and reverberant characteristics of the space, along with the frequency response of the sources and loudspeaker elements; these will affect the audio quality of the system. Packaging also should play a part in your design. Must controls be user-adjustable, for example, or should they be tamper-proof? Will the mixer/amplifier be rack-mounted with other equipment? Must the system operate in a dusty or moist environment, or where users will have dirty hands or be inexperienced? Finally, remember the bottom line: what percentage of the cost of materials sold on the job is represented by the cost of the unit? Once you have initially specified the requirements of the system, you can narrow down your choice of mixer/amplifiers for the job.

ElectroVoice offers two mixer/amplifiers designed to suit a wide variety of sound contracting needs. The MA1212 or 1206 units are mixer/amplifiers with either 120 or 60-watt output capabilities. Mountable on a shelf or in a rack, these units accommodate up to 12 different input sources to permit larger complex configurations with off-the-shelf convenience. Tone controls are provided, along with outputs to drive supplementary systems such as booster amps or tape recorders.

When To Specify Mixer/Amplifiers

The performance of integrated Mixer/Amplifiers is more than adequate for the vast majority of distributed sound installations.

Consider, for example, the question of frequency response. The MA models exhibit a low- frequency limit of 50 Hz, and an upper-frequency limit between 15 and 20 kHz. Is this good enough? To answer this question, we need to examine the performance of other components in the signal chain.

Let’s examine the response of a typical 8-inch ceiling speaker in a 1.8 ft3. sealed box. At the low end, the unit begins rolling off below about 150 Hz and is nearly 20 dB down by 50 Hz. Its response also rolls off above 5 kHz, and is nearly 10 dB down at 20 kHz. Paging projectors, used in a wide variety of industrial applications, will introduce even more severe response limitations, along with greater harmonic distortion.

Similar conditions apply at the system input. Background music players and FM stations, for ex- ample, are limited to the bandwidth from 50 Hz to 15 kHz, at best. Push-to-talk paging microphones, such as the ElectroVoice US600EL, are tailored for the voice range and thus feature far more limited frequency response. Telephone paging lines are further limited in response, and can introduce greater distortion.

Remember that using an amplifier with significantly wider bandwidth than the signal source and/or output devices will serve primarily to increase noise and distortion in the system. An amplifier which reproduces low frequencies below 200 Hz when driving a paging projector with a 300 Hz to 500 Hz lower cutoff will be wasting power (reducing headroom in the pass-band) and may cause intermodulation distortion if not premature failure in the paging projector. In this context, the audio specifications of the utility units in our example begin to look appealing. In fact, whenever the program source, paging mic or loudspeaker system exhibits limited bandwidth, there is a diminished need for an audiophile-quality mixer/amplifier system. The majority of practical installations fall within this category. In professional buildings, office structures, warehouse paging

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