High Impedance/Constant-Voltage Systems - Electro-Voice MA-1212 User Manual

3. Balancing relative speaker levels: There is no easy, electrically
efficient way to balance and adjust the sound levels among multiple
speakers in a low impedance installation. Constant-voltage high
impedance systems ease this process because they typically employ
speakers equipped with transformers with multiple input taps marked in
watts.
Low Impedance Speakers. The low impedance ohm output terminal is
provided for direct connection of one or more standard low impedance
speaker systems. For example, connect a single speaker system to the
common and 4 ohm terminals. Correct connection of several low
impedance speakers must follow the rules of proper series and parallel
impedance summation.

High Impedance/Constant-Voltage Systems

Installations that require high impedance speaker loads permit smaller
diameter wire for a given power loss in the speaker lines. Usually, low
impedance speakers are still used in such systems, but transformers are
used at the speaker locations to increase the impedance to the desired
value. Multiple transformer taps, labeled in watts, permit easy adjustment
of the individual speaker levels. Use simple parallel connection of the
transformer primaries (inputs). This eliminates the determination of load
impedance and series-parallel speaker connections.
For proper operation, all speakers should have transformers with the
same voltage rating, i.e. 25, 70.7, or 100 volts. The total of all the power
taps should be equal to or less than the amplifier’s rated output power.
When the total equals the rated power, the amplifier may deliver rated
power to the load, depending on the input signal levels. Totals less than
the rated amplifier output will not damage the amplifier since the
resultant load impedance is higher than the amplifier’s rated impedance
and thus only reduces the power delivered at maximum amplifier level.
How ever, avoid power totals greater than rated amplifier power, since
the total load impedance drops below the amplifier’s rated load.
Preamp Out: The Preamp Out jack provides a line-level mix of the input
signals, fed through a 1/4 inch phone plug wired with a single-conductor
shielded cable.
Use the signal at the Preamp Out jack to feed an external equalizer or
signal processor whose output would connect to the Power Amp In jack.
MA-1206 and MA-1212 User Manual
Electro-Voice
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