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AC Power Considerations
Ensure that the pm125 or pm420 is plugged into an outlet capable of supplying the correct voltage
specified for your model and enough current to allow full-power operation of all the amplifiers
plugged into it. The current demand of a power amplifier varies depending on several factors,
including the impedance of the load, the output level of the amplifier, and the crest factor and duty
cycle of the program material. Under typical conditions reproducing rock music, with both channels
driven into a 4 ohm load to the point where musical peaks are just at the clipping point, the amplifiers
require the following average currents:
pm125: 2 amps for 120V versions, 1 amp for 230V v e r s i o n s
pm420: 10 amps for 120Vversions, 5 amps for 230V v e r s i o n s
Magnetic Leakage Considerations
The pm125 and pm420 may be mounted without concern for magnetic flux leakage, within the confines of
common sense. For example, it’s not a good idea to mount any power amplifier near a microphone input
transformer or magnetic storage media.
Input Wiring
The 1/4-inch phone jacks (XLR connectors on pm420 export versions) for the input signal can be used with
either unbalanced 2-conductor or balanced 3-conductor cables. Use shielded coaxial cable to conduct the
signal from the source (i.e. mixer, equalizer, CD player) to the amplifier.
For balanced operation:
U.S. Version: Use a 3-conductor TRS 1/4˝ phone plug. The tip of the plug carries the (+) (hot, non-inverting) side
of the signal, the ring carries the (–) (low, inverting) side of the signal, and the sleeve is ground (see Figure 3A).
Export Version: Use a male XLR connector. Pin 2 carries the (+) (hot, non-inverting) side of the signal, Pin 3
carries the (–) (low, inverting) side of the signal, and Pin 1 is ground (see Figure 3B).
For unbalanced operation:
U.S. Version: Use a 2-conductor (Tip-Sleeve) 1/4˝ phone plug. The tip of the plug carries the signal and the
sleeve is ground. The ring connection in the jack is automatically grounded by the sleeve (see Figure 4A).
Export Version: Use a male XLR connector. Pin 2 carries the signal and Pin 1 is ground. Short Pin 3 to Pin 1
in order to reference the input differential amplifier for the correct gain. Failure to do this will result in a loss
Installation
Location and General Precautions
Observe the following precautions when choosing a location for your amplifier.
A . Do not expose the unit to rain or moisture. If a fluid or foreign object should enter the unit,
disconnect the power plug and contact an authorized dealer or service center. Do not pull out the plug
by pulling on the cord; grasp the plug firmly.
B . Protect from heat and allow adequate ventilation. Place away from direct sources of heat, such as
heating vents and radiators. All components produce some heat during operation, so make sure that
the ventilation holes are not covered and that air is allowed to circulate freely behind, beside and above the
unit. Excessive heat is the single greatest source of both short-term and long-term component failure.
Mechanical Considerations
The pm125 requires one rack space (1.75˝), while the pm420 requires two rack space units (3.5˝) and a depth
of 13.25˝ inside the rack, including the rear supports. Secure the unit mechanically using four screws with
washers to prevent marring the front panel. Neoprene rubber washers are a good choice because they grip the
screw head and prevent them from backing out when vibrated or transported.
Rear Support for Road Applications
If the pm125 or pm420 is rack mounted and the rack is transported, mechanical support for the rear of the
amplifier is required. This could take the form of a shelf across the rear of the amplifier or brackets that engage
the rear of the unit. This practice is recommended for all electronic instruments.
Thermal Considerations
When the pm125 or pm420 are used in a free standing configuration, no thermal considerations are necessary
other than keeping the ventilation holes open. If the amplifiers are rack mounted, ensure that adequate
ventilation exists around the sides of the amplifier. When several amplifiers are mounted together in a rack,
you may need to provide air inlets from the outside of the rack. The pm125 and U.S. versions of the pm420
are convection cooled. Export versions of the pm420 contain a fan that is mounted on the rear panel. It draws
in air from the rear and exhausts it out the front, which provides additional cooling for internal components.
The pm125 and pm420 may be stacked directly on top of each other without spacer panels. If the amplifiers
are used with other amplifiers, ensure that the heat output from the other amplifiers doesn’t interfere with the
ventilation of the pm125 and pm420 (or vice versa).
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