JBL MPC - Manual (Page 8)

7
Driving Distributed Lines
Make sure the sum of the power settings of all the speakers does not exceed the power rating of
the amplifier. Be aware that the actual power draw on some loudspeakers can vary considerably
from their tap labels. The tap labelings on some speakers refers to their power draw from the
distributed line, while on others they refer to the power delivered from the transformer into
the loudspeaker. In the latter case, the insertion loss of the transformer means that the actual
power draw from the amplifier is higher than the tap setting. Also, loudspeaker impedances
commonly vary with frequency, drawing more power at some frequencies than at others.
It is a good practice to allow a 20% safety margin. For example, if the amplifier has a power rating
of 250 watts, it is good practice to make sure the sum of the loudspeaker loads on the
distributed line is 200 watts or less.
Low-Impedance and Distributed Speakers on T-Version Amplifiers
MPC amplifiers are among the very few amplifiers that allow simultaneous use of low-impedance
speakers (connected to the LOW IMPEDANCE terminals) and distributed lines (connected to the
AUDIO TRANSFORMER terminals) not only on the same amplifier, but also on the same channel.
Applications—This versatility can be very useful in some applications. Examples 1 through 3
illustrate combining distributed lines and low-impedance speakers on the same channel, while
example 4 describes using them on separate channels of the same amp.
Application Example #1: A theater or performance venue—The main loudspeakers
can be operated from the low-impedance output while speakers in a restroom, lobby area,
or green room can be driven from the 100V or 70V output of the same amplifier channel.
This saves money, eliminating the need for an additional dedicated distributed amplifier.
Application Example #2: A club or restaurant—The main loudspeakers can be driven
from the low-impedance output while the 100V or 70V output drives backstage monitors
or restaurant loudspeakers.
Application Example #3: Extending low frequency performance—Adding a low
impedance subwoofer (4 Ω or 8 Ω ) to a distributed speaker system on the same channel
extends the sound spectrum with less risk of low frequencies saturating the transformers.
Balancing subwoofer and full-range speaker sensitivity—The sensitivity balance between full-range
speakers and subwoofers (the relative output level of each device) can be adjusted by choosing
appropriate tap settings on the speaker transformers and by positioning the subwoofer(s). For
example, suspending a subwoofer outdoors or in the center of a room, with no nearby boundary such
as a wall, floor, or ceiling, is acoustically the least efficient positioning; this is called placement in “free
space” because the sound radiates freely in all directions. Placing the subwoofer on one boundary—
on a wall or in the center of the ceiling—adds 3 dB of acoustic output because the boundary reflects
half the acoustic energy back into the listening space, reinforcing the other half. Moving the subwoofer
to a two-boundary junction (of 2 walls, a wall and ceiling, or a wall and floor) adds 3 dB more. Yet
another 3 dB can be obtained by placing the subwoofer in a corner (the junction of two walls and
the ceiling or two walls and the floor). Selective subwoofer placement therefore gives an adjustment
range of 9 dB, which represents an approximate doubling of its perceived acoustic volume.
Application Example #4: Using two channels for delay applications—When you need
delay for some, but not all, of the speakers (for example, to time-align speakers located in the
back of a venue), use both channels of the amplifier. The low-impedance output of one amp
channel can drive the main loudspeakers, while the same input signal is routed through a delay
and into the second channel to run a line of speakers for under-balcony or delayed-lobby use.
The delayed speakers can be either low-impedance, or more typically, 70V or 100V. This
eliminates the expense of an additional distributed amplifier.