The Power Master does not use speaker output match-
ing
transformers and thus avoids the distortion, power loss
and added weight inherent in such transformers. The
speaker output voltage is 28.3 volts, to an 8-ohm load, for
100 watts. This allows long speaker lines with wiring prac-
tices consistent with those used for 25-volt speaker lines.
Up to 30 m (100 ft) of #18 gauge two-conductor cable
(such as Belden #8452, 8478, 8460, or 8461) may be used
to connect from the Power Master to each 16-ohm
(VA300-S) speaker. Greater cable lengths require heavier
gauge wire to avoid appreciable power loss in the speak-
er cable. For 30 to 51 m (100 to 170 ft), use #16 gauge;
51 to 81 m (170 to 270 ft), use #14 gauge; and 81 to 128 m
(270 to 425 ft), use #12 gauge wire. To maximize the
power to each speaker, a separate cable should be used
to connect each speaker to the Power Master.
Speakers:
Selection of speakers for use in a vocal music system is
most critical. The factors which most significantly con-
tribute to an outstanding vocal speaker system are correct
frequency range, distortion-free reproduction, and enough
sound power to fully penetrate the audience area.
The Shure VA300-S Speaker Column has been designed
to provide all of these features. It is recommended that
VA300-S Speaker Columns be used in pairs, each column
having a nominal impedance of 16 ohms, which provides
an impedance of 8 ohms when two are used. Each column
utilizes two special 10-inch speakers and four special 8-
inch speakers, and has a total speaker cone area of
2,310 cm² (358 in²) per column. The VA300-S Speaker
Column delivers virtually uniform penetrating power over
a 140° angle in the horizontal plane and a 65° angle in
the vertical plane. The rear-ported enclosure of the
VA300-S contributes to its highly directional pattern,
which is critical in achieving maximum audience penetra-
tion and reduction of feedback.
The Shure VA301-S Speaker Column is intended pri-
marily for use as an “on-stage monitor.” While this speaker
meets all of the criteria for an excellent vocal system
speaker, its area of coverage is more localized than that
of the VA300-S. The nominal impedance of the VA301-S is
32 ohms, so that when used in conjunction with two
VA300-S Speaker Columns, the total system impedance
becomes 6.4 ohms. An integral volume control on the
VA301-S permits its use on-stage as a monitor at the high-
est possible sound level without feedback.
The following list shows various speaker combinations
and the resultant impedance loads which are suitable for
use with the Power Master:
IMPEDANCE
QUANTITY and SPEAKER MODEL
OHMS (NOMINAL)
1 VA300-S
16
2 VA300-S
8
3 VA300-S
5.3
1 VA301-S
32
2 VA301-S
16
3 VA301-S
10.6
4 VA301-S
8
5 VA301-S
6.4
6 VA301-S
5.3
1 VA300-S and 1 VA301-S
10.6
1 VA300-S and 2 VA301-S
8
1 VA300-S and 3 VA301-S
6.4
1 VA300-S and 4 VA301-S
5.3
2 VA300-S and 1 VA301-S
6.4
2 VA300-S and 2 VA301-S
5.3
4
FIGURE 3. Typical output power vs. speaker load impedance for the
Power Master
PM300 Power Requirements:
The PM300 Power Master is furnished with a three con-
ductor power cable and three-prong plug. Connect the
power cable to an outlet which supplies 120 ±10% volts
A.C., 60 Hz power. The three-position toggle switch on
the front panel controls power to the amplifier. This
switch (S1) is also used to reverse line polarity, for mini-
mum hum. If extension cords are required to supply
power to the PM300, a high quality #18 gauge or larger
cord should be used.
PM300E Power Requirements:
The PM300E Power Master is furnished with a three-
conductor power cable without a plug. To connect a plug
to the power cable, attach the brown lead to the live (+)
terminal of the plug, attach the blue lead to the neutral (-)
terminal of the plug, and attach the green-yellow lead to
the earth-ground terminal of the plug.
The PM300E is factory wired for operation from a nom-
inal 240 volt power supply. Connect the power cable to an
outlet which supplies 240 ±10% volts A.C., 50 to 60 Hz
power. The three-position toggle switch on the front panel
controls power to the amplifier. This switch (S1) is also
used to reverse line polarity, for minimum hum. If exten-
sion cords are required to supply power to the PM300E,
a high quality cable with conductors of 0.75 square milli-
meters cross-section or larger should be used.
An internal modification performed by competent per-
sonnel will allow the PM300E to be used with 120 volt, 50
to 60 Hz power. To change the power supply wiring for
120 volt operation, do the following:
1.
2.
3.
4.
5.
UNPLUG THE AMPLIFIER POWER (MAINS) CABLE.
Remove the bottom plate as described on page 6.
W ith the amplifier upside down, note the six wires
from the power transformer T1.
Unsolder the yellow-black transformer lead wire and
the red-black transformer lead wire from their solder-
lug.
Resolder the yellow-black lead wire to the solder-lug
to which the all-black transformer lead wire is already
attached.
