Service manual

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.
With 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.