Maintain a consistent polarity with respect to the ‘hot’ and
‘ground’ wires to the speakers. Most dual banana plugs have a
tab identifying one side,
which is customarily made the
‘ground’ side. In stereo speaker installations it is important to
keep consistent phase relationships to preserve low frequency
response and stereo ‘imaging’ perspective. This means that the
same wiring ‘sense’ should be followed when wiring each
speaker.
D U A L 6AWAMA PLUG
S P A D E
S I N G L E BANANA
PLUG
The black output terminals may be connected together
(common ground) when accessories utilizing a common ground
system, such as headphone junction boxes, are employed.
However, the
red
terminals should
never
be connected
together. Thus you should carefully check connections when
using a common ground device, to be sure there is no reversal of
the ground and high side.
Connecting Cables
The permissible cable length for an unbalanced input is
determined by two factors: the cable capacitance per foot, and
the output impedance of the signal source, which is a function of
the design of the output stage of the source (control center).
Cable capacitance is part of the ‘load’ on the signal source. As
the capacitance increases, the effect is that of a progressive filter
which rolls off the high frequencies before they reach the power
amplifier. The amount of the roll off increases as the output
impedance goes up. With most control centers and normal input
cables (under 20 feet) the filter effect will be above the audio
range. If the output impedance of the source exceeds 500 ohms
at 1000 Hz, a high cable capacitance may cause oscillation in
the source equipment. Shortening the cable, or choosing a
special low capacitance cable type may solve the problem. If
very long signal cables are required, converting to a balanced
input is the best solution. The Hafler BL500 balanced input
option may be installed in the P505, and avoids the need for
hum-sensitive input transformers.
The choice of shielded wire for input cables may involve
more than just the capacitance per foot. Where resistance to
abuse is a factor, the rubber-clad cables with woven shielding
outperform vinyl coverings. The vinyl insulation is usually
supplied with a foil shield which can be easily tom, so plugs
must be wired with great care. If permanent wiring involves
‘fishing’ cables through conduits, the vinyl cable is easier to use
because it has less surface resistance, but the foil shield is
susceptible to splitting with undue stress.
Some thought should also be given to selecting the output
cables. When high power is demanded from the amplifier, the
impedance of the output cables becomes part of the loudspeaker
load. The desirably high damping factor of Hafler amplifiers,
which is an index of the amplifier’s ability to control the
speaker, is reduced by excessive cable impedance. Even more,
long runs of inadequate wiring waste power. The following
chart illustrates typical power dissipation with a 4 ohm load.
The lowest gauge, or heaviest practical wire size is always
preferred. When very long output cables are required, the use of
balanced 70 volt lines is recommended. The Hafler LX250
transformer can provide a 70.7 volt output from one channel of
the P505. The rated available power per channel is the
equivalent of its 8 ohm output of 255 watts. Predrilled holes in
the amplifier’s cover can accomodate two transformers. The
output voltage of a P505 operated in the monophonic bridged
mode is sufficient to drive a 70 volt line directly without the
need for a transformer.
Wire Gauge
Power Loss in Watts @ 4 ohms
(A.W.G.)
25’
cable
100’ cable
6
1.8
6.9
10
4.4
17.3
14
11.1
41.3
18
26.8
91.2
Monophonic Operation (Bridged Mode)
When very high power is desired, the P505 can be switched to
a single channel amplifier, capable of delivering more than 800
watts into an 8 ohm load. For this, the Stereo-Mono slide switch
is moved to Mono, and an input signal is applied to the Left
input jack only. The two output leads are connected only to the
Red terminals, and no connections are made to the black
terminals. The output must be ‘floating’ - that is, neither side
can be grounded, as both are ‘hot’ with respect to chassis
ground. The red terminals are identified ( + ) and ( - ) simply for
phasing reference. In this mode, the P505 can drive a 70 volt
distribution line directly. Sensitivity is adjusted by the Left
input level control.
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