if higher quality transformers were used. Thus,
"economy" transformers, may actually cost more in
the long run than higher quality professional types.
For an existing system with lower quality 70-volt
transformers, a capacitor in series with the output of
the P-2200 can limit the current at low frequencies
(see Page SEVEN 6), and thereby avoid the possibility of
constant protection circuitry operation on the
P-2200, or damage to the 70-volt transformers from
excessive output power from the P-2200.
Fig. 52 - Impedance of Poor Quality 70-Volt Speaker
Transformer (Connected to
8
Speaker, Tapped for "5 Watts,"
looking into Primary).
ACTIONS OF THE P-2200 PROTECTION CIRCUITS
The P-2200 has several features that contribute to the
protection of the amplifier and its loudspeaker load:
Fuses
The AC line fuses protect the P-2200 from excessive
AC line voltage and, in the unlikely event of an internal
failure, the AC line fuses protect the amplifier from
severe damage. Always replace blown fuses with the
same size and type. If the fuses blow consistently, the
P-2200 should be checked by a qualified technician.
Grounding
The third wire on the AC line cord is a "ground" wire.
This wire connects the chassis of the P-2200 to AC
ground for safety. Do not defeat this safety feature
unless other methods have been employed to ensure a
good earth ground.
Thermal Protection
There is a thermal fuse, located inside the P-2200's
power transformer, that shuts down the AC power to
the P-2200 if the temperature of the transformer wind-
ings reaches 130° Centigrade. A thermal warning light,
on the front panel, turns on when the P-2200's heat
sink temperature reaches 100° Centigrade. Special heat
compensating circuits in the P-2200 insure that the
amplifier will perform properly within its operating
temperature limits.
Overload Protection
The P-2200's overload protection circuits limit the
maximum power available to drive any load. The effect
of these circuits is to smoothly limit the power to loads
below 2.5 ohms. The overload protection circuit action
is virtually inaudible, even when driving difficult, multi-
speaker loads. Figure 4, Page FOUR 1 and Figure 15,
Page FOUR 3 graph the power output of the P-2200 for
varying load impedances.
Transients and DC Protection
The P-2200 displays virtually no turn-off transient,
and the turn-on transient is minimal. A DC voltage at
the input will not be amplified (Figure 27, Page
FOUR 4), thus protecting speaker loads against damage
from DC at the output of the P-2200.
GROUNDING AND SHIELDING
Definitions
Ground: A general term, used in various ways through-
out the audio industry. It can mean the same as "com-
mon," "earth," "chassis" or "return."
Earth: A connection made to the actual soil or dirt.
Also a connection made to a cold water pipe or any
other device that ultimately enters the soil (and that can
provide a very low impedance path to the soil).
Common: The "return" wire of an audio pair; any
point where several such return wires connect with each
other. There can be "signal commons," "DC power
supply commons" or "AC power supply common"
(neutral). A common wire may or may not be con-
nected to ground or earth. Similarly, the AC power
supply ground may or may not be connected to the
audio system common or to earth.
Shield: A metallic shell around a cable, amplifier, or
other device that helps prevent the entrance of un-
wanted interference.
Grounding: The process of careful connection of
common, shield, ground, and earth connections to
avoid unwanted hum and noise.
Ground Loop: If a common or return signal can travel
from one point to another via two or more paths, the
resulting circular path is called a "ground loop."
Figure 53 shows two possible ground loops in an
audio system.
Fig. 53 - Two possible Ground Loops in an Audio System.
RFI: RFI (radio frequency interference) comes from
any number of sources, including radio stations, CB
radios, SCR (electronic) light dimmers, neon lights
and others, RFI may show up in a sound system as a
radio program, as a hum or buzz, or other noise. RFI
often enters a sound system at a low level preamplifier
stage. Many RFI problems can be cured by careful
