Yamaha P-2200 - Manual (Page 35)

grounding and shielding, and by the use of balanced,
twisted pair cables.
EMI: EMI (electro-magnetic interference) usually comes
from power transformers (either in a sound system, or
a building's electrical supply), motors, or cables carrying
large amounts of current. EMI usually shows up in a
sound system as a hum or buzz. Twisted pair, balanced
lines effectively reject most EMI. Whenever possible
avoid placing sensitive equipment near motors or trans-
formers (keep input transformers several inches away
from the P-2200's power transformer), and use twisted
pair balanced lines.
Careful grounding and shielding can minimize
externally caused hum and noise. These techniques,
in essence, are to use balanced lines, use shielded cables,
and eliminate ground loops.
Use of Balanced Lines
Balanced lines are discussed in the Appendix. This
paragraph summarizes their advantages over unbalanced
lines for noise rejection. Balanced lines reject RF and
electro-magnetic interference by phase cancellation
between the conductors; twisted conductors aid the
rejection. Balanced lines help avoid grounding problems
because the shield does not carry any signal current
(this is explained further in following paragraphs). Also,
any noise currents entering the shield cannot directly
enter the signal path because the shield is not part of
the signal path (in contrast to an unbalanced line, where
the shield is the signal "return" wire).
Use of Shields
An effective shield also aids noise rejection. The
shield effectiveness of many types of cable is specified
in percentage of density. A close braided shield can be
highly effective, but may be more expensive and is
harder to work with than foil shields. Foil shields, in
most cases, are more suitable for permanent cable con-
nections since they are easier to prepare. Many guitar
cables, especially the coiled type, have poor shields and
are the source of much of the hum common in guitar/
amplifier systems. A poor quality cable may also exhibit
"microphonics," a condition where movement of the
cable can cause noise in the sound system.
Fig. 54 - Poor Quality Shielded Cable
Metal equipment racks, and metal electrical conduits
are also effective shields against RF noise. However, few
shields offer really effective protection against electro-
magnetic interference (EMI). Solid iron conduit and,
possibly to a lesser extent, steel conduits and racks do
offer some protection. Fortunately, however, most EMI
interference can be effectively avoided by keeping
sensitive wiring and equipment away from large power
transformers, electric motors, etc., and by using
balanced, twisted pair cabling whenever possible.
Ground loops, are a common source of noise pickup.
Figure 55 shows the way noise enters a system through a
ground loop. One common source of ground loops in a
sound system is the double grounding path between
equipment caused by AC grounding the chassis of each
piece of equipment, and then making a second ground
connection between the two chassis via the signal cable
shield. Figure 53, Page SIX 13 shows this problem. Figure
56 shows a method of avoiding this type of ground loop
in a system by using what is known as a "telescoping
shield" connection where each piece of equipment is
AC grounded for safety, but a potential ground loop is
avoided by connecting the signal shield at one
end of the cable only. Traditionally, the shield is con-
nected at the "far" end of the cable, so that shield
currents "drain" in the same direction as the signals
flow. Figure 58 shows a similar connection using un-
balanced lines. The AC grounds on each device have
been "lifted" so that the only ground connection
between two pieces of equipment is the shield of the
signal cable. Since, in an unbalanced cable, the shield
carries signal current, it cannot be disconnected. More-
over, in this type of unbalanced grounding scheme, if
the shield becomes disconnected inadvertently at some
point along the signal path, some pieces of equipment
will not have an AC ground, so safety is compromised.
Fig. 55 - Noise Entering System through Ground Loop
Fig. 56 - Telescoping Shield
Fig. 57 - Feeding the Input of the P-2200 from a Balanced
Source without a Balancing (Bridging or Isolation) Transformer.
Unbalancing the source at the P-2200's input (CHAN A
Diagram) will usually result in lower hum levels than unbalancing
the source at the source (CHAN B Diagram).
been "lifted" so that the only ground connection