Yamaha P-2200, P-2200/2201, P-2201 cable can cause noise in the sound system.

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

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"

the

same

direction

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"

that

the only ground connection