Handbook for Sound Engineers

1306 Chapter 34

Both transformer-coupled and active-balanced
equipment can provide excellent hum and noise rejec-
tion. As a general rule, however, high-quality trans-
former-coupled inputs and outputs will outperform all
but the best active-balanced designs in terms of hum
and noise rejection. In addition, transformers offer the
benefit of protection against stray dc.

The choice should be made based on a careful exam-
ination of the device’s specifications, especially the
noise rejection performance of the input stages. In addi-
tion, remember that lower-quality transformers and
simple-circuit, low-performance active balanced inputs
and outputs will probably not provide the expected level
of hum- and noise-rejection or of audio performance.

34.5.1.9.12 Using Proper Shielding to Reduce Noise
Pickup

Proper grounding helps prevent pickup of noise that is
transmitted magnetically and noise that is coupled
through a common impedance. Magnetically transmit-
ted noise most often comes from motors or, more com-
monly in audio, from large ac power transformers
(either building transformers or the power transformers
in a power amplifier or other piece of audio equipment).
Proper shielding, on the other hand, helps prevent
pickup of noise that is transmitted capacitively. Capaci-
tively transmitted noise may be in the form of radio
waves from a radio station or citizens band radio, or it
may be in the form of static from certain types of
motors or lighting dimmers. Noise from lighting dim-
mers may also come through the ac lines.

Proper shielding, except in severe noise situations, is
straightforward. Use high-quality shielded cables on all
microphones and on all line-level equipment, and, if at
all possible, install the electronics in a metal equipment
rack (preferably steel since this also provides some
protection against magnetically coupled noise). Some
very low-cost audio cables including guitar cables have
poor quality shields. Watch for these potential sources
of noise pickup, Fig. 34-70.

It is seldom necessary to use shielded cable for loud-
speakers, since they operate at a very high level and a
very low impedance. The noise picked up by a loud-
speaker cable is actually at the same level as the noise
picked up by a microphone cable. However, because the
loudspeaker operates at a much higher level than the
microphone, the SNR is vastly better, and the noise is
seldom a problem.

34.5.1.9.13 Reducing Noise Pickup from ac Lines

Some types of noise, notably noise from lighting dim-

mers, enter audio equipment from the ac power lines.

There are four ways to reduce this problem (see Chapter

32). A licensed electrician must perform this trouble-

shooting and any needed modifications.

1. Install filters on the dimmer circuits (filters at the
   audio equipment won’t help as much and probably
   will cost a lot more).


2. Make sure the dimmer circuits are properly loaded.
   In other words, if the dimmers are rated for
   1500 W loads, make sure they have 1500 W worth
   of lighting connected to them. (Or add a suitable
   dummy load to simulate a full-rated load on the
   dimmer.) The reason for doing this is that the noise
   filters (if there are any) will only work properly
   when the dimmer is loaded properly (this is an
   example of impedance matching).


3. Be sure the lighting circuits are properly grounded
   (improper grounding can increase noise levels at
   the source as well as at the audio equipment).


4. Use a different ac circuit.

34.5.1.9.14 More Tips on Reducing Noise Pickup

Rack Mount the Equipment. Rack mounting, espe-

cially when the rack mount rails are made of metal, con-

nects the chassis of all the equipment into a unitized

shield. Perhaps more important, rack mounting allows

the use of shorter connecting cables and keeps them

closer together. When rack mounting large power

amplifiers, however, do not place sensitive, low-level

equipment right next to them in the rack. The power

transformer in a large power amplifier can produce a

large alternating magnetic field that can induce hum in

low-level equipment.

Figure 34-70. Poor-quality shielded cable. Courtesy

Yamaha International Corp.

Noise entering center

conductor through shield

Dielectric

Center

conductor

Loosely wrapped shield

Noise entering center

conductor through shield