Handbook for Sound Engineers

1206 Chapter 32

STEP 4: Leaving the dummy in place on the cable, plug
the dummy into the output of Box A as shown below.

i Output quiet?

No—The problem is common-impedance coupling

(see Section 32.4.4). Install a ground isolator at

the input of Box B.

Ye—The noise is coming from (or through) the

output of Box A. Perform the same test

sequence on the cable(s) connecting Box A to

upstream devices.

32.6.2.2.2 For Balanced Interfaces

STEP 1: Unplug the cable from the input of Box B and
plug in only the dummy (switch open or NORM) as
shown below.

i Output quiet?

No—The problem is either in Box B or farther

downstream.

Yes—Go to next step.

STEP 2: Leaving the dummy in place at the input of
Box B, plug the cable into the dummy (switch open or
NORM) as shown below.

iOutput quiet?

N—Box B has a Pin 1 problem (see hummer test,

Section 32.4.2, to confirm this).

Yes—Go to next step.

STEP 3: Remove the dummy and plug the cable directly
into the input of Box B. Unplug the other end of the
cable from the output of Box A and plug it into the
dummy (switch open or NORM) as shown below. Do
not plug the dummy into Box A or let it touch anything
conductive.

i Output quiet?

No—Noise is being induced in the cable itself by

an electric or magnetic field. Check the cable for

an open shield connection, reroute the cable to

avoid the interfering field, or replace the cable

with a starquad type (see Sections 32.2.5 and

32.4.3).

Yes—Go to next step.

STEP 4: Leaving the dummy in place on the cable, plug

the dummy (switch open or NORM) into the output of

Box A as shown below.

i Output quiet?

No—The problem is shield-current-induced noise

(see Section 32.4.3). Replace the cable with a

different type (without a drain wire) or take

steps to reduce current in the shield.

Yes—Go to next step.

STEP 5: Leave the dummy and cable as for step 4, but

move the dummy switch to the CMRR (closed) position.

i Output quiet?

No—The problem is likely inadequate

common-mode rejection of the input stage of

Box B. This test is based on the IEC common-

mode rejection test but uses the actual

common-mode voltage present in the system.

The nominal 10ȍ imbalance may not simulate

the actual imbalance at the output of Box A, but

the test will reveal input stages whose CMRR is

sensitive to source imbalances. Most often,

adding a transformer-based ground isolator at

the input of Box B will cure the problem.

Yes—The noise must be coming from (or through)

the output of Box A. Perform the same test

sequence on the cable(s) connecting Box A to

upstream devices.

Output

Box A Box B

Cable Input

From

upstream

devices

To

downstream

devices

Output

Box A Box B

Cable Input

From upstream

devices

Todownstream

devices

Output

Box A Box B

Cable Input

From

upstreamdevices Todownstream

devices

Output

Box A Box B

Input

From

upstream

devices

To

downstream

devices

Output

Box A Box B

Input

From

upstream

devices

To

downstream

devices

Cable