The Dummy Test
Simple test adapters or “dummies” allow each interface to be tested in the system without any additional instruments.
Temporarily placing the dummies at strategic locations in the interface reveals not only the exact location of the noise coupling but also precise information about the nature of the coupling.
The tests can specifically identify:
Common-impedance coupling in unbalanced cables (the most frequent problem by far).
Induction of noise into the interconnect cable itself by nearby magnetic or electric fields. Or…
Common-impedance coupling inside poorly designed equipment (also called the “pin 1 problem”).
Test dummies for unbalanced interfaces are made from standard connectors wired as shown in Figure 2.
Because they don’t pass signal, make sure dummies are clearly marked so they aren’t accidentally left in a system.
Figure 2: Be smart by using test “dummies” for unbalanced interfaces.
Also be very careful not to damage speakers or ears as cables are disconnected and re-connected!
The surest way to avoid this problem is to turn off the power amplifier(s) before reconfiguring cables for each test step.
Each signal interface is tested using the following four-step procedure: