Step 1: Unplug the cable from the input of Box B and plug in only the dummy. (Figure 3) Is the output quiet?
No: the problem is either in Box B or further downstream.
Yes: go to the next step.
Step 2: Leaving the dummy in place at the input of Box B, plug the cable into the dummy. (Figure 4) Is the output quiet?
No: Box B has an internal “pin 1 problem.” The “hummer test” can confirm this. (Reference 2)
Yes: go to the next step.
Step 3: Remove the dummy and plug the cable into the input of Box B. Unplug the other end of the cable from Box A and plug it into the dummy. Be sure the dummy doesn’t touch anything conductive. (Figure 5) Is the output quiet?
No: noise is being induced in the cable. Re-route it to avoid interfering fields.
Yes: go to the next step.
Figures 3 through 6.
Step 4: Leaving the dummy in place on the cable, plug the dummy into the output of Box A. (Figure 6) Is the output quiet?
No: the problem is common impedance coupling. Install an isolator in the signal path.
Yes: the noise must be coming from the output of Box A. Perform the test sequence at the next upstream interface (inputs to Box A).
Now that we know what the problem is and which interface is afflicted, how do we correct it without degrading audio performance or creating a safety hazard?
We’ll tackle that next time in “Pesky Ground Loop Problems.”
Reference 1: Robert A. Pease, Troubleshooting Analog Circuits, Butterworth-Heinemann, 1991.
Reference 2: Jensen Transformers, Build a “Hummer” to Help Find “Pin 1” Ground Problems, Application Schematic AS032, download at jensentransformers.com.
Additional installments of “Clear Path” by Bill Whitlock are available here.
Bill Whitlock has served as president of Jensen Transformers for more than 20 years and is recognized as one of the foremost technical writers in professional audio.
More than semantics, equalization has very real practical consequences.
