Previously (see here and here), we’ve looked at how unbalanced audio interfaces work, as well as how grounding-related noise can actually couple into the signal path.
Of course, under fortuitous conditions, audio systems may be acceptably quiet in spite of poor techniques.
But physics will ultimately rule and noises may later appear for no apparent reason – remember, noises that disappear by themselves also tend to re-appear by themselves!
Most systems consist of more than two pieces of interconnected equipment, so there are multiple signal interconnection cables.
When there’s a noise problem, figuring out which interface is the culprit can be a daunting and time-consuming task.
Troubleshooting with a logical, methodical approach can save both time and sanity.
Perhaps the most important aspect of troubleshooting is how one thinks about the problem:
Don’t fall into the trap of thinking something can’t be the problem just because you’ve always done it that way. Remember, things that “can’t go wrong” do!
Don’t start by changing things. Because many problems reveal themselves if we just gather enough clues, gather as much information as possible before you change anything.
Ask questions! My favorite troubleshooting guru, Bob Pease, suggests these basics: Did it ever work right?
What symptoms tell you it’s not working right? When did it start working badly or stop working? What other symptoms showed up just before, just after, or at the same time? (Reference 1)
Be alert to clues from the equipment itself. Operation of the equipment’s controls, along with some simple logic, can provide very valuable clues.
For example, if the noise is unaffected by the setting of a volume control or selector, logic dictates that it must be entering the signal path after that control.
Figure 1: Be sure to show all signal interconnecting cables, and note any pieces of equipment grounded by either a three-prong power plug or any other ground connection.
If the noise can be eliminated by turning the volume down or selecting another input, it must be entering the signal path before that control.
Write everything down — less than perfect memory can waste a lot of time!
Sketch a block diagram of the system! (Figure 1) Show all signal interconnecting cables, including digital and RF, and indicate their approximate length.
Stereo pairs generally can be indicated with a single line. Note pieces of equipment grounded by either a three-prong power plug or any other ground connection such as for cable TV or a DSS dish.
Mark any balanced inputs or outputs. (We’ll talk about proper transitions between balanced and unbalanced in the future.)
As a general rule, and unless clues suggest otherwise, always begin testing interfaces at the inputs to the power amplifiers (or powered loudspeakers) and sequentially test interfaces backward toward the signal sources.