Working backwards

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In any good detective novel we start with the crime scene first and work our way backwards to find the culprit. It's no different when discovering sources of hum. As I did with the garage door remote control problem, we want to isolate the issue down to what works, then branch out to find what doesn't. Just about every time I am asked to help fix a hum problem the person asking for help has started at the wrong end. It's the natural path to take when you have hum: start with where the most likely culprit will be and try and fix it. But if that doesn't work and you've exhausted the likely suspects, it's time to start over and do the opposite. What can we say for certain that will not hum in our system? I would suggest the loudspeakers, sitting by themselves without connection to an amplifier would qualify. Yes, I know, it might sound silly. We all know a loudspeaker sitting in the room will make no noise at all, but this idea is important. We'll be using the process of elimination but we'll be doing it in reverse order of what people normally do, and for good reason. Let's look to see what typically is done. Imagine our system is a simple one. Our component chain consists of a CD player and turntable for sources feeding a preamplifier, power amplifier and a pair of speakers. We have hum coming from the loudspeakers. Using the process of elimination we typically begin by blaming one of the sources and eliminating it to see if the hum goes away. Perhaps it does, perhaps it doesn't. All we've shown is that through a 'shotgun' approach we may or may not strike gold, eliminating the hum. But what if the preamp itself is the culprit? Or the interconnects between preamp and power amp? You can't just eliminate those in your shotgun approach, because the system will no longer function. No, a better way to diagnose hum is to start at the loudspeakers and work our way back. Let's try that on for size, remembering the cardinal rule of troubleshooting: the most difficult task is finding the problem. Solving it is secondary. Loudspeakers sitting disconnected make no noise. Check. Now, connect the speakers cables and power amplifier to them, but make SURE there's nothing else connected to the amplifier's inputs. And I mean nothing. Turn the amplifier on. Hum? No hum? It is at this very point we will stop for today to let this sink in. It's critical you understand the process and the rigor of the steps we take and why. What we have just shown is this small collection of equipment either does or does not hum. It's divorced from the rest of the system and isolated in a way that makes it easy to figure out what's going on. For example, let's imagine there is hum. What does that say to us? Several important things: there cannot be a ground loop, it is unlikely it is induced, it helps us point the finger at a specific component. A power amplifier sitting on the floor or shelf with no inputs attached and only speakers and their connecting cables tying things together points an accusing finger at the power amplifier itself. It's about as close to a smoking gun as you're going to get. Why? Let's go into some depth tomorrow.
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Paul McGowan

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