Using an Audio Amplifier

The surprises don't stop. The next toy that comes out of the PQ bag is an audio amplifier. What is fascinating to watch is people's reaction to this, and how this turns to amazement when they realize how a fault has been escaping them for years and how easy it would have been found with such a cheap instrument. "Basics" they are heard to say. But that's what I keep hammering on about! People are no longer taught the basics!

We're dealing with 50Hz, aren't we? Ah! An opportune time to remind a few that the audio bandwidth is known as 20Hz to 20kHz. That means mains frequencies fall well into the accepted audio spectrum, as well as dealing with anything up to the 400th harmonic! (333rd in 60Hz countries). That's quite some amount of data to process!

Why does one want to listen to mains hum? It's the bane of a Hi-Fi fanatic's life while trying to listen to a serious piece of classical music. As with music, the same note can be played by two different instruments but each will have its own unique sound. This comes from the different harmonic contents of each, together with non-harmonically related sounds (e.g. wind).

Putting a current clamp through a set of headphones will clearly indicate a varying load e.g. when a set of lights turns on. Further to this, intermittent faults, loose connections, or even the odd type of signalling equipment can be heard through the mains. Such faults are very quickly traced if the current waveform is listened to through the headphones at the time the cabling is moved. The typical sound is a lot of "scratching" and becomes louder when the current clamp is clipped to the offending circuit.

The audio amplifier's main role in PQ investigations is to help locate conditions that are related to one another, e.g. a mains borne signalling system causing a motor controller to stop or shudder. Such 'cause and effect' situations are easily tracked when listening to the current flow.

Its uses don't stop there. Another very useful application is tracing and/or determining the extent of ground loop currents, especially in data signal cables. These currents can easily exist between two pieces of equipment that are powered from separate power sources. A prime example is a main-frame computer in one location, and a computer terminal in another. In most instances the computer terminal would be fed from the nearest available power source with absolutely no regard given or attention paid to the voltages that may exist on the ground.

By simply using a small current clamp (10-100A) fed into an audio amplifier, the clamp then clipped around the signal cable (in its entirety), one can hear if there are any currents (not just limited to mains frequencies either) existing on the signal cables. Disconnecting the mains earth (with the equipment powered off!) will quickly point to whether or not the currents are induced through differences in ground voltages (if the equipment cannot be disconnected, then listen if there is any difference between each of the cables connected to the equipment).

Such audio amplifiers may be found in test instrumentation catalogues under "telecommunications" test equipment. Ensure you get one that has an external input (preferably with sockets that match your kit) and with a volume control.

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