Testing, Testing, 1, 2, 3
Page 1, 2, 3, 4, 5, 6, 7, 8, 9
 

FIG. 8: By sending a square wave through a piece of audio gear and reading its output on an oscilloscope, you can see whether the circuit is ringing. If so, a ripple appears in the flat part of the waveform.


Each waveform type is used to test different conditions. Sounding like a pure flute tone, the sine wave is very musical. Because of this waveform’s purity, you can easily hear harmonic overtones added by a piece of audio gear. For instance, if you feed a sine wave into a small amplifier that drives a speaker cabinet and then sweep the frequency in the 30 to 300 Hz range, you can listen for cabinet buzzes, handle rattles, and voice-coil rubbing (a light buzz that’s harmonically related to the input frequency). However, be careful not to output high-level sine waves at horn frequencies (above 1 kHz or so), because many horn drivers and tweeters will quickly burn out with a few tens of watts of continuous tone.

Square waves combine low-frequency information (the flat part of the waveform) and high-frequency information (the rising vertical part of the waveform), so they can be used to evaluate circuit stability at both ends of the audio spectrum. Square waves are most useful when teamed with an oscilloscope, a device that displays AC-signal waveforms (among other things). Hardware models tend to be expensive, but virtual oscilloscopes are a viable alternative. For example, a shareware program that can use any sound card for input is available on the Web at http://polly.phys.msu.su/~zeld/oscill.html. You also can find a commercial product with a hardware interface that connects to the PC’s parallel port and uses standard oscilloscope probes at www.designnotes.com/pcs64i.htm.

FIG. 9: Stop ringing in a piece of classic transformer-coupled, low-impedance audio gear connected to a high-impedance mixer input by placing a resistor across the output terminals. This works well with balanced outputs (above) and unbalanced outputs (below).

To use an oscilloscope, feed a test signal to the input of the device you want to evaluate and connect its output to the scope’s input. The most obvious thing you can see with square waves on an oscilloscope is circuit ringing, a ripple in the flat part of the waveform (see Fig. 8). This is caused by unterminated energy bouncing back and forth in a circuit.

One common thing to watch out for is unwanted ringing in transformer-coupled circuits. This can happen when you connect the output of a classic piece of gear, such as a UREI compressor, to a return on a modern mixing console that does not exhibit the expected input impedance of 600 ohm. To solve this problem, simply add a termination resistor across the terminal-strip output of the classic unit (see Fig. 9). The resistor should be the nearest value that is higher than the unit’s transformer impedance; if the transformer’s impedance is 600 ohm, the resistor should be set to 680 ohm. Adding this resistor can dampen the ringing, which improves the sound significantly.

A square wave is simply a symmetrical version of the pulse waveform, spending equal time at the low and high voltage values. Asymmetrical pulse waves are also useful; when combined with an oscilloscope, the waveforms can be used to determine if a particular circuit is inverting the polarity. If the input signal spends the majority of its time at the high voltage and the output signal spends most of its time at the low voltage, the device is reversing the polarity.


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Reprinted with permission from Magazine, March, 2001
2000, Intertec Publishing, A Primedia Company All Rights Reserved



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