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USB audio interface Edirol UA-5 * was used as the soundcard in the measurement. The left channel output of the soundcard was connected to the amplifier, and amplifier's output was connected to the left channel input of the soundcard as a test signal. The right channel output was directly connected to the right channel input as a reference signal. The characteristic of the amplifier can be measured by comparing with the reference signal.
* In the picture above, Edirol UA-3 instead of UA-5 is shown for illustration. But we don't recommend UA-3, because we have heard many problems in using this device.
The input and output waveforms of 1kHz sine tone are observed by the oscilloscope as below. In this configuration, a blue curve (left- channel) shows the test signal (amplifier's output), and a pink curve (right-channel) shows the reference signal (input to the amplifier). As we can see, two waveforms are almost the same except that the amplitude of the amplifier's output is slightly smaller than input.
When the input to the amplifier is overloaded, top and bottom of the waveform are clipped, generating distortion. To avoid such a situation, most amplifiers are equipped with the overload protection circuit. It is also important to check the proper output level of the signal generator itself, so that the measurement is performed accurately. See SIMPLE CHECK BEFORE A MEASUREMENT for detail.