Total harmonic distortion (THD) measurements are one of the most commonly quoted in audio. Contrary to belief in some circles, these can be very useful if performed properly, and reveal much about the overall performance of an amplifier.
There are a number of ways to measure distortion, none of which is perfect. Probably the best is a spectrum analyser, which shows the individual harmonics and their amplitudes. These are too expensive for the likes of you and me (well, me, anyway) and the next best thing is featured here.
There are other methods as well, one of which is to subtract the output of an amplifier from the input (with appropriate scaling). When the two signals are exactly equal and opposite they are cancelled out – any signal left is distortion created in the amplifier. This method seems easy, but is not, because there are phase shifts within the amp that can be very difficult to compensate for exactly, and the final accuracy of tuning the parameters – amplitude and phase – must be just as great as with this circuit for a meaningful result.
The standard tool for measuring THD is a notch filter. This is tuned to reject the fundamental frequency, and any signal that gets through is a combination of the amplifier’s noise (including any hum) and the distortion. The distortion shows up as a signal that is harmonically related to the signal fed into the amp, but is not the fundamental. Harmonics occur at double, triple, quadruple (etc) the input frequency. These are referred to as 2nd, 3rd, 4th (etc) harmonics, and are subdivided into odd and even. Even harmonics (2nd, 4th, etc) are claimed to sound better than odd (3rd, 5th, etc), but in reality we don’t want any of them.
The filter of Figure 1 is ‘normalised’ to 1uF and 1k Ohm, giving a frequency of 159Hz. The resistor and capacitor ratios are extraordinarily critical if a deep notch is to be obtained, and this is essential for distortion measurement. This notch filter is called a Twin-T, and works by phase cancellation of the input signal. When the phase shift is exactly +90° and -90° in the two sections, the tuned frequency is completely cancelled, leaving only those signals that are not tuned out. This residual signal represents total harmonic distortion + noise.
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