Distortion: A Show and Tell Page 2

What's your percentage?

Distortion is commonly expressed in percentages. Calculating it is a complex mathematical operation (explained in depth here), but to put it very simply, 1% THD is -40 dB below the original signal, 3% THD is about -30 dB below, 10% THD is -20 dB below, and so on.

This tone demonstrates how different levels of distortion sound. (Better put on your headphones for this; lower-level distortion will probably be hard to hear on your computer speakers.) The first 3 seconds is an undistorted 400 Hz tone. The next 3 seconds is the same tone at 0.5% THD, which some people can hear and some can't. The next 3 seconds is at 1% THD, which most people can hear. The next is at 5%, which anybody but Ted Nugent can hear. The final 3 seconds is at 10%, which sounds so grating because it's your amplifier's last desperate plea for mercy. Now when you see an amplifier's power specified at 0.5% or 1% THD, you'll have some idea of the real-world implications.

While I was making the tone, I also monitored the result through my computer running TrueRTA spectrum analyzer software. This let me grab some screen shots so you can see how different levels of distortion appear on an analyzer.

As we'll discuss later in this article, distortion of a certain percentage doesn't always sound the same. It varies depending on the level of the different harmonics. I made the tone and graphs here by cutting the supply voltage on my Bottlehead Quickie preamp from 36 volts to 12 volts so it would distort easily. Because the Quickie is a single-ended tube preamp, it has more 2nd-order harmonic distortion than a typical transistor preamp, thus the 5% THD you hear in this tone may sound different from the 5% THD you'd get from a transistor preamp.

Figure 2 gives you an idea of what 0.2% THD looks like. You can see the 2nd-order distortion harmonic appearing at 800 Hz to the right of the fundamental tone at 400 Hz. Again, with a solid-state preamp, you'd probably see more 3rd-order harmonic (1,200 Hz).

Figure 3 shows the Quickie preamp at 1% THD. The 2nd-order distortion harmonic at 800 Hz is a little higher in level, and now it's joined by the 3rd harmonic at 1,200 Hz and the 4th harmonic at 1,800 Hz.

Figure 4 shows 5% THD. At this amount of distortion, all hell breaks loose. Even the 8th- and 9th-order distortion harmonics (at 3,520 and 3,960 Hz, respectively) are clearly visible, and the lower-order harmonics are getting closer to the level of the fundamental tone.