Measure Audio Harmonics The Easy Way
Article By Larry Lisle
From Sound Practices Issue 12

  Designing, building and tweaking audio amplifiers can be a lot of fun. It can also be very frustrating unless you measure the effect of design changes or parts substitutions. In this article I'll show you an easy way to measure one of the most important characteristics of an amplifier, distortion caused by harmonics in the second, third, fourth and higher orders in the middle frequency range.

Every audio amplifier adds some distortion to the signal it amplifies. One type of distortion that's especially important is harmonic distortion. For example, if the input signal is 500 Hz, the output may also contain signals at multiples of 500 Hz, such as 1000, 1500, 2000, 2500 Hz and so on in addition to the fundamental frequency of 500 Hz. The relative strength of these harmonics to the fundamental and to each other can completely change the sound of the music amplified by the system.

There are many ways of measuring harmonics. One popular method cancels the fundamental with a null circuit and gives a value for what's left as total harmonic distortion or THD. This is useful but doesn't tell anything about the individual harmonics.

Other methods employ tunable filters and are tuned from harmonic to harmonic with the value of each being recorded. These devices are popularly called wave analyzers and tend to be very expensive.

My solution is to use a single, very sharp audio filter intended for ham radio operators for code reception. The better ones will give selectivity of 60 dB down at an octave away from the fundamental. Set the filter to a particular frequency such as 1000 Hz and tune the signal generator to submultiples of that frequency. The voltage passing through the filter will therefore be a harmonic of the input.

This method will measure distortion very accurately in the middle frequency range and give a good picture of the 'sound' of the amplifier. Of course the signal generator has to be relatively free of harmonics and its output and the voltage gain of the amp under test have to be uniform over the middle of the audio spectrum, but these are easily checked.

The equipment needed can often be found used at electronic flea markets. Older tube type gear is often dirt cheap. The signal generator shown cost $25 and the audio voltmeter cost $5. These two items are basic if you're going to do much work with audio. The prices are not unusual. The Autek audio filter cost $25. Some audio filters are tunable over a limited range but since the gain isn't constant they should be set at one frequency as described in the procedure outlined below. The only home-built item needed is the 50,000-Ohm potentiometer on a board which is used to set the meter to a convenient value, such as 1.00 Volt, and keep the audio filter from running out of head room.

To measure harmonics I use the following procedures, which will take longer to read than it will to make the actual measurements:

1. Measure the input voltage to the amplifier when used in its normal way.

2. Connect a dummy load with non-inductive resistors to the output of the amplifier.

3. Connect the outside contacts of the potentiometer across the dummy load.

4. Connect the input of the audio filter to the wiper contact and one end contact of the potentiometer.

5. Connect the voltmeter to the output of the audio filter.

6. Connect the signal generator to the input of the amplifier. Set the output voltage of the generator to a value determined in 1 above.

7. Set the frequency of the generator to the frequency of the audio filter, for example 1000 Hz. Rock the frequency back and forth slightly to make sure it's at the peak of the filter response curve.

8. Use the potentiometer to set the output of the filter to exactly 1.00-Volt or some other convenient value.

9. Without touching the audio filter or potentiometer tune the signal generator to 500 Hz and again rock the frequency slightly to be sure you're on the peak. The output voltage will give the amount of second harmonic distortion. For example, if the reading is .003-volt, .003 divided by 1.00 equals 3/10 of one-percent.

10. Now tune the generator to 333 Hz and take a reading. This is the third harmonic and may be greater or less than the second, depending on the design of the amplifier.

11. Other readings may be taken at 250 Hz for the fourth harmonic, 200 Hz for the fifth and so on, but the values for the second and third are the most significant.

Notice that the calibration of the signal generator and voltmeter doesn't need to be highly accurate in absolute terms, which makes this an ideal method for used of even home-brew equipment.

With this method you'll really be able to see and hear the effects as you tweak your amp towards your ideal sound.