Using an FFT spectrum analyzer to test audio amplifiers

Please note this application note is a few years old. The content is still valid but the ADC-216 used in the experiments has now been replaced by the PicoScope 4262.

Continuing our audio experiments with the ADC-216 spectrum analyzer we decided to test two audio power amplifiers. We chose a ‘budget’ model from Kenwood and a high-quality unit from Quad. One channel of the ADC-216 was connected via a x10 scope probe to the power amplifier speaker output connectors.

For all of the following tests, we used a low distortion signal generator from BlackStar. The PicoScope trace below shows a pure 1 kHz tone from the signal generator (Figure 1).

Firstly, we connected the Kenwood unit and attached an 8 ohm resistive load. The input signal was adjusted until the amplifier was supplying 25 watts into the load. We initially found very large amounts of distortion but these slowly reduced as the amplifier heatsinks warmed up. After a few minutes, the distortion had settled at about –60 dB as shown below (Figure 2).

We then connected the Quad unit and again adjusted the input until the amplifier was supplying 25 W into our attached 8 ohm load. As the trace below shows, the Quad unit is much improved. The distortion was immediately stable without any warm up time and did not appear to change as the unit slowly warmed. The most significant harmonic is the second and the overall noise is also much lower (Figure 3).

We then adjusted the input amplitude until the amplifier was supplying only 1.5 W into 8 ohms. When this is done, the effects due to crossover distortion can be more easily observed.

The results for the Quad unit are shown below (Figure 4).

audio amplifier

Figure 1

audio amplifier

Figure 2

audio amplifier

Figure 3

audio amplifier

Figure 4

We again connected the Kenwood unit and adjusted for the same 1.5 W into 8 ohms (Figure 5).

Frequency response is another important consideration when testing audio equipment. It is usual to specify a response ‘flat’ within a limit over the whole 20 Hz to 20 kHz audio spectrum, where the limit is usually –3 dB. We tested this by using PicoScope, in spectrum peak detect mode, and increasing the signal generator frequency from DC up to 40 kHz. We chose 40 kHz since many audiophiles are interested in the frequency response way beyond the human hearing range.

The responses from the two amplifiers were so similar that we have shown only the response of the Kenwood amplifier below (Figure 6).

audio amplifier

Figure 5

audio amplifier

Figure 6


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