Testing Audio Amplifiers Using an FFT Spectrum Analyzer (Part 1)

Let's start the experiment of ADC216 spectrum analyzer. This time we test two audio power amplifiers. One is Kenwood's economical model, and the other is Quad's high-quality model. One channel of ADC216 is connected to the speaker output connector of the power amplifier through an X10 probe.

In the following test, we used BlackStar's low distortion signal generator to generate the signal. The figure below shows the spectrum of a pure 1KHz signal generated by the signal generator.

    

 First, we connected the Kenwood amplifier and an 8 ohm resistor load. We adjusted the input signal until the amplifier was delivering 25W to the load. We initially saw a lot of distortion, but as the amplifier warmed up, the distortion slowly decreased. After a few minutes, the distortion stabilized at about -60dB, as shown in the figure below.

                                                                        

We then connected the Quad amp and adjusted the input until the amp was delivering 25W into an 8 ohm load. As shown below, the Quad amp performed significantly better. No warm-up time was required, the distortion stabilized immediately, and did not change as the unit heated up. The largest harmonic was the second harmonic, and the overall noise was much lower.