Using a FFT spectrum analyzer to test audio amplifiers

This application note looks at the use of the Pico ADC-216 pc based oscilloscope / spectrum analyzer for audio amplifier spectrum analysis.

See also out our other audio related pages :

Using the ADC216 to test power amplifiers

Continuing our audio experiments with the ADC216 spectrum analyser we decided to test two audio power amplifiers. We chose a 'budget' model from Technics and a high quality unit from Quad. One channel of the ADC216 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 1kHz tone from the signal generator.

Firstly, we connected the Technics 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 -60dB as shown below.
The window showing measurements and harmonics is a program that takes data from PicoScope (using DDE) to automate audio measurements. If you would like a copy, it can be downloaded free of charge.

We then connected the Quad unit and again adjusted the input until the amplifier was supplying 25W 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.

We then adjusted the input amplitude until the amplifier was supplying only 1.5W into 8 Ohms. When this is done, the effects due to crossover distortion can be more easily observed.
Here, on the Quad unit, the third harmonic has increased slightly but is still more than 100dB below the fundamental.

We again connected the Technics unit and adjusted for the same 1.5W into 8 Ohms. This time the third harmonic has decreased in amplitude. This would tend to indicate that the third harmonic distortion is being mostly generated by the large signal non-linearity of the unit. Consequently, the crossover distortion contribution is being swamped. See below.

Frequency response is another important consideration when testing audio equipment. It is usual to specify a response 'flat' within a limit over the whole 20Hz to 20kHz audio spectrum, where the limit is usually -3dB. We tested this by using PicoScope, in spectrum peak detect mode, and increasing the signal generator frequency from DC up to 40kHz. We chose 40kHz 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 Technics amplifier below.

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