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FEATURED WHITEPAPER: Measuring Switch-mode Power Amplifiers by Bruce Hofer.

Class D, or switch-mode power amplifiers, are rapidly replacing traditional linear power amplifiers in the marketplace. The advantages are many, including lower weight, smaller size, less cost, higher efficiency, and less heat. They do, however, present special testing challenges - if they are tested using the same techniques used for testing linear power amplifiers, the results can be highly inaccurate.

Class D amplifiers use a switching process that adds fast-rising edges at the switching frequency (typically 250 kHz to 750 kHz) to the audio output signal, presenting a difficult signal for audio analysis. Filtering out this high frequency energy without impacting measurement accuracy in the audio band requires a carefully designed filter with high quality parts. AP's two channel AUX-0025 and eight channel AUX-0100 are precision external filters made for this specific application. Inserting the filter into the signal path essentially eliminates this switching energy before it reaches the analyzer and makes accurate measurements possible.

In addition, class D amplifiers may contain elevated noise in the region above 20 kHz. Normally, a 20 kHz low-pass filter is used to limit noise measurements to the audible frequency range. However, the 3 to 6 pole low pass filter used in most audio analyzers may not be sharp enough to eliminate all of this extra noise, so the measurement accuracy may be impacted. A better filter to use is a low-pass filter conforming to the AES-17 standard. Originally intended for measuring D/A converters, this sharp low-pass filter is also ideal for measuring class D amplifiers. For Audio Precision's 2700 Series, the optional S-AES17 filter is available. All of the built-in digital filters present in APx analyzers already provide a sharp cut-off, so this issue is no longer a concern.

Traditional distortion measurements may not catch high frequency distortion problems, which can be present in class D amplifiers. That's because the usual THD+N measurement, using a low-pass cut-off frequency of 20 kHz, can't measure 3rd harmonics of fundamentals over about 6 kHz. The usual SMPTE IMD measurement is also insensitive to high frequency distortion, as it uses a fundamental of only 7 kHz. A twin-tone IMD measurement, using two high frequency fundamentals at 18 kHz and 20 kHz, solves this problem and reveals high frequency distortion problems. Both the 2700 Series and the APx Series include twin-tone DFD IMD testing capability.

Common Challenges

  • High frequency switching artifacts can cause distortion on an analyzer's inputs. A high quality external filter, such as AP's two channel AUX-0025 or eight channel AUX-0100, is essential to reduce high frequency switching noise that can affect measurement accuracy.
  • Out-of-band noise can affect noise measurements when using a standard low-pass filter. A sharp low-pass filter will keep significant out-of-band noise from affecting the results. The filters on the APx Series already have the necessary sharp slope. The optional S-AES17 filter is needed for the 2700 Series.
  • High frequency distortion is missed with traditional THD+N and SMPTE IMD tests. The twin-tone (DFD) IMD test, available on the APx and 2700 Series, will reveal high frequency distortion missed by the other tests.

Tips for Optimum Testing

  • Don't use a variac for bringing up the AC mains power. Some switching power amplifiers won't function properly or turn on at low voltage settings, so check with the manufacturer beforehand.
  • Determine if you need a filter. The APx Series has a high tolerance for out of band noise, however a filter may still be required. To find out if you need a filter, the best way is to compare results with and without a filter in line between the amp and the analyzer.
  • Some class-D amplifiers (and some linear amplifiers) have considerable common-mode DC voltage on the outputs. When testing these amps with an APx585 Family analyzer, the AUX-0100 Switching Amplifier Measurement Filter should always be used, as it contains coupling capacitors to block the DC voltage from reaching the analyzer. On the other hand, the APx525 Family, the 2700 Series, and the ATS-2 analyzers all have internal coupling capacitors that can be selected by choosing AC Input Coupling in the control or measurement software.

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