When measuring the power output of multichannel amplifiers according to the EIA/CEA-490-A standard, it is necessary to reduce the level of the non-measured channels by 9 dB (sec 6.1). For example, when measuring channel 2 of an 8-channel receiver, channels 1 and 3–8 are reduced by 9 dB.
The APx500 software has a Maximum Output measurement for measuring power amplifier output levels. This measurement regulates the generator output level until a target THD+N value is measured at the analyzer input of the selected channel. All the channels, however, are driven at the same output level.
In order to drive the non-measured channels 9 dB lower than the measured one, we instead use the Auto Gen Level feature in Reference Levels. Auto Gen Level also provides generator regulation, but adds the ability for us to load a wave file as the generator signal source. The wave file has a 1 kHz tone on one channel at full level, and a 1 kHz tone on the other channel at –9 dB. Since Reference Levels doesn’t behave like a standard APx500 measurement, we follow it with a Level and Gain measurement so that we can capture the measured output level into a report.
Accompanying this article are two APx project files that are already configured using the method explained above. The APx526 project requires an APx526 analyzer and one Audio Precision SWR-2755M Output Switcher or SWR-2755U Unbalanced Switcher (configured for output). The other project is for an APx585 or 586 and does not require a switcher. The projects are designed as a starting point for your own APx multichannel power amplifier measurement projects. You can enhance them with additional features, like delay steps for amplifier preheating. For full compliance with all the details of EIA/CEA-490-A, please consult the standard.
APx sample project for measuring the power output of a multichannel amplifier according to EIA/CEA-490-A, section 6.1, where non-measured channels are attenuated by 9 dB. A related KB article includes an explanation of the measurement.