AP - The Recognized Standard In Audio Test

Power amplifiers range greatly in size and form, from headphone drivers to large professional power amps for stadiums and concert halls that can put out thousands of watts. The testing methods are similar, however, with most power amps being tested for the six basic audio measurements (download TN-104 "Introduction to the Six Basic Audio Measurements"). Depending on performance and speed requirements, one of Audio Precision's APx Series, 2700 Series, or ATS-2 will offer the best solution.

The APx585 has the ability to drive and measure 8 channels simultaneously, a huge advantage for testing today's multichannel amplifiers. The APx586 measures 16 channels, while the APx525 provides two channel testing at a lower cost. One unique feature of the APx Series is continuous sweep, which makes 14 key measurements across all input channels in just 7 seconds. Another feature is a sine-wave burst signal for compliance testing of automotive stereo amplifiers to the CEA-2006 standard. The burst signal allows testing of amplifiers that may not be able to sustain a continuous full-power output. An additional feature of the APx Series is the ability to measure DC only, useful for adjusting DC offset and for detecting harmful levels of DC voltage on the outputs.

For true state-of-the-art amplifiers with noise floors even lower than the APx's -108 typical dB, the only choice is AP's flagship 2700 Series. With a dedicated analog generator and analyzer (in addition to full digital analysis capability), it has the highest performance of any analyzer in the world.

The APx Series, 2700 Series, and ATS-2 all include regulated frequency sweeps, for automatically finding maximum power output across multiple frequencies.

For testing class D digital switching amplifiers, you'll also need an outboard filter unit, such as the AUX-0100 (for the APx585/586) or the AUX-0025 (for the 2700 Series, ATS-2, or APx525). These filters prevent extraneous out-of-band high frequency energy from overloading the inputs of the analyzer and affecting the test results. The filter is also necessary to block the DC voltage on the output of class D amplifiers from the inputs of the APx585/586. For more details, see AP Solutions: Testing Class D Amplifiers.

Common Challenges

• Use caution when running an amplifier at full output. While most pro amps can take the heat, lesser amps may shut down, go into limiting, or fail.
• If the amplifier has just been repaired, is suspected of being defective, or has not been used for a long time, bring the AC mains voltage up slowly with a variac while monitoring current. Some new digital amplifiers can not be brought up this way, so check with the manufacturer.
• Maximum power output and distortion may drift as an amplifier gets hot under a continuous full output signal. You'll get different test results, depending on how long the signal is applied. This may indicate a defect, inadequate cooling, a design deficiency, or simply that the amp isn't designed for that kind of stress. The APx Measurement Recorder can be used to trace measurements over time and quantify drift over a specified period.

Tips for Optimum Testing

• Use non-inductive dummy load resistors of sufficient wattage (such as Vishay/Dale NH-250) for standard specification tests. Putting two 8 Ohm resistors in parallel to make 4 Ohms, or four 8 Ohm resistors in parallel to make 2 Ohms, is a convenient way to decrease the load resistance while inversely increasing power handling capability.
• Before proceeding, check for excessive DC on the outputs of linear power amplifiers, which may indicate a failed output transistor. Class D amplifiers have common mode DC on their outputs at all times.