When to Use a Pre-analyzer Filter

Created on 2008-07-11 12:53:00

Q: When would I need a pre-analyzer filter?

A: In conventional audio analyzers, signal filtering is implemented late in the measurement path, following several analyzer gain stages. This approach provides repeatable and accurate results for devices whose noise spectrum is somewhat constant or diminishes slightly with frequency, typical of a wide range of audio devices. Audio Precision's analog analyzers and DSP-implemented analyzers follow this model.

However, the output signal from some devices can exhibit high levels of energy at high frequencies beyond the audio band. This is characteristic of digital-to-analog converters (DACs) and switch-mode or Class D amplifiers outputs. This high-level out-of-band energy can overload analyzer input stages and produce inaccurate measurements.

To reduce this out-of-band energy before the signal is applied to the analyzer input circuits, we recommend using a low-pass pre-analyzer filter.

The S-AES17 pre-analyzer filter for DAC testing
The Audio Engineering Society has published its recommendations for the measurement of digital audio equipment in the document AES17. Recognizing the problem in measuring signals in the presence of out-of-band noise, AES17 recommends the use of a pre-analyzer filter when measuring DAC outputs.

The Audio Precision S-AES17 pre-analyzer filter option satisfies the recommendation in AES17. The filter is comprised of two components: a low-pass filter module inserted before the analyzer gain stages, and a "brick-wall" low-pass filter inserted later in the measurement path.

S-AES17 is a factory-installed internal option for analog-equipped System Two and 2700 Series instruments. An S-AES17 kit is available for field installation.

The AUX-0025 filter for switch-mode amplifier testing
The Audio Precision AUX-0025 is a two-channel low-pass pre-analyzer filter mounted in a 1U standard rack mount chassis. AUX-0025 can be used with any Audio Precision analyzer that has analog inputs, including System One, System Two, the 2700 Series and the ATS-2.

The AUX-0025 has been carefully designed to reduce switch-mode (Class D and other) amplifier out-of-band noise before the signal is applied to the analyzer inputs. The filters in the AUX-0025 use custom-wound inductors that are optimized for extremely low distortion at the levels and impedances typical of amplifier outputs.