Verification Testing

This document defines a methodology for testing Pascal amplifiers. It is based on verification procedures and is intended to ensure repeatability, accuracy, and comparability across measurements.

01 Introduction

This document defines a methodology for testing Pascal amplifiers. It is based on verification procedures and is intended to ensure repeatability, accuracy, and comparability across measurements.

02 Test philosophy and measurement principles

Audio amplifier testing must separate different performance domains: power capability, distortion, noise, and frequency behavior. These are independent characteristics and must be measured under controlled and clearly defined conditions.

03 Recommended test equipment

Audio analyzer

Audio Precision APx555 / APx525
Prism dScope Series III
Rohde & Schwarz UPV

Load equipment

High-power resistive load bank (low inductance)
Dedicated amplifier test loads (e.g., NH Research, AE Techron)

Power source

Programmable AC source (e.g., Chroma, Keysight)
For power testing on high power amplifiers, use 3-phase connected power sources if needed and use an AC power source matched properly with the amplifier's power draw. Ensure facility breakers/fuses are not the limiting factor.

Additional equipment

True RMS multimeters
Oscilloscope
Thermal monitoring equipment

04 Test environment and setup

It is recommended to perform tests at ≤ 25 °C ambient temperature. The device under test (DUT) should be stabilized at idle temperature before measurements begin. Connections should use balanced cabling.

05 Load considerations and best practices

Load selection is critical for accurate amplifier testing.

Use non-inductive, high-power load banks designed for audio testing
Ensure adequate thermal dissipation
Avoid small discrete resistors which can introduce inductance and thermal drift

Using inappropriate loads can significantly affect THD measurements, as impedance behavior varies with frequency and temperature.

06 Output power testing

Output power is measured at 1 kHz using defined load impedances (e.g., 2 Ω, 2.7 Ω, 4 Ω, 8 Ω, and equivalent load resistance for constant voltage). Output power may also be tested during different durations; the standard burst duration is 100 ms.

Procedure

Apply a sine wave at 1 kHz and record power at the final cycle of the burst duration
Increase input signal gradually
Monitor output voltage continuously
Record power at the specified distortion threshold (e.g., 1%)

Measurement conditions

Single channel driven
All channels driven
Different mains voltages (e.g., 100 VAC, 120 VAC, 230 VAC)

07 Total harmonic distortion (THD+N)

The DUT inputs are connected to the balanced outputs of the audio analyzer with XLR cables. The DUT outputs are connected to the hot and cold terminals of the balanced inputs of the audio analyzer with XLR cables.

THD+N: The distortion is measured at 1.0 kHz with a 1-second average.
THD vs. frequency: The THD performance is measured with a continuous sweep from 20 Hz to 20 kHz, with a sweep time of 5 seconds.
THD FFT: The THD is measured with 64K and 10 averages.

The test is performed twice — once into open load at −3 dBFS and once into 4 Ω at 1/8th full power.

08 Frequency response, gain and linearity

Test of the DUT's frequency response and gain. The test must cover three measurements: frequency response, gain, and linearity. The test is performed twice — once into open load at −3 dBFS and once into 4 Ω at 1/8th full power.

Frequency response: Measured with a continuous sweep from 20 Hz to 20 kHz, with a sweep time of 5 seconds.
Gain: Measured at 1.0 kHz — may be derived from the frequency response measurement.
Linearity: Measured as the ± deviation in dB, referenced to 1.0 kHz — may be derived from the frequency response measurement.

09 Noise and idle performance

Noise measurements include:

RMS noise (A-weighted and unweighted)
FFT noise spectrum
Noise vs. time stability

Inputs should be shorted during measurement, and results should be averaged over time.

10 Crosstalk

The DUT input 1 is connected to the balanced output of the audio analyzer with XLR cables; input 2 is shorted at the connector. The DUT outputs are connected to the hot and cold terminals of the balanced input of the audio analyzer with XLR cables. Channel 1 is also connected to a 4 Ω load in parallel to the sense cables as a 4-point measurement.

Crosstalk: Measured at 1.0 kHz with a 1-second average, referenced to channel 1.
Crosstalk vs. frequency: Measured using a sweep from 20 Hz to 20 kHz, referenced to channel 1.

11 Output level (Vrms)

Vrms: The level is measured at 1 kHz. The level is increased and noted when 1% THD+N is reached.

12 Measurement filters and standards

All measurements should follow industry standards:

AES17 filtering for bandwidth limitation
Defined averaging times
Consistent FFT sizes and windowing
Controlled sweep durations

13 Common pitfalls and measurement errors

Using inductive or thermally unstable loads: Can introduce impedance variation and phase shift, leading to inaccurate power and THD measurements.
Improper grounding or cabling: May introduce hum or measurement errors.
Insufficient averaging: Leads to noisy data and reduced measurement repeatability.
Ignoring mains voltage influence on performance: Can cause over- or underestimation of output power and distortion due to supply sag or variation.
Testing at ≥ 40 °C ambient temperature: Can reduce output power and increase distortion due to thermal effects and activation of protection mechanisms.
Not using AES17 filtering: Can include out-of-band noise in measurements, resulting in artificially elevated THD+N and non-comparable results.

14 Conclusion

A structured and standardized approach to amplifier testing ensures reliable and repeatable results. Proper equipment selection and measurement methodology are essential for accurate characterization.