The QA 401 from QuantAsylum (superseded by a newer, improved QA 402 and now the QA403) has emerged as a popular, comprehensive audio test instrument in the DIY audio fraternity and the go to instrument in many cases for commercial production testing in the audio industry. It combines a first rate DAC and ADC with a simple user friendly PC interface to realize a fully integrated measurement and characterization system.
In a good set-up, line level distortion measurements of under 3ppm (1 kHz) are readily achievable while the instrument’s noise floor is close to -155 dBV, allowing accurate noise characterization in things like phono and microphone amplifiers. Combine this with a host of built in test scripts and its easy to see why this $450 instrument was such a hit.
However, to get the best results from the QA 401 and it’s more recent upgraded siblings the QA402 and QA403, the user must spend some effort on the measurement set up. I’ve seen many test plots with excessive mains noise, thermal noise or subpar distortion results that have nothing to do with the QA401’s performance, and everything to do with the measurement set-up.
The short presentation below explains how noise arises in a typical measurement set-up, and then shows the reader how to set their measurement system up to exploit the full capability of the QA 401.