The QA 401 from QuantAsylum (superseded by a newer, improved QA 402 and now the QA403) has emerged as a popular, comprehensive audio test instrument in the DIY audio fraternity and the go to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.
really enjoy the KX2 amplifier that I build last year. Fantastic sound from a relatively small amplifier.
I have in the meantime build some VFA amplifiers with MOSFET’s and BJT output stages.
I have the QA401 (and an old RE201 audio analyser) and is sometime struggling with THD measurements. It seems that the load resistors (both wirewound and high power metal types) can have some voltage dependencies. Is this something you have expirence with? We are taking 0.0005% so it is really low distortion values.
Claus from Denmark
I’ve never had this problem. I use Arcol 200W non-inductive power resistors for the load.