Hi all,
I’m an enthusiastic owner of QA401 and QA451.
I’m searching for the best settings for precision distortion measurements for evaluate the best bias setting of a Class AB amplifier.
This measurement is carried out on the critical first watt area of a Class AB amplifier.
All suggestions are appreciate.
Thanks in advance.
Other than determining theoretical bias settings using Ltspice I’ve not tried it the real world using the QA401. It’s something I’ll try when I next get the chance.
Personally I’d start with a short fft length so you get the responsiveness and lengthen as you hone it.
There is an article here on fft settings which may help.
Hi Antonix, I think the article SABristol is referencing might be below. In short, use the ability of the QA401’s sweep to learn the best settings for the measurements. When distortion is bad, SABristol is spot on that long FFTs just waste your time. As distortion levels drop, you’ll need larger FFTs to reduce the noise floor and see the harmonics. But the sweeps will help familiarize yourself with the tradeoffs to be made.
In short, if speed is important, start with a small FFT and increase it until you see the 2H, 3H fully emerge. And then go from there.
Hi @Antonix
I’d be interested is seeing your results posted on here.
Certainly when I modelled it, increasing the bias current reduced the distortion with a law of diminishing returns, so in practice my decision where to set it in practice was mainly driven by how hot it got when idling.