# Very low distortion measurements

**URL:** <https://forum.quantasylum.com/t/very-low-distortion-measurements/246>\
**Category:** QA401\
**Created:** [January 30, 2021, 12:02pm UTC](https://forum.quantasylum.com/t/very-low-distortion-measurements/246 "2021-01-30T12:02:27Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Antonix](https://avatars.discourse-cdn.com/v4/letter/a/919ad9/32.png) [@Antonix](https://forum.quantasylum.com/u/Antonix)\
**Post date:** [January 30, 2021, 12:02pm UTC](https://forum.quantasylum.com/t/very-low-distortion-measurements/246/1 "2021-01-30T12:02:27Z")

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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.

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**Author:** ![SABristol](https://yyz2.discourse-cdn.com/flex032/user_avatar/forum.quantasylum.com/sabristol/32/380_2.png) [@SABristol](https://forum.quantasylum.com/u/SABristol)\
**Post date:** [February 1, 2021, 8:00am UTC](https://forum.quantasylum.com/t/very-low-distortion-measurements/246/2 "2021-02-01T08:00:37Z")

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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](https://forum.quantasylum.com/t/fft-sizes-with-the-qa401-qa450/124) on fft settings which may help.

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**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://forum.quantasylum.com/u/matt)\
**Post date:** [February 1, 2021, 10:55pm UTC](https://forum.quantasylum.com/t/very-low-distortion-measurements/246/3 "2021-02-01T22:55:47Z")

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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.

> [@FFT Sizes with the QA401 & QA450](https://forum.quantasylum.com/t/fft-sizes-with-the-qa401-qa450/124):
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> The QA450 is designed for really fast burst testing of amplifiers. Because of its size, the QA450 can handle an average power of 5-10 watts, and a peak power of 200W or so. The QA450 is NOT the tool you want if your aim is a load that can soak up lots of power while you tune in real time. The manual goes into a lot of detail on the power levels and time durations for testing. A lot of engineers like to use really long FFTs for their measurements. But beyond a certain point, there’s not much to …

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**Author:** ![SABristol](https://yyz2.discourse-cdn.com/flex032/user_avatar/forum.quantasylum.com/sabristol/32/380_2.png) [@SABristol](https://forum.quantasylum.com/u/SABristol)\
**Post date:** [February 6, 2021, 11:45am UTC](https://forum.quantasylum.com/t/very-low-distortion-measurements/246/4 "2021-02-06T11:45:14Z")

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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.