# QA403 and Tube Amp setup

**URL:** <https://forum.quantasylum.com/t/qa403-and-tube-amp-setup/2067>\
**Category:** QA40x\
**Created:** [October 14, 2025, 7:42pm UTC](https://forum.quantasylum.com/t/qa403-and-tube-amp-setup/2067 "2025-10-14T19:42:09Z")\
**Posts on this page:** 4\
**Page:** 2

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**Author:** ![MarkT](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@MarkT](https://forum.quantasylum.com/u/MarkT)\
**Post date:** [December 26, 2025, 1:32pm UTC](https://forum.quantasylum.com/t/qa403-and-tube-amp-setup/2067/21 "2025-12-26T13:32:15Z")

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The transformer will saturate when the magnetizing flux reaches a threshold - the magnetizing flux goes up with lower frequencies and higher voltages, since the magnetising current sees the inductance of the core. Larger cored transformers can handle lower frequencies and/or higher powers. The sudden rise of distortion at the low-end is thus magnetic saturation in the transformer core.

The baseline increase of distortion with power is likely to be circuit-related as its independent of frequency. That might indeed indicate class A to AB transition somewhere above 1W, but I think you’d need to plot in more detail at lower powers to get a better feel for this.

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**Author:** ![LAPC123](https://avatars.discourse-cdn.com/v4/letter/l/6de8d8/32.png) [@LAPC123](https://forum.quantasylum.com/u/LAPC123)\
**Post date:** [December 26, 2025, 10:00pm UTC](https://forum.quantasylum.com/t/qa403-and-tube-amp-setup/2067/22 "2025-12-26T22:00:52Z")

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Thanks all for the input.

What type of plot should I need for more details on low frequencies impact?

I anticipated more THD at lower end frequency with increases in power but didn’t expect it has a such steep increases as shown.

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**Author:** ![MarkT](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@MarkT](https://forum.quantasylum.com/u/MarkT)\
**Post date:** [January 2, 2026, 12:49pm UTC](https://forum.quantasylum.com/t/qa403-and-tube-amp-setup/2067/23 "2026-01-02T12:49:30Z")

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Core saturation is a limiting phenomenon, similar to voltage clipping, so it will rapidly jump to 10% distortion or more as the waveform is truncated.

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**Author:** ![LAPC123](https://avatars.discourse-cdn.com/v4/letter/l/6de8d8/32.png) [@LAPC123](https://forum.quantasylum.com/u/LAPC123)\
**Post date:** [January 2, 2026, 5:16pm UTC](https://forum.quantasylum.com/t/qa403-and-tube-amp-setup/2067/24 "2026-01-02T17:16:06Z")

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If I rewire it for Triode mode in pure Class A, does the amp still suffer the same/similar THD on low freq end as shown for the UL mode?

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