# QA451B issue with external loads

**URL:** <https://forum.quantasylum.com/t/qa451b-issue-with-external-loads/2171>\
**Category:** QA45x\
**Created:** [January 26, 2026, 2:56pm UTC](https://forum.quantasylum.com/t/qa451b-issue-with-external-loads/2171 "2026-01-26T14:56:44Z")\
**Posts on this page:** 1\
**Page:** 2

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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 26, 2026, 10:41pm UTC](https://forum.quantasylum.com/t/qa451b-issue-with-external-loads/2171/21 "2026-02-26T22:41:08Z")

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> [@Mudjock](#):
>
> The article touted the QA451 with an external load as a way to get 12 dB of attenuation and the high pass filter and be able to test higher power amps than would be normally possible going straight into the 451. This wound up working for the Behringer NX1000 because it has a balanced output topology, but as your later spice modeling showed, it doesn’t work the same way with a high power, single ended output amp (like the Samson here).

Ahh, yes, OK I get it now. Noted, thanks.

> I can use the center point between the resistors to get 6 dB of attenuation if I need to.

Yes, just be mindful of the bal/unbal limitations you might run into. The issue should be lessened becuase of the low value you have on the lower leg. But run it in SPICE if you plan to use unbalanced.

The amp dynamic plots are super informative and interesting. Is the 10mS increase in gain in the last plot the supply rail ramping up and disturbing the loop??? Or, asked another way, what test would stress a class H amp the most?

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