Sorry if this has has been discussed already, but I ran into something that might be of interest here.
Context: applying an EQAPO filter set on the 3.5mm headphones out of my Windows 11 Acer laptop, to equalize a pair of IEMs.
Windows uses audio processing objects (
APOs) to provide software based digital signal processing for it's audio streams.
First Surprise: Some hidden non defeatable APO made a mess of it,
even with "all effects off' in Windows. Naughty Windows! My music system and work PCs didn't show this issue, so it may be caused by an APO for the Dell Realtek soundcard. To verify the measurement, I retested with RMAA's spectrum analyzer and the tones at a lower level (to make sure it wasn't intermediate clipping or the CAudioLimiter). Same outcome.
To try and fix this, I then installed (free) Equalizer APO (EAPO). This brilliant software supports a trouble shooting mode that turns off all "original" (stock) system APOs:
On my laptop, this setting at 3) is grayed out.
It is also unchecked. So initally I thought all was good.
Thing is, EQAPO requires Audio enhancements to be switched on.
(Image taken from
here for the English UI.)
However, switching between Device Default Effects and Off resulted in a very noticeable sound difference. Like operating a loudness button - really nothing subtle. As I had all EQAPO filters disengaged, and the EQAPO configuration settings were as outlined above, I did not expect to hear any sound change.
Turns out that the difference was caused by Acer TrueHarmony. I found it in the Realtek Audio Console.
Long story short, I now have this switched off as well, and hear no more difference between the two Windows Audio enhancements settings.
I am not sure what this all means. It appears that despite the findings shared by the OP there may still be things going on outside of what is covered by their recommendations in the first post. Sadly I don't have the gear (and frankly the skills) to establish for sure that I killed all additional processing. But hearing no differences between enhancements on and off does give me some peace of mind.
In any case I'm glad to eventually have found this TrueHarmony switch and hope this info is of help to some.