external DAC suddenly sounds quieter or “compressed” on iOS 18 and 26

[forumlurker]

TL;DR
On iOS 18/19/26 (tested on iPhone 17), my external DAC (Fiio BRT7, usb DAC mode) was recognized as Headphones. This triggered Apple’s Headphone Safety path → lower max volume + audible “flattening” of peaks.

Fix: disable “Treat USB Devices as Headphones” in Settings → Sounds & Haptics → Headphone Safety.

Subjectively, this restores full dynamics — I’d love to hear if anyone with proper measurement gear can confirm.

clearly a user error

...

After upgrading to a new iPhone on iOS 26 (beta), I noticed something strange when connecting my external USB DAC:

• overall output volume seemed lower compared to iOS 17
• dynamic peaks felt noticeably reduced, as if soft limiting/compression was applied
• it didn’t sound “bit-perfect” anymore, more like some system gain management was in the chain, Telegraph Road (Dire Straits) less dramatic than in my memory.
• and most surprisingly: 100% volume on iOS was less loud than 95%.

At first I suspected Apple had changed the USB audio pipeline starting with iOS 18/19. But after digging through settings, I found the actual culprit:

• In Settings → Sounds & Haptics → Headphone Safety there’s a toggle called “Treat USB Devices as Headphones.”
• When this is enabled, iOS classifies the DAC as a headphone and applies Headphone Safety features (like Reduce Loud Sounds and gain limiting).
• Once I disabled the toggle, playback immediately sounded “normal” again — no more flattened peaks, full output level, and dynamics returned.

I only went by ear — no lab gear, no bit tests. To me, the difference is obvious, but I’d love to know if anyone can verify/falsify this.

Would be great if someone can confirm (or falsify) whether iOS actually alters the digital signal when this toggle is on.

Keywords / Search terms
“iOS 18 external DAC volume low”
“iOS 19 USB DAC compression limiting”
“iPhone USB Audio Accessories Headphone Safety"
“iOS Treat USB devices as headphones toggle”
“iOS DAC bit perfect problem fix”

 

[ampguy]

I think the non US versions may have lower output on the audio devices.