Possible USB Audio Quality Regression on Recent iOS/iPadOS Versions with External DACs

Dear Apple Audio Engineering Team,

I am a long-time Apple user and a serious music enthusiast who values high-quality audio reproduction.

I would like to report a possible audio quality regression affecting external USB DACs on recent versions of iOS and iPadOS.

Over

the past few days, I conducted several listening comparisons using the

same external DAC and headphones across different devices. The results

were surprising and concerning.

1. Audio Equipment Used

• DAC/Headphone Amplifier: Resonessence Labs Herus+
• Headphones: beyerdynamic DT 1990 PRO and Bang & Olufsen H9 3rd Generation
• Music Service: Apple Music
• Current iPhone: iPhone 16 Pro running iOS 27
• Current iPad: Recent iPad mini running iPadOS 27
• Reference Source: Windows PC running Apple Music

Both headphones were tested using wired connections through the same Herus+ DAC/headphone amplifier.

2. Previous Experience with iOS 17

Around

the iOS 17 period, I used an iPhone 11 connected to the Herus+ through

Apple’s original Lightning to USB Camera Adapter and a USB-A to USB-B

cable.

At that time, the sound quality was excellent.

In

fact, I preferred using my iPhone over my Windows computer because the

iPhone delivered a more enjoyable and emotionally engaging listening

experience.

This was one of the reasons I trusted Apple devices as high-quality digital audio sources.

3. Current Experience with iOS 27

With

my current iPhone 16 Pro and iPad mini, both connected to the same

Herus+ DAC, the listening experience is noticeably different.

The sound seems less full-bodied, with weaker perceived impact and less musical engagement.

When

using the DT 1990 PRO, increasing the volume can make the treble

uncomfortable before I achieve the desired sense of musical impact.

The B&O H9 also sounds less engaging than I remember from my previous iPhone setup.

For clarity, Dolby Atmos and Sound Check are disabled on my iPhone.

4. Comparison with Windows Apple Music

I then connected the same Herus+ DAC and headphones to my Windows computer.

The difference was immediately noticeable.

Even with the Apple Music equalizer disabled, Windows provided a significantly more satisfying listening experience.

With the DT 1990 PRO, I experienced:

• Excellent detail retrieval even at relatively low listening volumes.
• Fuller perceived sound and stronger musical impact.
• Clear instrument separation.
• Comfortable treble without the harshness I experienced on my iPhone.
• A much more engaging overall listening experience.

I repeated the comparison using my B&O H9 3rd Generation headphones.

The H9 also regained the sound quality and musical enjoyment I remembered from years ago.

Although

I personally preferred the DT 1990 PRO for its superior perceived

detail and separation, both headphones sounded substantially more

enjoyable through the Windows setup.

This suggests that my experience is not limited to one particular headphone.

On Windows, the Apple Music equalizer, system audio enhancements, and spatial audio were disabled during the comparisons.

5. Additional Observation

My

AirPods Pro 2 continue to provide a satisfying listening experience,

and I feel their sound performance has improved over time.

However, my experience with an external USB DAC and wired headphones has become less satisfactory.

I

understand that AirPods use a different audio processing architecture,

and I am not suggesting that Apple intentionally reduces the quality of

third-party audio products.

Nevertheless,

I am concerned about a possible compatibility or audio output issue

affecting certain external USB DAC configurations.

6. Request for Technical Investigation

I

respectfully ask Apple’s audio engineering team to investigate whether

recent iOS or iPadOS updates have changed any aspect of external USB

audio output, including:

1. USB Audio Class compatibility and device initialization.
2. Digital audio output levels and gain management.
3. PCM audio processing or sample-rate handling.
4. USB power management affecting bus-powered DACs.
5. Potential differences in external DAC behavior between Lightning-based and USB-C-based Apple devices.

I understand that subjective listening comparisons alone cannot establish the technical cause.

However,

the consistency of my experience across two different headphones,

combined with my previous positive experience using the same DAC with

older Apple devices, makes me believe this issue deserves investigation.

My hope is simply to enjoy the same high-quality wired audio experience on my iPhone that I previously enjoyed.

Thank you for your time and consideration.

Sincerely,

An Apple Music subscriber and high-fidelity audio enthusiast from Taiwan