Your Phone Is the Weakest Link: DAC Dongles, DAPs and Output Impedance

Spotify Premium now streams lossless FLAC. Spotify started rolling it out in September 2025, at up to 24-bit/44.1 kHz, at no extra cost. Many people then play that stream through a phone’s Bluetooth or a cheap adapter. The file is no longer the weak link. The playback chain is.

This part covers that chain: what turns a file into sound, what to buy, and the one number that decides whether an IEM sounds as it was tuned.

What a DAC and an amp do

Music files are numbers. Earphones need a changing voltage. Two stages sit between them:

Stage	Job	What goes wrong when it is poor
DAC (digital-to-analogue converter)	Turns the samples into an analogue signal	Noise, distortion; rarely audible in modern chips
Amplifier	Gives that signal enough voltage and current to drive the earphone	Hiss, clipping at volume, tuning changes from high output impedance

With IEMs, the amplifier stage usually matters more than the DAC chip. Modern DAC chips measure well below audible limits. Output impedance, noise and power vary much more.

Phones without jacks

Most flagship phones dropped the 3.5 mm jack years ago. You now have two options: Bluetooth, which moves the DAC and amp into the earphones, or a USB-C dongle.

Apple’s USB-C to 3.5 mm adapter is a better starting point than its price suggests. Reference Audio Analyzer measured about 0.3 Ω output impedance and about 0.5 V RMS maximum output. That is low impedance and low power: fine for most IEMs, short of headroom for hard-to-drive headphones.

Dongle DACs

A dongle DAC is a small DAC and amp in a cable or stick. It plugs into the phone’s USB-C port and powers itself from the phone.

Option	Price	Outputs	Notes
Apple USB-C adapter	Low (Apple list price varies by region)	3.5 mm	About 0.3 Ω, about 0.5 V RMS (measured)
FiiO KA11	About $30	3.5 mm	Under 0.7 Ω output impedance (FiiO spec at 32 Ω)
Moondrop Dawn Pro	About $60	3.5 mm and 4.4 mm balanced	Output impedance not checked

Dongles drain the phone battery faster. Some phones also resample all audio to 48 kHz before it reaches the dongle. Neither matters much for listening; both are worth knowing.

Dedicated players (DAPs)

A digital audio player does one job. Its advantages are practical more than audible:

• Battery: music playback no longer drains your phone.
• Storage: a local FLAC library, no streaming or data needed.
• Power and outputs: more voltage, often a 4.4 mm balanced jack.
• Focus: no notifications in the middle of an album.

Sony’s current high-end Walkmans show the range. The NW-ZX707 launched at a US suggested price of $899.99. The NW-WM1AM2 sells for about $1,400. An iPod Video 5.5G running Rockbox sits at the other end: a Wolfson WM8758 DAC, 16-bit/44.1 kHz, and a few ohms of output impedance. Later parts of this series cover that iPod in detail.

The 1/8 output-impedance rule

This is the number that matters most for IEMs.

Every source has an output impedance. It sits in series with the earphone, so the earphone and the source form a voltage divider. If the earphone’s impedance changes across frequency, as multi-driver IEMs do, the divider changes the tuning. The part on IEM drivers explains why.

The rule of thumb, popularised by the audio engineer and blogger NwAvGuy:

Keep the source’s output impedance below one eighth of the earphone’s impedance.

Earphone impedance	Maximum source output impedance
32 Ω	4 Ω
16 Ω	2 Ω
8 Ω	1 Ω

The 1/8 ratio keeps the tuning change under about 1 dB, close to the smallest difference most people hear. It is a rule of thumb, not a law. A single dynamic driver with a flat impedance curve tolerates far more. A multi-BA set with a curve that dips to a few ohms needs a source well under 1 Ω to sound as tuned.

Use the lowest point on the IEM’s impedance curve, not the single figure on the box, when you apply it.

Power and sensitivity

IEMs need very little power. A high-sensitivity IEM reaches loud levels with a fraction of a milliwatt. For IEMs, two things matter more than raw power:

• Noise floor. A sensitive IEM makes a noisy source hiss between tracks.
• Volume steps. Some sources jump too far between steps at low volume.

Power matters for big over-ear headphones and some planar IEMs. If your IEM gets loud enough with volume to spare, more power will not improve it.

Balanced 4.4 mm

A balanced output drives each earpiece with two amplifier channels instead of one, and does not share a ground between left and right. The 4.4 mm Pentaconn plug is the common connector on DAPs and dongles.

In practice, balanced outputs often give more voltage and slightly better channel separation. On many devices the balanced side also has a different output impedance, sometimes higher. Check the spec for the output you will actually use.

Balanced is useful, not essential. Many IEMs ship with swappable 3.5 mm and 4.4 mm plugs, which makes trying both cheap.

Hi-res and MQA, honestly

CD quality, 16-bit/44.1 kHz, already covers the range of human hearing; part 1 explains why, and why hi-res helps in the studio rather than at playback. MQA, the “folded” hi-res format once used by Tidal, added licensing and complexity without a clear audible benefit, and Tidal has since moved to plain FLAC. Spotify’s 24-bit/44.1 kHz lossless is enough. Mastering, the IEM and the fit matter more than the number after the slash.

Bluetooth codecs, briefly

Bluetooth compresses audio again, even when the file is lossless; part 1 shows what that means for AirPods Pro and the Sony WH-1000XM4. Codecs differ in how much data they send:

Codec	Typical bitrate	Notes
SBC	Varies	The baseline every device supports
AAC	About 256 kbps	Apple’s default; good on iPhones
aptX HD	576 kbps	Qualcomm; Android only
aptX Adaptive	About 279–420 kbps, variable	Scales with signal conditions
LDAC	330, 660 or 990 kbps	Sony; 990 kbps drops out easily in busy radio areas

Good AAC or aptX is hard to tell from wired in a noisy street. For quiet, careful listening, wired still removes a variable. The iPod has no Bluetooth at all, which settles that question for it.

What to buy

• Phone only, IEMs under about $200: a low-impedance dongle such as the Apple adapter or the FiiO KA11. That fixes the weakest link for the least money.
• Multi-BA or tribrid IEMs: confirm the source is well under 1 Ω on the output you use.
• You want a separate device and an offline library: a DAP. The next parts cover the iPod as a purist player and building a FLAC library you own.

Sources

• Spotify lossless launch, Stereophile
• Spotify 24-bit lossless, audioXpress
• Apple USB-C adapter measurements, Reference Audio Analyzer
• FiiO KA11, Apos Audio
• Moondrop Dawn Pro, Frieve audio review
• The 1/8 rule, audioreviews.org
• Sony Walkman prices, SlashGear
• Bluetooth codecs, Audioengine