HD 650 from a Galaxy S25: every 2 V dongle lands one decibel short
The HD 650 needs 2.24 V. A FiiO KA17 gives 1.993 V and reaches 109.0 dB against a 110 dB peak target; the cheaper KA13 stops at 104.7 dB. The only source here that clears it is a battery-powered DAC at 5.2 V.
Almost. The KA17 reaches 109.0 dB where the target is a 110 dB peak — a shortfall of 1 dB, which the calculator counts as «enough, no headroom» and which in practice means the volume control lives at maximum.
Sennheiser rates the HD 650 at 103 dB per 1 V into 300 Ω, so a 110 dB peak needs 2.24 V, 16.71 mW and 7.46 mA. Against that, the whole class of phone dongles clusters in one place: KA17 and Qudelix 5K at 2 V (1.993 V delivered, 109.0 dB), Hidizs S9 Pro Plus Martha and DragonFly Cobalt at 2.1 V (109.4 dB), TRUTHEAR SHIO at 2 V. The cheap end fails outright — the KA13 at 1.22 V reaches 104.7 dB, 5.3 dB short. The only source in this catalogue that takes an HD 650 to a genuine 110 dB peak from a phone is the Chord Mojo 2, which publishes 90 mW into 300 Ω as 5.2 V RMS: 117.3 dB, 7.3 dB spare, at eight times the price of the KA13.
Why they all sit at 2 V. Every bus-powered dongle in this catalogue lands between 1.22 and 2.1 V, and the only source that goes past that from a phone carries its own battery — although a battery is no guarantee either: the Qudelix 5K is battery-powered and still stops at 2 V. The gap between 2 V and the 2.24 V the HD 650 wants is the whole argument, and it is 1 dB wide. A 1 dB shortfall on peaks is not audible as distortion — it is audible as never being able to turn it up on a quiet recording.
The pearl: Samsung never publishes the port current. The specification sheet gives the port version — USB Type-C, USB 3.2 Gen 1 — and stops. The 4.5 W in this catalogue is derived from what the USB Type-C specification defines as Default for a single-lane USB 3.2 port, 900 mA at 5 V, so it is a floor rather than a measurement; the phone may advertise 1.5 A or 3 A and only a measurement would say. And the switch that actually stops most people is not power at all: USB host mode has to be enabled, and some carrier builds hide the setting.
What we cannot check. FiiO publishes <1 Ω for the KA17, which against 300 Ω is a ratio of 300 and safe. Qudelix, Hidizs and AudioQuest publish no output impedance at all for their 3.5 mm outputs, so for those three the 1/8 rule — 37.5 Ω or less for a 300 Ω headphone — cannot be verified in either direction. It matters more than usual on this headphone: the HD 650's impedance peak near 100 Hz means a high-impedance source adds bass instead of merely taking level away.