Headphone amps demystified — a measurement-based shootout
- https://www.youtube.com/watch?v=v5TrcO210To
- Original title: Stop plugging your headphones into your audio interface!! Headphone amps demystified.
A mastering engineer runs objective, measurement-based tests on headphone amplifiers — from a $4,000 Benchmark HPA4 down to the built-in headphone jack of a MacBook Pro — to cut through hi-fi marketing hype and identify what actually matters for professional studio monitoring. He argues three metrics count: low output impedance (which prevents an amp from coloring headphone frequency response by up to 2–3 dB), a precisely recallable calibrated volume level (which cheap potentiometer-based interfaces and mixers can't provide), and adequate output voltage. His counterintuitive conclusion: typical audio interfaces and console headphone outs are unfit for critical monitoring, the Benchmark is the only unit technically perfect on every metric, the budget FOSI Audio ZH3 ($199) is outstanding value, and — with caveats — the built-in headphone jack on an Apple MacBook Pro or Mac Mini objectively outperforms much professional gear.
Setup
Units compared: Benchmark HPA4 (~€4,000, loan), Austrian Audio Full Score One (~£1,500, loan), FOSI Audio ZH3 ($199, free review unit, also a DAC), Focusrite Clarett 8Pre and a Behringer interface, TC Electronic BMC2 monitor controller, plus two "mystery" amps revealed at the end. Headphones for real-world tests: Austrian Audio Composer (~£2,500), OLLO Audio X1 (~€600), Sennheiser HD 660S (~150 ohm). The framing: high-end audio is full of snake oil and marketing, so test purely on measurable facts rather than subjective adjectives.
Output impedance is the headline metric
The three dedicated amps measured effectively zero output impedance; the Focusrite and Behringer interfaces had significant impedance (Behringer worst); the TC BMC2 monitor controller surprisingly measured ~80 ohm.
Impedance mismatch colors the sound
Frequency-response sweeps showed good low-impedance amps produce near-identical curves, but the Behringer and BMC2 shifted response by 2–3 dB (especially bass/low-mids), altering tonality. This is not the amp's own frequency response (all are ruler-flat 20 Hz–20 kHz) but the impedance interacting with the headphone drivers. He considers 2–3 dB a dealbreaker for consistent studio work. Oddly, the BMC2 barely affected the low-impedance OLLO X1 — which he couldn't explain.
Recallable calibrated level is essential
Interfaces and mixers with a plain potentiometer can't return precisely to a reference level and are disqualified for studio monitoring. The Benchmark uses relays for exact 0.5 dB steps (hence its weight); the FOSI uses digital control with ~2 dB steps but is fully recallable by number; the Austrian Audio uses a high-quality marked pot (fine for hi-fi, borderline for studio). Cheap pots wander badly at low levels — the Behringer showed up to 7 dB channel imbalance. FOSI and Benchmark had no measurable imbalance; the Austrian demo unit showed 0.4 dB (likely worn).
Output voltage
FOSI ~14 V (enough), Austrian ~28 V, Benchmark ~30 V. Diminishing returns beyond "enough."
The mystery amps are Apple hardware
Revealed as a 2015 MacBook Pro and an M4 Mac Mini. Their built-in headphone jacks have near-zero output impedance, perfect channel balance, 0.01 dB volume granularity, and an integrated DAC — objectively beating much pro gear. Caveats: volume must be set via text entry in Audio MIDI Setup; the DAC rolls off highs above ~16 kHz at 44.1 kHz (use higher sample rates); output is under 3 V so very high-impedance headphones may lack headroom; and this applies only to quality Apple hardware, not random PC laptops.
Conclusions
Behringer is junk; the TC BMC2 is a great monitor controller but poor headphone out; the FOSI ZH3 is exceptional value and industry-disrupting; the Austrian Audio is a beautiful hi-fi piece but not his studio first choice; the Benchmark HPA4 is the engineering pinnacle and the only unit perfect on every metric. Any interface or device with an analog volume pot is unfit for critical headphone monitoring — he would sooner use an Apple laptop's built-in jack.