Methodology

Target SPL and required power

Every pairing is computed against a target of 110 dB SPL at the headphone driver. This is a high but realistic ceiling — roughly "very loud with clean peaks." It is not a continuous listening level; it is a headroom reference: an amp that can reach 110 dB has comfortable reserve for peaks without clipping.

The required power in milliwatts is:

For example, the Sennheiser HD 600 at 100.5 dB/mW requires 10^((110−100.5)/10) = 10^0.95 ≈ 8.9 mW to reach 110 dB SPL.

Headroom and verdict thresholds

Power headroom is the margin (in dB) between what the amplifier delivers and what the headphone requires:

Verdicts are assigned by headroom:

• Ample (≥ 10 dB): substantial reserve; the amp drives the headphone comfortably at any listening level with clean transient dynamics.
• Adequate (3–10 dB): sufficient for most listening; limited margin for peaks at maximum volume.
• Marginal (0–3 dB): barely reaches 110 dB SPL; no meaningful reserve; dynamics may sound compressed at loud levels.
• Insufficient (< 0 dB): the amp cannot reach the 110 dB target with this headphone.

Specification sourcing: measured vs. rated

Manufacturer-rated specifications are convenient but frequently optimistic. Where independently measured data is available from Reference Audio Analyzer (RAA), we use it in preference to the manufacturer figure. Both rated and measured values are shown on every pair page and labeled explicitly. The value actually used in computation is always identified.

Measured impedance matters for two reasons: (1) it determines which amp power-output table row to use (nearest published load), and (2) it affects the damping ratio. The HD 600, for example, measures 352 Ω despite a 300 Ω rating — the difference shifts it into the 300 Ω load bucket, so we use 300 Ω power figures, which is correct.

Output impedance and damping

A mismatched output impedance between amplifier and headphone can alter the headphone's frequency response and perceived tonal balance. The traditional "rule of eighths" (output impedance should be ≤ 1/8th of headphone impedance) applies to solid-state and low-output- impedance amplifiers, but not universally to tube output transformerless (OTL) designs.

Strict path (output impedance < 2 Ω)

For amplifiers with low output impedance — solid-state, most DAC/amps, dongles, and tube hybrids that run low Zout — we flag damping caution when the ratio of headphone impedance to amplifier output impedance is below 8× (the rule of eighths). This protects frequency-response integrity.

Relaxed path (output impedance ≥ 2 Ω): OTL tube amps

Genuine output-transformerless tube amplifiers — Bottlehead Crack (120 Ω), Darkvoice 336SE, Schiit Valhalla 2 (14 Ω) — are intentionally designed to interact with high-impedance (250 Ω+) headphones. The impedance interaction is the intended tube character, not a flaw. Applying the rule-of-eighths to a Crack + HD 600 pairing would flag it as "caution" despite being a textbook-compatible match.

Therefore, for amplifiers with output impedance ≥ 2 Ω, we apply a relaxed threshold: damping caution fires only when the impedance ratio falls below 2×. This correctly identifies genuinely problematic low-impedance mismatches (e.g. a 120 Ω OTL amp into a 14 Ω planar headphone, ratio ≈ 0.12×) while allowing the intended high-impedance pairings to render as compatible.

The damping rule is driven by measured output impedance, not by whether an amp contains a tube. A tube hybrid running 0.3 Ω output impedance (e.g. Schiit Lyr 3) lands on the strict path regardless of its tube stage.

Derived power estimates

Many amplifiers publish power figures at only one or two load impedances. When a headphone's impedance doesn't match a published load, we estimate the power at the headphone's impedance from the published voltage ceiling at the nearest published load:

This estimation is only performed upward in impedance (from a lower-impedance published figure to a higher impedance), where voltage limiting is the dominant constraint. Downward derivation (low impedance from a high-impedance figure) is not performed because current limiting may invalidate the assumption. All derived figures are marked with an asterisk (*est.) on spec tables and in cautions.

What we don't measure

Distortion (THD, IMD), noise floor, channel balance, frequency response measured at the headphone, and subjective sound quality are not assessed. Verdicts are purely driveability estimates — can the amplifier reach the target SPL with adequate reserve — not holistic audio quality assessments.