Output impedance and damping: the Zout/8 rule

Every source has an output impedance (Zout) in series with your headphone. The headphone's impedance and the source's Zout form a voltage divider, so some of the signal is dropped inside the source instead of driving the headphone. If the headphone's impedance were constant across frequency this would only cost a little loudness.

But headphone impedance is rarely flat. Multi-driver IEMs with crossovers swing from 8 Ω at one frequency to 60 Ω at another; dynamic drivers have an impedance hump at their bass resonance. Against a high-Zout source, the voltage divider then varies with frequency. The headphone gets more level where its impedance is high and less where it is low. The result is a real, sometimes multi-dB tonal shift: typically boosted bass hump and skewed crossover behavior.

The community rule of thumb (popularized by NwAvGuy) is damping factor 8: keep Zout below the headphone's minimum impedance divided by 8. A 16 Ω IEM wants Zout under 2 Ω; a 300 Ω dynamic tolerates almost anything. This is also the classic OTL tube amp mismatch: output-transformerless tube amps have output impedances around 100 Ω or more, which suits 250-600 Ω dynamic headphones they were designed around, but audibly reshapes and underdamps low-impedance planars and IEMs.

One nuance: planar-magnetic headphones have nearly flat, purely resistive impedance, so a high Zout costs them loudness and damping-factor purism matters less for tonality, but the voltage-divider loss on an already power-hungry planar is exactly what you don't want. Either way, low output impedance is the safe default, which is why it is flagged in our power calculator whenever Zout exceeds Z/8.