Symphonium Helios: how much amplifier do you need?

The Symphonium Helios is 8.5Ω with a sensitivity of 83.3 dB/mW, which is 104.0 dB/V. To reach 100 dB SPL — genuinely loud, louder than most people listen — it needs 47 mW, or 0.63 V across the drivers.

That is a moderate load. Most USB dongles handle it without complaint; the weakest phone outputs will run out of volume before they run out of quality. 134 of 280 sources here have enough headroom. The cheapest portable source that does the job is the FiiO KA11 at $29.

Sensitivity here is in dB per milliwatt. Manufacturers often quote dB per volt instead and leave the unit off, which is the single most common reason online calculators give answers that are 10–15 dB wrong. The converter explains the difference.

Symphonium publishes this as 104 dB/Vrms (@1kHz) — a figure per volt, not per milliwatt. At 8.5Ω the two differ by 20.7 dB, so the 83.3 dB/mW above is that published number converted, not a different measurement.

This is the conversion that online calculators skip. Quote the per-volt number as if it were per-milliwatt and this headphone looks 21 dB easier to drive than it is — which is roughly the difference between a dongle being enough and not being enough.

Symphonium prints the impedance as 8.50 Ohms @ 1 kHz, not as a bare number. Everything on this page is calculated with the nominal 8.5Ω, and the rest of that line is quoted here rather than tidied away, because how a figure is qualified is part of the specification too.

Source: symphoniumaudio.com. How this data was built.

Most people listen between 75 and 85 dB. The 100 dB row is the one worth planning around: it gives you headroom for dynamic peaks without designing your system for a volume that would damage your hearing in under fifteen minutes. The safe listening calculator puts numbers on that.

Every source in the database, judged at 100 dB SPL into 8.5Ω. “Headroom” is how much power is left over above what the headphone needs — 10 dB means ten times the required power, which is what you want for dynamic peaks.

Hiss. At 104.0 dB/V these are sensitive enough to reveal a source's own noise floor. The noisiest source in the database, the Schiit Mjolnir 3, would put its hiss at roughly 12 dB SPL in these headphones — clearly audible in quiet passages. The hiss calculator lets you check your own amplifier's noise figure.

Output impedance. At 8.5Ω these want a source output impedance below 1.06Ω to satisfy the one-eighth rule. Of the 192 sources that publish one, 50 miss it, the worst being the Bottlehead Crack (OTL kit) at 120.0Ω; the other 88 publish no output impedance at all, so they cannot be checked either way. Above that threshold the source starts shaping the frequency response. Check yours here.

Worked out in advance against its own stable: vs Giant · vs Meteor · vs Titan · vs Triton. Each of those pages counts the sources that drive one and not the other.

Or open the comparison tool to put any two models side by side, or browse all 578. Or see every Symphonium headphone in the database, and how much of what this site says about them came from Symphonium’s own documents.