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. Why the unit matters.
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.
| As it is labelled | As printed | Tolerance | Standard |
|---|---|---|---|
| Frequency response | 12Hz - 24kHz, ± 2 dB | ±2 dB | not printed |
Symphonium prints 12Hz - 24kHz, ± 2 dB for the Symphonium Helios, under the heading “Frequency response”. Source: symphoniumaudio.com.
This is one of only 58 models — out of the 490 whose maker prints a range at all — that also print the tolerance. That is what makes it a specification rather than a headline.
Only what the maker printed; maker by maker.
| Level | What it is | Power | Voltage | Current |
|---|---|---|---|---|
| 85 dB SPL | quiet background listening | 1.5 mW | 0.11 V | 13.2 mA |
| 95 dB SPL | normal listening | 15 mW | 0.35 V | 41.7 mA |
| 100 dB SPL | loud listening | 47 mW | 0.63 V | 74.2 mA |
| 105 dB SPL | very loud | 148 mW | 1.12 V | 131.9 mA |
| 110 dB SPL | peaks on dynamic music | 468 mW | 1.99 V | 234.6 mA |
Most people listen between 75 and 85 dB; the 100 dB row leaves room for peaks.
The 15 cheapest of the 280 sources here that reach 100 dB SPL into 8.5Ω with at least 10 dB of headroom and no known hiss or output-impedance problem with these headphones. All 280 sources, judged for these headphones.
| Source | Type | Price | Power into 8.5Ω | Headroom | Verdict |
|---|---|---|---|---|---|
| Schiit Magni Heresy | desktop amplifier | $99 | 1.5 W | +15 dB | Comfortable |
| Schiit Magni+ | desktop amplifier | $109 | 531 mW | +11 dB | Comfortable |
| Schiit Magni Unity | desktop amplifier | $119 | 1.6 W | +15 dB | Comfortable |
| JDS Labs Atom Amp 2 | desktop amplifier | $129 | 704 mW | +12 dB | Comfortable |
| Topping L30 II | desktop amplifier | $149 | 1.9 W | +16 dB | Comfortable |
| SMSL HO100 | desktop amplifier | $160 | 1.6 W | +15 dB | Comfortable |
| xDuoo MT-604 | desktop amplifier | $169 | 531 mW | +11 dB | Comfortable |
| Geshelli Archel Pro | desktop amplifier | $200 | 478 mW | +10 dB | Comfortable |
| Schiit Magni Piety | desktop amplifier | $200 | 638 mW | +11 dB | Comfortable |
| SMSL C200 | desktop DAC/amp | $219 | 1.4 W | +15 dB | Comfortable |
| Schiit Midgard | desktop amplifier | $219 | 2.9 W | +18 dB | Comfortable |
| Topping L50 | desktop amplifier | $229 | 1.9 W | +16 dB | Comfortable |
| Topping A30 Pro | desktop amplifier | $249 | 3.2 W | +18 dB | Comfortable |
| Schiit Asgard 3 | desktop amplifier | $279 | 2.7 W | +18 dB | Comfortable |
| SMSL SH-9 | desktop amplifier | $289 | 3.2 W | +18 dB | Comfortable |
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. Check your own source.
Output impedance. At 8.5Ω these want a source output impedance below 1.06Ω to satisfy the one-eighth rule; 50 of the 192 sources that publish one miss it. Check yours here.
8.3Ω · 86.2 dB/mW · Easy
See requirements →9.7Ω · 90.9 dB/mW · Easy
See requirements →7.0Ω · 88.5 dB/mW · Easy
See requirements →43Ω · 88.3 dB/mW · Moderate
See requirements →18Ω · 87.6 dB/mW · Moderate
See requirements →16Ω · 88.0 dB/mW · Moderate
See requirements →