The Moondrop Chu II is 18Ω at 101.6 dB/mW; the Moondrop S8 is 16Ω at 104.0 dB/mW. This page is only about what they ask of an amplifier — not how they sound, which no specification on either sheet can tell you.
The Chu II needs 2.4 dB more than the S8 to reach the same loudness — 0.69 mW against 0.40 mW.
| Specification | Chu II | S8 |
|---|---|---|
| Impedance | 18 Ω | 16 Ω |
| Sensitivity | 101.6 dB/mW | 104.0 dB/mW |
| Sensitivity (voltage) | 119.0 dB/V | 122.0 dB/V |
| Power for 100 dB | 0.69 mW | 0.40 mW |
| Voltage for 100 dB | 0.11 V | 80 mV |
| Current for 100 dB | 6.2 mA | 5.0 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 2.25 Ω | 2.00 Ω |
| Sources that reach 100 dB out of 280 | 280 | 280 |
Every source in the database that drives one of them drives the other. On power, the choice between these two changes nothing about what amplifier you need.
2 sources stay below the audible-hiss threshold with the Chu II but cross it with the S8 (FiiO BTR11, Rupert Neve RNHP).
280 of the 280 sources here reach 100 dB on both. That count is about power only — the output-impedance and hiss lines above are separate questions, and a source can pass this one and fail those.
The gap between these two in decibels does not change with volume — it is a property of the two headphones. What changes is the size of the numbers, and it changes fast: every 10 dB costs ten times the power. The row that matters is the one nearest the level you listen at, and for most people that is nearer the top of this table than the bottom.
The Chu II is more demanding than 37% of the 567 non-electrostatic models in this database; the S8 than 26%. Both figures use the same measure as the rest of the site: whichever runs out first against a 2 V, 100 mW dongle, voltage or power.
| dB SPL | Chu II power | Chu II voltage | S8 power | S8 voltage |
|---|---|---|---|---|
| 80 | 6.9 µW | 11 mV | 4.0 µW | 8 mV |
| 85 | 0.02 mW | 20 mV | 0.01 mW | 14 mV |
| 90 | 0.07 mW | 35 mV | 0.04 mW | 25 mV |
| 95 | 0.22 mW | 63 mV | 0.13 mW | 45 mV |
| 100 | 0.69 mW | 0.11 V | 0.40 mW | 80 mV |
| 105 | 2.2 mW | 0.20 V | 1.3 mW | 0.14 V |
105 dB is included because forum power figures are usually quoted there or higher. It is also a level that damages hearing in minutes — how many minutes is its own calculator.
| Source | Price | Headroom, Chu II | Headroom, S8 | Output impedance | Noise |
|---|---|---|---|---|---|
| JadeAudio JA11 | $20 | 13.9 dB | 15.8 dB | 0.5 Ω | 1.7 µV |
| FiiO KA11 | $29 | 25.3 dB | 27.9 dB | 0.7 Ω | 1.7 µV |
| FiiO BTR13 | $59 | 22.2 dB | 24.8 dB | 0.6 Ω | 1.0 µV |
Of the 280 sources here, 280 reach 100 dB on both. That is the easy axis. 67 of them also publish an output impedance low enough for both under the one-eighth rule and a noise figure that stays below the audible-hiss threshold with both — the rest either fail one of those or never printed the number at all, and 208 of them are in the second group.
The cheapest of the 67 is the JadeAudio JA11, at $20. It leaves 13.9 dB of headroom on the Chu II and 15.8 dB on the S8, its output impedance is 0.5 Ω against a one-eighth limit of 2.25 Ω and 2.00 Ω, and its noise figure puts the hiss floor 16 dB and 13 dB below the 20 dB SPL this site treats as the point where a quiet room stops hiding it.
Cheapest that satisfies three published numbers is not the same as best, and none of it is about how either headphone sounds. To put either model against a source this page does not mention, or to change the target loudness, use the amplifier matcher.
| Model | As printed | Tolerance | Standard |
|---|---|---|---|
| Chu II | 15Hz - 38kHz | not printed | not printed |
| 20Hz - 20kHz (IEC60318-4, -3dB) | -3 dB | IEC60318-4 | |
| S8 | 20-40kHz | not printed | not printed |
| 20-20kHz | not printed | not printed |
Moondrop prints two ranges, “15Hz - 38kHz” and “20Hz - 20kHz (IEC60318-4, -3dB)”, for the Chu II and two ranges, “20-40kHz” and “20-20kHz”, for the S8.
Only one of the two is a specification. The range printed for the Chu II comes with a tolerance; the one printed for the S8 does not, and a range without a ±dB figure says nothing about how loud that model is at either end of it. The two cannot be compared number against number, however similar they look in the table. Of the 490 models whose maker prints a range, 58 print the tolerance.
None of this is a measurement. What each maker publishes, and what it is worth, is set out in the frequency response audit.
Each model has a page of its own with the full source list, the hiss estimate and the provenance of every figure above: Moondrop Chu II and Moondrop S8. Every figure on this page comes from what Moondrop published, or is derived from it — what that maker publishes, counted.