The 64 Audio Aspire 1 is 11Ω at 104.0 dB/mW; the 64 Audio U4s is 11Ω at 107.0 dB/mW. This page covers what they ask of an amplifier, not how they sound.
The Aspire 1 needs 3.0 dB more than the U4s to reach the same loudness — 0.40 mW against 0.20 mW.
| Specification | Aspire 1 | U4s |
|---|---|---|
| Impedance | 11 Ω | 11 Ω |
| Sensitivity | 104.0 dB/mW | 107.0 dB/mW |
| Sensitivity (voltage) | 123.6 dB/V | 126.6 dB/V |
| Power for 100 dB | 0.40 mW | 0.20 mW |
| Voltage for 100 dB | 66 mV | 47 mV |
| Current for 100 dB | 6.0 mA | 4.3 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 1.38 Ω | 1.38 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.
12 sources stay below the audible-hiss threshold with the Aspire 1 but cross it with the U4s (Lake People G103-S Mk II, Schiit Valhalla 3, Grace Design m900, 9 more).
280 of the 280 sources here reach 100 dB on both.
The Aspire 1 is more demanding than 20% of the 567 non-electrostatic models in this database; the U4s than 10%.
| dB SPL | Aspire 1 power | Aspire 1 voltage | U4s power | U4s voltage |
|---|---|---|---|---|
| 80 | 4.0 µW | 7 mV | 2.0 µW | 5 mV |
| 85 | 0.01 mW | 12 mV | 6.3 µW | 8 mV |
| 90 | 0.04 mW | 21 mV | 0.02 mW | 15 mV |
| 95 | 0.13 mW | 37 mV | 0.06 mW | 26 mV |
| 100 | 0.40 mW | 66 mV | 0.20 mW | 47 mV |
| 105 | 1.3 mW | 0.12 V | 0.63 mW | 83 mV |
| Source | Price | Headroom, Aspire 1 | Headroom, U4s | Output impedance | Noise |
|---|---|---|---|---|---|
| JadeAudio JA11 | $20 | 14.1 dB | 17.1 dB | 0.5 Ω | 1.7 µV |
| FiiO KA11 | $29 | 26.3 dB | 29.3 dB | 0.7 Ω | 1.7 µV |
| FiiO BTR13 | $59 | 23.2 dB | 26.2 dB | 0.6 Ω | 1.0 µV |
Of the 280 sources here, 280 reach 100 dB on both. 51 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 cheapest of the 51 is the JadeAudio JA11, at $20. It leaves 14.1 dB of headroom on the Aspire 1 and 17.1 dB on the U4s, its output impedance is 0.5 Ω against a one-eighth limit of 1.38 Ω for both, and its noise figure puts the hiss floor 12 dB and 9 dB below the 20 dB SPL this site treats as the point where a quiet room stops hiding it.
| Model | As printed | Tolerance | Standard |
|---|---|---|---|
| Aspire 1 | 20Hz - 18kHz | not printed | not printed |
| U4s | 10hz – 20kHz | not printed | not printed |
64 Audio prints 20Hz - 18kHz for the Aspire 1 and 10hz – 20kHz for the U4s.
Neither range has a tolerance printed beside it, so a wider range on one sheet is not deeper bass or more treble.
Not a measurement; maker by maker.
Both have a page of their own. In full: 64 Audio Aspire 1 and 64 Audio U4s. Every figure on this page comes from what 64 Audio published, or is derived from it — what that maker publishes, counted.