On power: 50 of the 51 USB dongles here reach 100 dB SPL on the Beyerdynamic DT 990 Pro (250Ω). It needs 2.5 mW at its 250Ω, and 1 of them do not get there.
Power is one of three questions and the one that goes wrong least often. The other two — whether the source’s output impedance leaves the frequency response alone, and whether you will hear its noise floor — are answered further down, separately, because a source can pass one and fail another.
The 1 that fall short are Apple USB-C dongle (EU). The closest of them, the Apple USB-C dongle (EU), delivers 2.4 mW into 250Ω against the 2.5 mW needed — 0.2 dB short.
The cheapest one that does reach it is the Apple USB-C dongle (US) at $9, which delivers 4.8 mW into 250Ω — 3 dB of headroom over the 2.5 mW this headphone needs for 100 dB SPL.
100 dB SPL is a peak level, not a listening level. If you listen at 85 dB the same headphone needs 0.08 mW, which is 15 dB less, and the answer above changes for most of the category. The model page has the full ladder.
Under the one-eighth rule the Beyerdynamic DT 990 Pro (250Ω) wants a source below 31.25Ω. Of the 18 USB dongles here that publish an output impedance, 18 are under it and 0 are not.
The other 33 publish nothing at all on this, so they are not on either side of that count. This site leaves them blank rather than assuming a figure.
Hiss depends on the source’s own noise floor and on how sensitive the headphone is. At 96.0 dB and 250Ω, the Beyerdynamic DT 990 Pro (250Ω) turns one microvolt of source noise into about -18 dB SPL — and the threshold where that stops being noticeable in a quiet room is around 20 dB SPL.
Of the 16 USB dongles here that publish a noise figure, 16 stay below that threshold with this headphone and 0 cross it.
The remaining 35 publish no noise figure, so they cannot be placed on either side.
| Source | Price | Into 250Ω | Output impedance | Hiss in these | Power verdict |
|---|---|---|---|---|---|
| FiiO KA11 | $29 | 26 mW | 0.7 Ω | -13 dB SPL | Comfortable |
| ddHiFi TC35AL | $46 | 27 mW | not published | not published | Comfortable |
| ddHiFi TC35Air | $46 | 27 mW | not published | not published | Comfortable |
| Tempotec Sonata BHD Pro | $60 | 36 mW | not published | not published | Comfortable |
| ddHiFi TC44 Pro E2 | $99 | 32 mW | not published | not published | Comfortable |
| Tanchjim LUNA | $140 | 59 mW | not published | -16 dB SPL | Comfortable |
| Moondrop Moonriver 2 Ti | $190 | 58 mW | not published | -14 dB SPL | Comfortable |
| Qudelix T71 | $229 | 64 mW | not published | not published | Comfortable |
| Questyle M15 | $250 | 66 mW | not published | not published | Comfortable |
| Generic CX31993 dongles | $12 | 11 mW | not published | not published | Enough |
| Tanchjim Stargate II | $40 | 9.6 mW | not published | not published | Enough |
| Hidizs SD2 | $50 | 9.0 mW | not published | not published | Enough |
Sorted by how many of the three questions the source answers well, then by price. A blank in a column is a blank in the manufacturer’s own documentation, not a zero.
The Beyerdynamic DT 990 Pro (250Ω) on other categories: portable players · desktop amplifiers · audio interfaces.
The Beyerdynamic DT 990 Pro (250Ω) page has every source in the database against this headphone, and the matcher takes your own numbers.