The Fostex T50RPmk4Q is 28Ω with a sensitivity of 97.0 dB/mW, which is 112.5 dB/V. To reach 100 dB SPL — genuinely loud, louder than most people listen — it needs 2.0 mW, or 0.24 V across the drivers.
That puts it in the easy category. A phone, a laptop headphone jack or the cheapest USB dongle will drive it to painful levels. Of the 280 sources in the database, 274 have at least 10 dB of headroom and another 6 have enough. The cheapest portable source that does the job is the Apple USB-C dongle (US) at $9.
Sensitivity here is in dB per milliwatt. Why the unit matters.
Fostex publishes this directly as 97 dB/mW, per milliwatt, so nothing was converted.
The maker adds a power recommendation of its own: 最大入力 (max. input): 3,000 mW
Source: fostex.jp. How this data was built.
| As it is labelled | As printed | Tolerance | Standard |
|---|---|---|---|
| Frequency Response | 10–40,000 Hz | not printed | not printed |
Fostex prints 10–40,000 Hz for the Fostex T50RPmk4Q, under the heading “Frequency Response”. Source: fostex.jp.
No tolerance is printed with it, so “10–40,000 Hz” says nothing about how loud this headphone actually is at 10 Hz.
Only what the maker printed; maker by maker.
| Level | What it is | Power | Voltage | Current |
|---|---|---|---|---|
| 85 dB SPL | quiet background listening | 0.06 mW | 42 mV | 1.5 mA |
| 95 dB SPL | normal listening | 0.63 mW | 0.13 V | 4.7 mA |
| 100 dB SPL | loud listening | 2.0 mW | 0.24 V | 8.4 mA |
| 105 dB SPL | very loud | 6.3 mW | 0.42 V | 15.0 mA |
| 110 dB SPL | peaks on dynamic music | 20 mW | 0.75 V | 26.7 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 28Ω 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 28Ω | Headroom | Verdict |
|---|---|---|---|---|---|
| JadeAudio JA11 | USB dongle | $20 | 26 mW | +11 dB | Comfortable |
| FiiO SnowSky Tiny A | USB dongle | $22 | 35 mW | +12 dB | Comfortable |
| FiiO SnowSky Tiny B | USB dongle | $22 | 39 mW | +13 dB | Comfortable |
| FiiO KA11 | USB dongle | $29 | 204 mW | +20 dB | Comfortable |
| Tanchjim Stargate II | USB dongle | $40 | 75 mW | +16 dB | Comfortable |
| ddHiFi TC35AL | USB dongle | $46 | 184 mW | +20 dB | Comfortable |
| ddHiFi TC35Air | USB dongle | $46 | 184 mW | +20 dB | Comfortable |
| Fosi Audio N3 | Bluetooth receiver | $50 | 121 mW | +18 dB | Comfortable |
| Hidizs SD2 | USB dongle | $50 | 61 mW | +15 dB | Comfortable |
| FiiO BTR13 | Bluetooth receiver | $59 | 102 mW | +17 dB | Comfortable |
| iFi GO link | USB dongle | $59 | 61 mW | +15 dB | Comfortable |
| Kiwi Ears Allegro Pro | USB dongle | $60 | 60 mW | +15 dB | Comfortable |
| ddHiFi TC35Pro (Mountain2) | USB dongle | $66 | 57 mW | +15 dB | Comfortable |
| iBasso DC03 Pro | USB dongle | $69 | 86 mW | +16 dB | Comfortable |
| DUNU DTC 480 | USB dongle | $70 | 57 mW | +15 dB | Comfortable |
Hiss. At 112.5 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 21 dB SPL in these headphones — clearly audible in quiet passages. Check your own source.
Output impedance. At 28Ω these want a source output impedance below 3.50Ω to satisfy the one-eighth rule; 35 of the 192 sources that publish one miss it. Check yours here.
28Ω · 97.0 dB/mW · Easy
See requirements →28Ω · 96.0 dB/mW · Easy
See requirements →25Ω · 100.0 dB · Easy
See requirements →14Ω · 94.0 dB · Easy
See requirements →70Ω · 101.0 dB/mW · Easy
See requirements →70Ω · 101.0 dB/mW · Easy
See requirements →Compared with its stablemates: vs T20RP MK3 · vs T40RP MK3 · vs T50RP MK3 · vs T60RP · vs T60RPmk2 · vs TH1100RPmk2.