The Fostex T50RP MK3 is 50Ω at 92.0 dB/mW; the Grado SR80x is 38Ω at 98.0 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 Fostex T50RP MK3 needs 6.0 dB more than the Grado SR80x to reach the same loudness — 6.3 mW against 1.6 mW.
| Specification | Fostex T50RP MK3 | Grado SR80x |
|---|---|---|
| Impedance | 50 Ω | 38 Ω |
| Sensitivity | 92.0 dB/mW | 98.0 dB/mW |
| Sensitivity (voltage) | 105.0 dB/V | 112.2 dB/V |
| Power for 100 dB | 6.3 mW | 1.6 mW |
| Voltage for 100 dB | 0.56 V | 0.25 V |
| Current for 100 dB | 11.2 mA | 6.5 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 6.25 Ω | 4.75 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.
1 source stays below the audible-hiss threshold with the Fostex T50RP MK3 but crosses it with the Grado SR80x (Schiit Mjolnir 3).
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 Fostex T50RP MK3 is more demanding than 88% of the 567 non-electrostatic models in this database; the Grado SR80x than 66%. 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 | Fostex T50RP MK3 power | Fostex T50RP MK3 voltage | Grado SR80x power | Grado SR80x voltage |
|---|---|---|---|---|
| 80 | 0.06 mW | 56 mV | 0.02 mW | 25 mV |
| 85 | 0.20 mW | 100 mV | 0.05 mW | 44 mV |
| 90 | 0.63 mW | 0.18 V | 0.16 mW | 78 mV |
| 95 | 2.0 mW | 0.32 V | 0.50 mW | 0.14 V |
| 100 | 6.3 mW | 0.56 V | 1.6 mW | 0.25 V |
| 105 | 20 mW | 1.00 V | 5.0 mW | 0.44 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, Fostex T50RP MK3 | Headroom, Grado SR80x | Output impedance | Noise |
|---|---|---|---|---|---|
| JadeAudio JA11 | $20 | 4.8 dB | 12.0 dB | 0.5 Ω | 1.7 µV |
| FiiO KA11 | $29 | 13.1 dB | 20.3 dB | 0.7 Ω | 1.7 µV |
| FiiO BTR11 | $36 | 4.8 dB | 12.0 dB | 0.5 Ω | 8.0 µV |
Of the 280 sources here, 280 reach 100 dB on both. That is the easy axis. 69 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 69 is the JadeAudio JA11, at $20. It leaves 4.8 dB of headroom on the Fostex T50RP MK3 and 12.0 dB on the Grado SR80x, its output impedance is 0.5 Ω against a one-eighth limit of 6.25 Ω and 4.75 Ω, and its noise figure puts the hiss floor 30 dB and 23 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 |
|---|---|---|---|
| Fostex T50RP MK3 | 15〜35,000Hz | not printed | not printed |
| Grado SR80x | 20-20kHz | not printed | not printed |
Fostex prints 15〜35,000Hz for the T50RP MK3. Grado prints 20-20kHz for the SR80x.
Neither range has a tolerance printed beside it, and that is the whole of what this section can tell you. A wider range on one sheet is not deeper bass or more treble on that headphone: without a tolerance, neither number says how loud either model is at the ends of its own range, so the difference between the two is a difference between two pieces of marketing copy. A ±dB figure is what turns a range into a specification, and of the 490 models whose maker prints a range, 58 print it.
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: Fostex T50RP MK3 and Grado SR80x. Every figure on this page comes from what Fostex and Grado published, or is derived from it — what Fostex publishes and what Grado publishes, both counted.