The Fostex T20RP MK3 is 50Ω at 92.0 dB/mW; the Fostex T50RP mk4 is 28Ω at 97.0 dB/mW. This page covers what they ask of an amplifier, not how they sound.
The T20RP MK3 needs 5.0 dB more than the T50RP mk4 to reach the same loudness — 6.3 mW against 2.0 mW.
| Specification | T20RP MK3 | T50RP mk4 |
|---|---|---|
| Impedance | 50 Ω | 28 Ω |
| Sensitivity | 92.0 dB/mW | 97.0 dB/mW |
| Sensitivity (voltage) | 105.0 dB/V | 112.5 dB/V |
| Power for 100 dB | 6.3 mW | 2.0 mW |
| Voltage for 100 dB | 0.56 V | 0.24 V |
| Current for 100 dB | 11.2 mA | 8.4 mA |
| Highest source impedance that leaves them alone (one-eighth rule) | 6.25 Ω | 3.50 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 is low enough in output impedance for the T20RP MK3 under the one-eighth rule but not for the T50RP mk4 (Shanling H5).
1 source stays below the audible-hiss threshold with the T20RP MK3 but crosses it with the T50RP mk4 (Schiit Mjolnir 3).
280 of the 280 sources here reach 100 dB on both.
The T20RP MK3 is more demanding than 88% of the 567 non-electrostatic models in this database; the T50RP mk4 than 65%.
| dB SPL | T20RP MK3 power | T20RP MK3 voltage | T50RP mk4 power | T50RP mk4 voltage |
|---|---|---|---|---|
| 80 | 0.06 mW | 56 mV | 0.02 mW | 24 mV |
| 85 | 0.20 mW | 100 mV | 0.06 mW | 42 mV |
| 90 | 0.63 mW | 0.18 V | 0.20 mW | 75 mV |
| 95 | 2.0 mW | 0.32 V | 0.63 mW | 0.13 V |
| 100 | 6.3 mW | 0.56 V | 2.0 mW | 0.24 V |
| 105 | 20 mW | 1.00 V | 6.3 mW | 0.42 V |
| Source | Price | Headroom, T20RP MK3 | Headroom, T50RP mk4 | Output impedance | Noise |
|---|---|---|---|---|---|
| JadeAudio JA11 | $20 | 4.8 dB | 11.2 dB | 0.5 Ω | 1.7 µV |
| FiiO KA11 | $29 | 13.1 dB | 20.1 dB | 0.7 Ω | 1.7 µV |
| FiiO BTR11 | $36 | 4.8 dB | 12.1 dB | 0.5 Ω | 8.0 µV |
Of the 280 sources here, 280 reach 100 dB on both. 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 cheapest of the 69 is the JadeAudio JA11, at $20. It leaves 4.8 dB of headroom on the T20RP MK3 and 11.2 dB on the T50RP mk4, its output impedance is 0.5 Ω against a one-eighth limit of 6.25 Ω and 3.50 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.
| Model | As printed | Tolerance | Standard |
|---|---|---|---|
| T20RP MK3 | 15 to 35,000 Hz | not printed | not printed |
| T50RP mk4 | 10 to 40,000 Hz | not printed | not printed |
Fostex prints 15 to 35,000 Hz for the T20RP MK3 and 10 to 40,000 Hz for the T50RP mk4.
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: Fostex T20RP MK3 and Fostex T50RP mk4. Every figure on this page comes from what Fostex published, or is derived from it — what that maker publishes, counted.