T50RP (mk2)

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published: May-2-2013, updated: Jan-01-2022

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NO SMOOTHING is applied to the shown plots. Most measurement sites have some smoothing applied which ‘irons flat’ sharp peaks and ‘wiggles’. I do not use smoothing because some info about sound quality is lost when plots are smoothed.

Aside from a small correction of the microphone itself also some correction in the lowest frequencies is applied to the plots to compensate for the perceived loss of bass when using headphones. This is described HERE in more detail.
A ‘horizontal‘ frequency response curve on the shown frequency response plots on this website thus indicates a perceived ‘flat’ tonal signature.

ALL measurements are made with a good SEAL on a flatbed measurement rig.
shape of your head, bone structure, pad size, pad ‘softness,  (compliance), hair or no hair and or wearing glasses may (drastically) change the frequency response of some headphones, so… your personal experience may differ substantially from these plots.

Frequency response (tonal balance) is the most sound-determining aspect of headphones. A horizontal line shows audible neutral response in the plots on this website. Deviations in different severities at different frequency bands have an effect on the sound character.
The bigger the deviation the stronger the effect.

Below an aid to help determining the sound character of headphones with relation to the frequency response.


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Fostex T50RP

T50-940 pad

The Fostex T50RP is an oldie. It has a (square shaped) planar magnetic driver which is made from a relatively thick membrane. This is the mk2 version and a modified version as well.
The original version has very uncomfortable pads and the stock cable sometimes made intermittent contact. The headband is not comfortable and a bit limited in adjustability.
The T50RP is a semi-open headphone meant for usage in studio. It is well liked in the modding community because it is hard work to destroy the drivers (but possible) and with modifications the stock sound and comfort can be greatly improved. When driven with some power and EQ the sound quality can go from mediocre to very good.
This headphone is fitted with Shure SRH-940 velour pads which kind-off fit (but can fall off easily) but improve comfort and only change the sound signature a little.
So not a review of the original T50RP but a modified T50RP mkII.


Type: over ear (circum-aural), open
Usage: home, studio
Driver type: planar dynamic
Pads: replaceable, pleather (thick vinyl)
Collapsible: no
Headphone cup connector: 2x 3.5mm TRS angled, locking
Cable entry: dual sided.
Cable: 3m straight cable with split, 2x 3.5mm angled to 6.3mm TRS jack
Inner Pad dimensions: depth = 10mm, height = 56mm, width = 40mm.
Driver size: 40x40mm (effective 32x32mm)
Nom. power rating: 3W (estimated)
Max. voltage: 12.5 Vrms (34Vpp)
Max. current: 240 mA
Max. S.P.L.:  134dB
Impedance: 52Ω (measured)
Efficiency: 96dB @ 1mW
Sensitivity: 109dB @ 1V
Weight: 330 g. (without cable)
Clamping force: medium-high
Accessories: none


Below the measurements of a stock T50RP (Left, Right). The headphone in question had selected (by me) drivers because there was quite some channel imbalance out of the box in all 3 Fostex headphones I had available at the time (2013). I had two T50RP mk2 + one T40RP-mkII (same drivers) and created 2 well matched T50RP and one lesser matched T40RP from the available drivers.


Subbass is rolled off. The midbass hump gives it a powerful punchy bass which is somewhat elevated.
With thr original pad the upper midrange suckout (around 3kHz) is less severe but still audible.
This makes some shouty recordings less shouty. The treble is present but lacks extension and ‘air’ and while a bit elevated is not sibilant.

Below the phase response of the T50RP mk2 (Left, Right).
phaseOnly steep phase shifts in a narrow frequency band can be audible. At 600Hz and 8kHz this is the case but not in a severe and worrying amount.

Below the group delay plot (Left, Right) made with Shure pads.GDThe variance around 70Hz is ‘pad bounce’ (the pads storing an releasing low frequencies) and will be different with the original pads.

Below the distortion measurements of the T50RP (Right channel). Note that around 200Hz the actual level is around 100dB.
dist RThe plot above shows the level differences between the signal (upper trace) and the harmonics.
Most people prefer to see percentages instead of level differences so below the exact same plot except ‘normalised’ to the actual signal and level differences given in percentages.
dist R percent
Distortion levels are mainly 2nd harmonic and remain below 1%. Below 100Hz we also see higher harmonics which indicates linearity is becoming worse at larger excursions (low frequencies).

The actual 2nd harmonic distortion may well be lower than 0.2% . A shortcoming of my measurement rig.

Below the CSD (waterfall plot) of the T50RP (Left, Right) made with Shure pads.CSDBelow 2.5kHz there are a lot of resonances. Above 2.5kHz damping is quite good. Even the resonance around 8kHz is quite short.

A different plot is the spectrum plot. This basically is a CSD (Waterfall) plot but viewed from above where the level differences are colour coded instead of being in the vertical axis. Also the frequency range of the spectrum plot is wider (from 100Hz instead of 500Hz). The time span is also bigger in the spectrum plots and expired time is shown from below to top where in the CSD the time is shown from rear to front.

Below the spectrum plot of the T50RP (Left channel)
spectr.This plot also shows the resonances/lingering between 500hz and 2.5kHz which is not optimal.

The step response below (Left, Right) shows the low midrange/upper bass oriented sound and the lack of speed. The initial rise -5dB and a sizeable bounce (-12dB) and slow rise to -2dB indicate the slowish treble response. The steep drop after a few ms indicates the lack of sub-bass response.
The headphone can be improved with some damping in front and rear of the driver and EQ.
Below some results.

Below the FR of a modifiedFostex T50RP with Shure SRH 940 pads fitted versus the same modified T50RP playing from a Kameleon amplifier.
EXT T50RP Kam vs stock

As can be seen the Kameleon amplifier (with T50RP module) extends the lows, lowers the midrange just slightly, fills in the 3kHz dip just enough and extends the top end.

Below the distortion plots of the (modified) T50RP driver.

DIST stock T50RP

Below the distortion measurements of the same headphone but playing on the Kameleon amplifier.

DIST kam T50RP

Below the CSD of the (modified) T50RP. Note that the stock T50RP will look different because of the added damping materials in front and rear of the driver (as well as due to the pads).

CSD 30k stock

Below the CSD of the same headphone playing from the Kameleon amplifier.


A nice and clean CSD which also shows the improved frequency response and extension up to 20kHz.

Another clue for signal response is the square-wave and needle pulse response.
Shown below the plots for the modified T50RP on the left versus the same headphone playing via the Kameleon amplifier on the right.


Especially the impulse response shows a much improved transient response. Also visible in the 440Hz square-wave where the leading edge (where the ‘attack’ is shown) is higher.

Do note that the filter PCB mentioned in the modification article is no longer available, it has been replaced by the Kameleon amplifier.

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