(Last Updated On: July 10, 2019)

Today we are taking a look at the famous Fostex RP series of headphones where many headphone enthusiasts and audiophiles were introduced to the fun world of modding.

The Fostex RP series from Fostex International.
The Fostex RP series from Fostex International.

We will explore some of the standard recommended modifications and take a look at my journey in tuning a stock Fostex T50RP mk3. In addition, we will look at (and inside) some of the popular enthusiast and commercial modifications available to see what they all have in common.

Fostex T50RP Mk3 Mod
The finished product. My DIY modified Fostex T50RP mk3.
This is a long article. Use the Table of Content below to navigate around the article. You can jump straight to the “DIY Modifications” section for the mods itself.

Table of Content

Fostex Overview

The Fostex Company was founded in July 1973 by Foster Electric Co., Ltd. Originally established to sell components to the Japanese marketplace, Fostex expanded to include a range of headphones, monitors, PA speakers and microphones.

Fostex started making planar magnetic headphones in the mid-1970s and continues to manufacture them as the RP (Regular Phase, or as it is sometimes referred to, Regulated Phase) series. New RP models and partnerships have debuted in the last couple of years with Dekoni Audio and Drop (formerly Massdrop).

Fostex RP Series History

Planar magnetic drivers and headphones seem like a recent development due to increased popularity and press. However, as noted above, Fostex has been quietly making this line of orthodynamic headphones for decades.

The first generation Fostex T20 and T50 from the late 1970’s brochure.
The first generation Fostex T20 and T50 from the late 1970’s brochure.

Originally featuring round cups, it wasn’t until the 1980s that Fostex started producing the distinctive cup shape that the RP series still features today. Although the components have changed a few times over the last 40 years, the proprietary transducer design and technology has remained essentially the same over the years.

There have now been four generations (or versions) of RP headphones. This leads to some confusion, as there are different models in each generation. Even within a single model (such as the T20) the sound has evolved significantly with each version. From the (now defunct) ‘Wikiphonia’ headphone web database:

“It’s particularly important to be sure which T20 one is talking about, or about to buy, because they all sound different in stock form.

  • T20v1 has a decent but not outstanding low end, a flat midrange, then a big peak around 2kHz after which response drops off.
  • T20v2 is more efficient, is flatter than the v1, has better bass, but still has the upper mid/low treble peak.
  • T20v3 has weak bass (with version one pads) and a smooth response tilted upward toward the treble.”
The Fostex T20RP v2 from Pinterest.
The Fostex T20RP v2 from Pinterest.

Fostex RP Series Generations

VersionModelsYearChangesNotes
1T10, T20, T30, T501978Popular in LP jacket photos as common studio headphones.
2T20RP v2, T40RP, T50 v2, OEM1980sSimilar 45mm round driver as version 1, but with a new headband and ear cup construction.There were several probable Fostex OEM headphones:
  • Pacific Stereo Concept CE-H
  • Grundig GDHS 223
  • Maior RPT-50
  • NAD RP18
  • Sansui SS-100
3T20RP mk2, T40RP mk2, T50RP2002 - 2006Similar cosmetics to previous version but with a new 38mm square driver.The drivers were originally all round in shape until the mid 2000s when Fostex changed to square drivers. This change heralded the mk2 (mark two) designation.

Radical driver change meant that the sound changed quite significantly.
4T20RP mk3, T40RP mk3, T50RP mk3, T60RP, Dekoni Blue, Drop TX0 II2015 - presentNew padded headband, rim slot for pad retention, baffle changes with edge vents and no baffle port, driver filter density, no felt in front of the driver.The T60 features wooden cups for the first time (rather than the traditional black plastic cups).

Dekoni Blue features Dekoni pads and blue plastic cups.

TX0 II are produced in partnership with Drop (formerly Massdrop) and include two sets of damping foam and two different ear pads for basic customization and modding.

The RP mk3 Series

The current Fostex RP mk3 headphone series was released in 2015 as a successor to the popular mk2 series. It consists of 4 models: the T20RP mk3, T40RP mk3, T50RP mk3 and T60RP (note no mk3 designation as this is the first T60 model but it does utilize the mk3 driver).

The T60RP from kopfhoerer.de.
The T60RP from kopfhoerer.de.
All share the majority of the same components with the same band, driver, cables, etc. The T60RP is a more recent addition and the cup material is made from wood rather than plastic, albeit in the same shape and design. The T60RP also has a redesigned vertical cable jack.

The primary difference in the mk3 series is in how they sound.

“Offering… different sound characteristics; Open (T20 RP mk3) for “Deep Bass”, Closed (T40 RP mk3) for “Focused Bass” and Semi-Open (T50 RP mk3) for “Flat and Clear” sound.” – Fostex

Intended as professional studio headphones, the RP series were designed for reliability and accurate studio monitoring rather than home or portable use. The mk3 models signify a change in response to the current personal audio renaissance, with redesigned ear pads, housing and band to improve comfort and looks. The T60 is definitely a move towards a more attractive and higher-end personal audio market.

It is important to realize that Fostex lists the T20RP mk3, the T40RP mk3 and the T50RP mk3 under recording equipment. Only the T60RP is listed as consumer equipment. This is indicative of how Fostex intends the RP series to be used.

The RP Driver

At the heart of the RP series is the Regular Phase (RP) diaphragm driver, which uses copper foil etched polyimide film and a neodymium magnet. This is a unique planar magnetic (also known as orthodynamic) driver which Fostex claims has been redesigned to “achieve sharper audio reproduction for more accurate monitoring”.

The original drivers from the 1977 brochure.
The original drivers from the 1977 brochure.
“The RP system is an electrically driven electro-acoustic transducer using a plane coil diaphragm… a surface driving system which actuates the whole surface of the diaphragm with the same phase… so constructed that entire diaphragm is evenly driven by supporting extremely thin plane coils of high density between pairs of powerfully magnetic circuits with identical poles opposed, through a unique holding method.” – original 1977 brochure.
The technology behind the RP driver from Rin Choi’s Headphone Analysis Blog.
The technology behind the RP driver from Rin Choi’s Headphone Analysis Blog.

It is this time tested, tried-and-true driver that makes the Fostex RP series so interesting. Right from the beginning, Fostex heralded the benefits of their planar magnetic driver:

…offering superior performance and combining the merits of the electrostatic type and the dynamic type… such as sharp resolution, wide and dynamic range, and excellent transient response with low distortion… The outstanding features in regards tonal quality and performance are as follows:

  • Fully increased clarity of highs with flat response characteristics.
  • Extremely low distortion.
  • Excellent transient characteristics
  • Makes possible the manufacture of headphones with outstanding directional characteristics.
  • Extremely wide in the reproduction range of high frequencies.
  • Flat impedance characteristics.
The back of the Fostex T50RP mk3 driver attached to the baffle. Red tape is not stock.
The back of the Fostex T50RP mk3 driver attached to the baffle. Red tape is not stock.

Fostex RP mk3 Specifications

  • Driver: Regular Phase
  • Impedance: 50 ohm
  • Sensitivity: 91-92dB (at 1kHz, 1mW)
  • Frequency Response: 15Hz – 35kHz
  • Maximum Input Power: 3000mW
Regardless of modification or specifications, I find the RP series remarkably difficult to amplify properly. The Schiit Lyr version 1 tends to be my go-to for ample power.

Why Modify the RP Series?

For many years Fostex has been marketing different sounding models (at different price points) that are essentially identical headphones. They feature smaller or larger vents in the cups to change the sound. Once it was realized that small changes to the enclosure do make significant changes in the sound signature, the enthusiast community started to get very interested.

The period between 2011 and 2017 was the height of RP series modding. The release of the enhanced mk3 versions brought renewed excitement to the community. Several enthusiasts moved from modding their own headphones to starting companies to do mods for others and selling their own designs.

The T50RP mk3, T50RP mk2 and T20RP mk2 from Pro Sound Network.
The T50RP mk3, T50RP mk2 and T20RP mk2 from Pro Sound Network.

The mk2 was significantly less expensive than the mk3. It is likely Fostex took note of all the renewed excitement and made a few improvements and a new (almost twice as expensive) price tag. The success of the third party modification companies proved that this inexpensive planar could be sold for 5 times the price with the right modifications.

The timing of this interest mirrored the success of companies, such as HiFiMan, who started gaining widespread recognition for producing relatively inexpensive, but good sounding planar magnetic headphones. This sparked more momentum in this type of headphones, and that helped bring the RP series back to the forefront.

A few things make the RP series so attractive to modify:

  • Planar magnetic driver technology
  • Low price point – with the promise of top tier performance far below top tier prices
  • Ease of disassembly – real screws, no glue, means little chance of breakage
  • Simple cup design – lots of options to try damping and venting alterations
  • Noticeable and measurable changes to the sound signature with simple modifications
  • Not the ‘best’ stock sound or comfort – the flat stock ear pads are universally considered uncomfortable
Regardless of model, all current Fostex RP models share the identical driver and virtually the identical enclosure. A modder’s dream!

DIY Modifications

For all these reasons, the RP series are undoubtedly the world’s most modded headphones. The changes vary from mild to wild, with anything from home-made comfort straps, to 3D printed and custom wood replacement cups. Whether improving comfort, fixing flaws or tuning for a certain sound, there are plenty of options available.

There is a 175+ page thread on Head-Fi entitled Fostex T50RP Incremental Mods and Measurements that is incredibly detailed and a wealth of information. While impressive, it is frankly overwhelming to someone new to exploring the world of Fostex RP mods.

In 2015, Tyll Hertsens of InnerFidelity posted measurements of many of the popular Fostex RP variants including both commercial and DIY modifications.

In general, the modifications largely fall under three broad categories (with some overlap):

  1. Durability modifications
  2. Comfort modifications
  3. Sound modifications

Durability Modifications

The number one durability issue with the RP mk3 series is the cable and headphone socket. The stock cables are thick, unwieldy and oddly fragile. The jack is a custom 3.5mm TRS socket with a locking slot that is acknowledged to have widespread issues including causing channel imbalances, static and audio dropouts.

“…noise may occur when the headphone plug is connected and turned 90 degrees to lock it. Unless noise is heard with the plug in the locked position, it is not a problem. However, if noise or uneven reproduction volume occurs, there is a possibility that the headphone plug has become dirty. In such a case, wipe the plug with a dry cloth.” – Fostex
The stock T50RP mk2 and orange cable from Head-Fi.
The stock T50RP mk2 and orange cable from Head-Fi.
My stock short orange cable caught on a kitchen drawer knob, which I’ve done countless times with countless headphones. Unfortunately, this is the first time the cable stopped working after a light tug. A bit disappointing, since I liked the cheeky orange color.

The easy solution for cable replacement is the angled Kevlar wrapped, “Audio Only” cable from V-Moda. While technically the 45 degree angled end is intended for plugging into the source, it functions perfectly when that end is plugged into the Fostex cup. It’s tougher (can withstand over 1 million bends – 100x industry standard), less expensive ($12), available in 5 colors (black, light grey, dark grey, purple and red), thinner and lighter than the original cables.

A red V-Moda Audio Only cable from Head-Fi.
A red V-Moda Audio Only cable from Head-Fi.

The V-Moda cable is simply better.

Comfort Modifications

Comfort Band

While the stock band was upgraded between mk2 and mk3 versions, from an unpadded soft rubber to a lightly padded faux-leather, the overall comfort of the modern RP series is more utilitarian than luxurious. An easy fix is to add a fabric or leather suspension strap beneath the existing headband, attached via the two screws on either side.

The purpose of a suspension strap is to distribute the headphone’s weight across the top of the head evenly, eliminating hot spots and making the headphone feel lighter.

Installation of a strap requires replacing the two screws on the headband with longer screws. These screws control the motion of the slider bars and the cups. Since they are threaded into plastic, it is easy to strip the screw holes unless care is taken when tightening.
My DIY comfort band installed.
My DIY comfort band installed.

The MrSpeakers Comfort Strap is the basic epitome of this modification. Picture 10 inches of a black leather belt and you have the general idea. Mod House Audio offers an upgraded (and almost twice as expensive – $25 vs $15) Suspension Strap out of hand-stitched deerskin suede or Alcantara.

Rather than adding a strap, another option is to replace the headband itself with a padded version such as the Fostex Comfort Headband from Mayflower Electronics. ZMF Headphones offers a Pilot Pad in suede or lambskin that wraps around the existing headband and provides additional cushioning.

My DIY Leather Comfort Band

A DIY leather comfort strap was the obvious (and easy) starting place for my own modifications of a stock T50RP mk3.

Since ‘frugal DIY’ is the name of the game around here, I make a quick trip to the local used clothing store and bought a black leather coat for a few dollars. Clearly more than enough leather for very many comfort straps or future DIY projects.

Rough leather piece cut from the jacket and covered in contact cement glue.
Rough leather piece cut from the jacket and covered in contact cement glue.

I cut out an approximately 12”x4” rectangle out of the coat and covered the inner suede surface with low odour contact cement. After 40 minutes, I folded it in half lengthwise to adhere the leather to itself, and rolled it flat with a rolling pin.

Excess material cut from the strap using a sharp blade.
Excess material cut from the strap using a sharp blade.

Using a metal ruler and a fresh X-Acto blade, it is easy to get a very clean edge on the leather. I cut the strap 10” long and 1 5/8” wide. I notched the ends (1 3/8”) and then cut a gentle angle on each edge for aesthetics.

Replacement silver screws to hold the comfort strap in place.
Replacement silver screws to hold the comfort strap in place.

A quick dig through my parts bin netted a couple of replacement screws that were about ¼” longer and with slightly larger heads than stock. This meant a washer was not required to keep from pulling through the leather. These worked perfectly.

Sound Modifications

Ear Pads

Earpads are where the rubber meets the road as far as modifications go. Not only are they a must for comfort, ear pads also play a major role in sound quality. Most folks find the stock earpads much too shallow and thin for long term listening sessions. There are a huge number of options out there as many pads from other manufacturers will fit the RP series.

When you select replacement pads, you must balance comfort (size, breathability, thickness, and firmness) with the corresponding changes in sound that is created by each of those factors.

Brainwavz HM5 Hybrid Pads (top) and stock ear pads (below).
Brainwavz HM5 Hybrid Pads (top) and stock ear pads (below).
Honestly, you could start and end your Fostex RP modification journey just with ear pad changes.
The Dekoni Blue version from Addicted to Audio.
The Dekoni Blue version from Addicted to Audio.

For instance, the extremely well reviewed Dekoni Blue version is often described as surpassing the sound quality of a stock T50RP mk3:

“…the addition of the Dekoni Hybrid Ear Pads and other proprietary changes makes this headphone a better version of the original by being less fatiguing and smoother all around with an extended bass response.” – Dekoni Audio
A Dekoni Blue empty cup.
A Dekoni Blue empty cup.

I assume by their unspecified “proprietary changes” they mean “we made the cups blue and wrote Dekoni on the headband”. Beyond that, to justify almost twice the price, the only real alteration from the Fostex T50RP mk3 appears to be the change to the Dekoni Elite Hybrid Pads (plus a pair of included Elite Velour Pads).

The Dekoni Blue are produced by Fostex in their manufacturing facilities.
The interior of the Dekoni Blue driver baffle.
The interior of the Dekoni Blue driver baffle.
“While I won’t go into detail about the specific changes within the headphone components we made I will say the addition of the asymmetrical ear pads we made for this headphone were designed in such a way to obscure the outer edges of the driver which, in turn, caused the “fatigue peak” to go away.” – Dekoni Audio

What’s most remarkable about the Blue is the overwhelmingly positive critical response to this minimal modification. This is a completely safe and non-invasive modification (you don’t have to unscrew a single screw) that yields tangible results. Proof that absolutely anyone can modify the RP series!

The front side of the Dekoni Blue driver baffle.
The front side of the Dekoni Blue driver baffle.

Here are a few examples of reviewers comparing the Blue to the stock T50RP mk3:

“Dekoni has smoothed off a lot of the rough edges of the t50 and while some will dismiss it as something they could do at home for half the expense and a couple hours effort, I don’t believe that is the case.” – Audiofool Reviews
“…have created a completely new sound signature than the original base T50RP MKIII. Every single change to the Blue has increased the level of personal usability, comfort, and sound from the originals to a new plateau. They are on a whole different level.” – Headphone Guru
“One reason I didn’t get the TR50 MKIII is that, though I liked its basic nature, I didn’t jive with its highs, which, to me, are grainier than Blue’s.” – Headphonia

Ear pads do many things beyond comfort. They change the distance, shape and volume of the space between the driver and the ear. Some pads are angled or asymmetrical which correspondingly changes the angle of the driver to the ear.

The exterior material (cowhide or lambskin leather, synthetic protein leather, alcantara, velour, etc.), interior foam properties (memory, stiff, soft, etc.) and finish (smooth, fenestrated, hybrid, etc.) all have different reflective and absorptive sound properties.

All these changes directly impact the perceived sound of the headphone. In general, smoother finishes, like leather and protein (synthetic leather) tend to increase bass quantity, although perhaps at the expense of quality (loose vs tight bass). There are reports that cowhide is brighter sounding that lambskin or protein leather.

Softer velour finishes tend to decrease the quantity of bass. Perforated pads increase air flow and clarity but can decrease bass response. Increasing pad thickness directly affects soundstage. Hybrid pads (smooth, perforated sides with a soft velour top) like the Dekoni pads, tend to be popular as a good sounding, and well designed, compromise of all these factors.

A proper seal is critical to good sound. A gap between the ear and pad will dramatically reduce low frequency response due to changes in acoustic pressure. The seal is a function of both ear and pad size and shape, and the compliance of the inner foam material. In addition, the quality of the seal influences isolation from outside sounds.

Frequency measurements of a variety of pads on the T50RP from DIY Audio Heaven.
Frequency measurements of a variety of pads on the T50RP from DIY Audio Heaven.

Clearly not every ear shape is going to be comfortable with, nor gain the same acoustic benefit of all ear pads. This is likely why there are so many different recommendations floating around.

Based on the positive reviews of the Dekoni Elite Pads (and the Dekoni Blue headphones) they are a safe, albeit expensive, option for a stock mk3 headphone.

My Experiences with Ear Pad Rolling

It is beneficial to explain my preferences before you take any advice from my personal experiences. Most important to consider is what I was hoping to achieve by modifying the T50RP mk3.

Since I have a plethora of open back headphones, I was looking to create something more v-shaped and fun (rather than accurate) to listen to. I wanted to capitalize on that potentially big but fast, planar bass and end up with something very different than what I was used to.

And, of course, to make them more comfortable for long term listening.

So if your personal tastes or modding goals differ from my own, then please just take this article as a discussion of what is possible. A single journey with a wide variety of potential destinations.
A large angled lambskin leather MrSpeakers Alpha Dog ear pad.
A large angled lambskin leather MrSpeakers Alpha Dog ear pad.

I’m going to minimally discuss the very subjective changes in sound that each pad provides. It just isn’t universally applicable, except in the broad spokes we discussed above. Each step of internal modification impacts the interaction with the chosen pads. In other words, what you decide to do inside the cups will likely change what ear pads you select in the end.

Ear PadManufacturerPriceMaterialNotes
110*90MM Leather Replacement Pillow Cushion - AngledZGBC Renensin Store - AliExpress$9Protein LeatherSoft density. Thick.

Foam quick to recover shape, less ‘memory foam’ feeling. Does not compress easily on head.

Mid-sized hole. Larger than stock.

Available in black, blue, red and white.

Technically not left and right (both sides identical) as seam is on top of one and bottom of other when installed.

Superficially tore the mounting lip when trying to install. Material does not feel tough.

Low end and sound stage increase.
Flat Oval HybridBrainwavz$30Exterior edge: protein leather

Interior edge: perforated protein leather.

Top: velour.
Low density. Soft

Much larger hole than stock. Largest hole of all tested.

Sound becomes sharper and treble is less pleasant.
Stock T50RP mk3Fostex$30Protein LeatherVery thin. Low density memory foam (easy to compress).

Long term comfort lacking due to thinness.

The stock sound is clear and bright.
HPAEC1540Shure$40AlcantaraLow density. Soft.

Slightly wider than stock sized hole.

Alacantara is a really nice material for being less hot than leather but with many of the positive audio properties. Somewhat less bass than stock pads.

Compresses easily so driver distance is more similar to stock pads.
Elite Velour (included with Blue)Dekoni$50VelourFirm density. Does not compress much.

Shorter but wider hole than stock. Smallest hole size of all tested.

Smooth and balanced sound. Very high quality and comfortable.
Alpha Dogs PadsMrSpeakers$60Lambskin leatherVery thick.

Large rectangular holes.

Extremely soft and comfortable. Creates a big seal around the ear.

Bass response fuller and lower, although can tend towards less controlled, and sounds a bit overwhelming depending on music.

Midrange and high end slightly recessed but reasonably balanced.

Soundstage appears to increase.

I guess, as they say, the proof is in the pudding. What pads have stayed the longest on my modded Fostex?

Believe it or not, the sub $9 AliExpress angled wonders have seen the most use. They are a cheap knockoff of the Alpha Dog pads. I can say nothing good about the quality of build or materials (except perhaps it is appropriate at this price point). However, they boost the low end and seem to best suit what I’m looking for.

Note, I use the Alpha Dog pads almost exclusively with the Alpha Dog headphones as intended.

MrSpeakers Alpha Dogs pad (top left), AliExpress pad (top center), Dekoni Elite Velour pad (top right), Stock T50RP mk3 pad (bottom left), Shure 1540 pad (bottom center), Brainwavz Flat Oval Hybrid pad (bottom right).
MrSpeakers Alpha Dogs pad (top left), AliExpress pad (top center), Dekoni Elite Velour pad (top right), Stock T50RP mk3 pad (bottom left), Shure 1540 pad (bottom center), Brainwavz Flat Oval Hybrid pad (bottom right).

I’m also a fan of the Dekoni pads. They are extremely well made and comfortable and seem to give a nicely balanced feel to the sound. Not so much a bass-head pad, but it’s impossible to complain about the quality.

My least favorites are the Brainwavz Hybrid pads. They seem to make the high end sharper and much less pleasant for me. Build quality and comfort are quite good though.

While the mk2 and mk3 series are very similar in appearance, modifications and ear pad sound should not be considered to be identical across both series. The series does sound different.

Dampening Material Modifications

The stock RP series cup is a thin plastic shell, empty of any sort of damping material. This means it is prone to internal reflections and vibrations. It’s an easy place to start improving things.

My basic set of modding tools. All I needed to do most of the upgrades discussed here.
My basic set of modding tools. All I needed to do most of the upgrades discussed here.
“The most common modification usually takes care of the boomy bass and housing reverb problems which resulted from improper acoustics in the housing design. – Headfonia

There are two categories of damping materials with two different purposes.

1. Reducing unwanted air vibrations. Adding dampening material to the interior of the cups is done in an effort to eliminate reflections and resonance. This includes:

The dual material layer damping material found in a popular commercial mod. Unnamed to preserve ownership of intellectual material.
The dual layer damping material found in a popular commercial mod. Unnamed to preserve ownership of intellectual material.

2. Reducing unwanted mechanical vibrations. Adding mass to the interior of the driver baffle is done in an effort to reduce mechanical motion. This includes:

The cup interior found in a commercial modification, covered with clay and carbon rod pieces. Unnamed to preserve ownership of intellectual material.
The cup interior found in a commercial modification, covered with clay and carbon rod pieces. Unnamed to preserve ownership of intellectual material.
While it is possible to first line the interior of the cup with a mass loaded material and then cover it with acoustic foam, this is seldom done due to the unwanted increase in weight and lack of reported positive results.
MrSpeakers Mad Dogs internal modifications from hififorum.de.
MrSpeakers Mad Dogs internal modifications from hififorum.de.

My Experiences with Adding Damping Materials

Most of the modifications involve some form of the following:

  1. Fill all the spaces around the driver with Newplast clay.
  2. Line the cup with acoustic foam.
  3. Add loose cotton to the empty space behind the driver.
The Mayflower Mod Kit.
The Mayflower Mod Kit.

While you can source all these materials individually, Mayflower Electronics makes it very easy by having a preassembled Mod Kit. For $20, you receive:

  • Roughly 25 grams of Newplast clay
  • Stick on soft and stiff felt
  • SilverStone dampening foam
  • Rolled cotton

All you need for several attempts at basic damping modifications (or for a couple pairs of headphones). They even have an instruction video.

Removing four easily accessible Philips screws is all that is needed to liberate the driver baffle from the back of the cup. If you are careful, there is sufficient play in the internal wires to complete all modifications without removing the driver or wires.

Black Newplast clay flattened into all the gaps on the interior driver baffle. Small Phillips screwdriver used to make a pattern in the surface.
Black Newplast clay flattened into all the gaps on the interior driver baffle. Small Phillips screwdriver used to make a pattern in the surface.

Clay is added to all the indentations surrounding the driver on the interior of the baffle. A flat tool is used to scrape the surface of the clay flat and many modders recommend adding random indentations with a sharp point to further minimize the chance of reflections.

My only complaint about the addition of clay is that this noticeably increases the weight of the T50RP mk3 on my head. I definitely prefer the weight without the clay, but I am getting used to it.
The measurements and template for cutting the foam pieces.
The measurements and template for cutting the foam pieces.

Mayflower Electronics includes a rough template for cutting out the foam shapes for adhesion to the cup interior.

Larger foam pieces are adhered to the cup walls while smaller segments are stuck on top of the large pieces of foam.
Larger foam pieces are adhered to the cup walls while smaller segments are stuck on top of the large pieces of foam.

The amount of loose cotton is one place you can play around with how damping material affects the sound. In general, I found that the addition of cotton improved clarity and generally flattened out the sound (reduced the mid-bass hump present on the stock version). I added more until I was satisfied with what I was hearing.

I went with enough cotton to fill the space but not over the center raised section or vent material.
I went with enough cotton to fill the space but not over the center raised section or vent material.

There is some debate around whether the raised flat surface centered directly behind the driver should be covered with foam or cotton, as the theory is that it reflects certain frequencies directly back through the driver to the ear.

Vent Tuning Modifications

As discussed above, the mk3 versions of the RP series use identical components but vary in how sealed the ports are in the back of the cups. The stock mk3 models do sound different, and differ only in this vent opening:

  • The T20RP mk3 vents are essentially ‘open’ and has the most bass.
  • The T40RP mk3 vents are ‘closed’ and has the least bass response.
  • The T50RP mk3 vents are partially covered, and is typically considered the best sounding and most balanced of the mk3 lineup.

You will have to find the right amount venting based on your tastes. Many report that opening the vents fully (to increase bass) and then using cotton to dampen reflections gives the most low end performance. I ended up leaving the stock felt (covering the T50RP vents) in place. This yielded reasonable amounts of clean, and not overwhelming, low end.

Tape can easily be applied and removed to the outside of the cup (without opening it up) to test different amounts of coverage. Black electrical tape is all but invisible on the outside of the black cup.
Experimenting with covering the four front vents with tape and placing a bit of green felt on the front of the driver.
Experimenting with covering the four front vents with tape and placing a bit of green felt on the front of the driver.

There are also 4 vents on the front side of the baffle surrounding the driver. Covering these with tape noticeably increases bass response. Very easy to experiment with how many vents covered sounds best to you. The tape is undetectable once ear pads are installed.

Mayflower offers a $10 Baffle Upgrade to replace the foam circle on the outside of the cup surrounding the driver. It is made of a heavier neoprene-like material that additionally blocks air from escaping.

After much experimentation, I ended up covering all the front vents with tape and using the Mayflower Baffle Upgrade.

Exterior Driver Covering Modifications

There is a thin fabric on the exterior of the driver (front ear-side). This can be removed or replaced with another material. Most pads have a fabric that covers the driver as well. This can also be removed, or another material (a single ply of toilet tissue, cotton, or foam for example) can be added underneath the pad over the driver to attenuate high frequencies.

Two adhesive MrSpeakers Doggie Treats installed on the center of the driver exterior.
Two adhesive MrSpeakers Doggie Treats installed on the center of the driver exterior.

MrSpeakers offered adhesive foam rectangles (called Doggie Treats) that were meant to adhere to the front surface of the driver. They were designed to be placed in a line across the center of the driver. One or two could be added to reduce harshness or treble as desired.

I found the treble could be harsh with some recordings listening to the stock T50RP. I played around with adding different materials and adhesive felt to the front of the driver surface (and in different positions).

In the end, I removed all of it and found ear pads that sounded the best to me. While excess treble was tamed, I discovered that adding anything to this area made the overall sound too muffled for my tastes.

Driver Modifications

There is a thin white material adhered to the back of the drivers. There are many opinions on what sounds best, and if you should remove it. Unfortunately, if you elect to remove the material, or adhere another substance to it, it is all but impossible to get it back to stock form.

“Some, like bluemonkeyflier (BMF), prefer the stock white material left in place. Some, like mrspeakers (rastapants) prefer replacing the stock material with stiff felt. Some like transpore tape, some like suede, some like their drivers completely naked.” – Head-fi

Material may be adhered to the back of the driver to reflect sound and increase treble response. Adhesive foam or felt dots are sometimes used, or Transpore surgical tape strips may be placed over the back (or along the edges) of the driver.

Felt added to the rear of the MrSpeakers Alpha Dog driver.
Felt added to the rear of the MrSpeakers Alpha Dog driver.
Since I did not want to damage the stock white material, nor increase the treble, this is one aspect of modification that I did not pursue.

Professional Modifications

To fully discuss the range of available modifications for the Fostex RP series, we need to take a look at the companies who are currently offering, or have offered, professional modification services. Many began simply as headphone enthusiasts that started small and went on to found their own businesses.

We will take a look at:

  • Cascadia Audio
  • Enigmatic Audio
  • Mayflower Electronics
  • Mod House Audio
  • MrSpeakers
  • Smeggy Thunderpants
  • ZMF

Cascadia Audio

Reddit user /u/Mad_Economist, located in the Pacific Northwest, formed Cascadia Audio and currently offers the Talos 2 (modified Fostex T50RP mk3) for $325. As expected, it is a successor to their previously offered Talos model. They describe the sound as:

“…tastefully boosted bass and treble for an energetic sound without excessively overshadowing the midrange….a tastefully V-shaped response which adds excitement to music without overly embellishing.”
The Talos 2 from Cascadia Audio.
The Talos 2 from Cascadia Audio.

The Talos 2 includes a leather comfort strap, rubber bumpers on the headband sliders and a pair of custom-made hybrid (sheepskin and velour) ear pads. Also included is a Cascadia branded hard case for transportation, storage and display.

The stock cups on the Talos 2 are painted a distinctive hammered copper color (the previous Talos used decals). The stock headphone jack on the cup is also replaced with a sturdier 3.5mm TRRS socket allowing for single ended or balanced connections with the appropriate cable.

Enigmatic Audio

Enigmatic Audio was created by Head-Fi member and sound engineer, Luis Flores, otherwise known online as LFF. His current lineup of modified T50RP headphones are the Paradox ($600) and the Starving Student Slant ($700). He also offers the Enigma (from $900), which is not a T50RP modification, but a completely new closed back headphone.

The Paradox from Enigmatic Audio.
The Paradox from Enigmatic Audio.

Enigmatic Audio started by modding customer-supplied headphones, and quickly garnered online praise as being the best sounding T50RP available. Perhaps to protect intellectual property, the website is disappointingly spartan and devoid of information. From the few pictures available, it appears that mk2 versions are being used.

“Light. Well-built. balanced sound. Sounds open but it’s closed. Quite easy to drive.” – Headphonia

Originally, the Paradox was available at three price points and modification levels. Level 1 included internal sound modifications plus HiFiMan or Fisher Audio ear pads. Level 2 added a dual entry cable, comfort strap and custom paint job. Level 3 included a glossy black or white paint scheme, storage bag and mini-XLR cable and sockets.

The Enigma from Enigmatic Audio.
The Enigma from Enigmatic Audio.

Unfortunately, it isn’t clear on the website what is currently included in the $600 Paradox, or what the $700 Slant upgrade includes. Both share the same photo, which, like their namesake, is a bit of an enigma.

Mayflower Electronics

In addition to the handy Mod Kit and Baffle Upgrade offered by Mayflower Electronics, they also offer two models of modified T50RP mk3 headphones for sale: the Version 1 (starting at $265) and the Version 3 Bass Model (starting at $255).

The Version 1 from Mayflower Electronics with Shure 1540 ear pads.
The Version 1 from Mayflower Electronics with Shure 1540 ear pads.

The Version 1 adds internal changes to mass loading and baffle alterations and includes Shure 1540 ear pads. The version 3 includes these internal modifications with pleather ear pads. They describe the Version 1 as:

“…our cleanest sounding model with a flat frequency response that does everything right without overdoing any of the lows, mids or highs. Great for gaming, general purpose audio listening and studio mixing.”

Version 3 is described as a:

“…bass heavy sounding model with a “V” shaped sound signature. This means the bass is elevated, mids are slightly lowered and has slightly higher treble than the stock T50RP.”

Mod House Audio

Mod House Audio was founded by Ryan Dietz. He currently offers both a modding service, entitled Argon Conversion, for mk2 ($60) or mk3 ($90) headphones (T20, T40, T50, T60 models accepted). In addition, a fully modified T50RP mk3 Argon is available for $245 and a T60RP Argon for $385.

The Argon from Mod House Audio.
The Argon from Mod House Audio.

The Argons include a hand-stitched deerskin suede comfort strap, ear-side baffle damping, custom driver modification, ZMF ear pads, V-Moda cable, metallic vinyl decals on the cups and 3D printed badges.

In addition, Mod House Audio offers a balanced cable upgrade option that replaces the stock socket with a new 3.5mm TRRS jack ($35) and a balanced Periapt cable ($62).

The Argon sound is described as:

“tuned to provide a balanced sound with tastefully elevated bass with the oval pads mounted in baffle lip, if you would like to add a little bass feel free to mount the pads around the outside of the cups similar to Mk2 style mods.”

MrSpeakers

MrSpeakers located in San Diego California, was founded in 2012 by Dan Clark, an electrical engineer. Unlike many of the other companies in this list, MrSpeakers has moved beyond a tiny one-man-show modding the Fostex RP series, to an industry recognized producer of high-end electrostatic headphones. Their new Ether 2 is the world’s lightest planar magnetic, over-ear headphones.

MrSpeakers no longer sells modified Fostex headphones and is entirely focussed on their own designed and manufactured products.

Dan started out humbly, creating the Mad Dog which was a fairly standard modified T50RP mk2. This progressed into the Alpha Dog ($600), which gained prestige by using the world’s first 3D printed cups. The Alpha Prime was the ultimate version, costing $1000 in 2015.

The MrSpeakers Alpha Dog and Mad Dog lineup from 6moons Audio Reviews.
The MrSpeakers Alpha Dog and Mad Dog lineup from 6moons Audio Reviews.
One of the main things that MrSpeakers offered was reputable quality control and repeatability often missing from the DIY community.
“…the Alpha Dog is easier to like with its more dynamic sound, but is also superior in terms of technicalities… This is the best T50RP-mod headphones I’ve heard… Let’s start with the best part first. The depth in the sound. The Alpha Dog is superior to every other T50RP-mod I know in terms of depth.” – Headfonia

In addition to the unique, and beautifully finished red cups (featuring 3D printed, double walled lattice) the Alphas were bundled with a metal stand, unbalanced Canare Quad Star cables with hirose cup connections (optional XLR were available), leather comfort strap, velvet bag and cleaning cloth kit.

The 3D printed port vent inside the Alpha Dog from Headfonics.
The 3D printed port vent inside the Alpha Dog from Headfonics.

Another unique feature of the Alpha Dogs was the bass tuning port located in each cup. With a tiny Allen (hex) key, the port opening could be screwed more or less open to change the level of bass. Dan created a handy video explaining the process.

“Stock Alpha Dogs are tuned neutral from the factory, and the default factory setting gives you one of the best flat monitoring tunings I’ve heard among any headphones. Extremely flat with amazing clarity.” – Headfonia
I can attest that the Alpha Dog sounds very different that anything I was able to create myself with standard modding methods. It has a very neutral tonality and a surprisingly deep soundstage for a closed back headphone. It sounds very, very good (as it should for the original $600 price tag)!

Smeggy Thunderpants

One of the originators of the Fostex modification scene was Head-Fi member Smeggy (Gary) and his (bizarrely named) Thunderpants TP1 headphones (£350 UK pounds). He’s since disappeared, much to the regret of many who owned or heard the Thunderpants.

Custom wooden cups from the (now defunct) Smeggy Thunderpants website.
Custom wooden cups from the (now defunct) Smeggy Thunderpants website.
“… The latest Mad Dog, however, is still not quite the level of the Thunderpants. I thought that the Thunderpants was so ahead beyond the other modded Fostexes and I was sad to hear that you can no longer buy them…” – Headfonia

The Thunderpants was not simply a standard modification and featured gorgeous, hand-made wooden cups. In 2011, his Smeggy’s blog chronicled his experimentation with engraving, sanding, and finishes.

“…none of the other modded T50RPs even got close of the quality of the Thunderpants. Crystal clean, clear, yet retaining the typical smoothness of Fostex orthodynamic drivers, all the instruments well arranged without the slightest hints of confusion in the soundstage.

The presentation is moderately laid back and the tonality is a very slight v-shaped.” – Headfonia

Beautiful finished cups on the Smeggy Thunderpants from Headphonia.
Beautiful finished cups on the Smeggy Thunderpants from Headphonia.

Eventually Smeggy stopped doing business and there were ultimately reports of poor buyer experiences. A very promising start to the commercial Fostex RP modding scene that didn’t quite pan out.

ZMF Headphones

ZMF Headphones, founded by Zach Mehrbach, currently offers the Classic ($300), Blackwood ($700) and Ori ($900). Every ZMF headphone is assembled, tuned, and inspected by Zach. Previous models included the Vibro and Omni.

ZMF Classics from ZMF Headphones.
ZMF Classics from ZMF Headphones.

ZMF modifications employ (as expected) a combination of driver enclosure alteration and physical damping material. Modifications are done to the baffle to change the venting and airflow. One of the biggest improvements is a modification to stabilize the driver, lowering distortion and cleaning up the sound, eliminating any unwanted bass rattle noises.

“The ZMF sound is known as a synergy between analytical and musical, with an uncanny ability to present detail without losing organic enjoyable attributes.” – Hearfidelity

The Classic includes a cowhide leather band (pilot pad available $20), replaced 3.5mm jack (balanced Audeze style available $60), vinyl decals, ZMF ear pads, and optional case ($50). Channels are matched to within 1db +/-.

“…this might be the only stock t50 mod I’ve made that has “just” the right amount of damping. It does suffer slightly from using the original enclosure which has thin plastic walls, but the resonances are minimalized and it still keeps an amount of decay that keeps things natural… Also, when you listen to a stock t50 next to a Classic, it’s pretty cool, because you can tell they are two completely different things.” – Hearfidelity

The Classic maintains the traditional Fostex T50 mk3 build, but Blackwood and Ori both add wooden cups.

The ZMF Blackwood from Head-Fi.
The ZMF Blackwood from Head-Fi.

The Blackwood is available with Manchurian Ash or Blackwood cups with choice of paint color on headband slider and cable connector type. The Ori is available with Burmese Padauk, Cherry or Wenge cups with similar cable and color options.

Conclusion

Whew! We made it!

Hopefully you now have a much greater knowledge and appreciation of why the Fostex RP series has been successful and popular for decades. I certainly do.

This was an extremely time consuming article to prepare and write. The sheer volume and variety of Fostex RP series tuning is overwhelming. Add that to the time invested in researching and testing my own modification process. It was lots of work, but it was a really interesting and fun task.

Is the Fostex RP mk3 series the finest sounding headphones in existence? Nope. Are they the best made, highest quality and most attractive headphones ever? Not even close. The stock pads, cables and socket are poor, and the aesthetics are much more suited for the recording studio than for public.

In the end though, it all comes down to one thing. Potential. The RP series can be made into almost anything you could want from a headphone.

My MrSpeakers Alpha Dogs powered by a Schiit Lyr.
My MrSpeakers Alpha Dogs powered by a Schiit Lyr.

Ultimately, I’m delighted with my two pairs of modified Fostex. My own T50RP mk3 has turned out just how I wanted it to sound. Feel like rocking out to Aphex Twin? Well, do I have the headphones for you. Want a much more balanced sound? The Alpha Dogs are remarkably civilized, detailed and they present a terrific soundstage.

On a recent trip to visit a music-loving friend, I brought along a variety of headphones and amplifiers for him to try. This included the Sennheiser HD58X, Meze 99 Classics, Beyerdynamic T1 (generation 1) and my own T50RP mk3 mod. Hands down his favorite sound were the Fostex.

Every headphone enthusiast owes it to themselves to check out a pair of the Fostex RP series. Hear for yourself what all the history, excitement and hype is about. Check out how small changes can dramatically alter the sound of this orthodynamic headphones. Get caught up in the fun of tuning the sound to your own tastes.

Now if I can just resist the compelling allure of the Fostex T60. Mmmmm… wooden cups.