Driving the Fostex T50RP MK3 headphones

After too many years of abstinence from headphones, I was given as a present a pair of Fostex T50RP MK3s.

These headphones have an excellent VFM, having true planar magnetic drivers at a price point of ~150€.

This is what they look like on the inside:

Fostex inside (Medium)

Picture borrowed from here: http://www.head-fi.org/t/763009/fostex-new-rp-headphones-t50rpmk3-t40rpmk3-and-t20rpmk3/375#post_12077324

There is a drawback though, and that is their low sensitivity. ~92dbs mean that my current cell phone has no hope of making them rock. No chance at all. Same goes for pretty much any USB-powered DAC/headphone amp. So anyone serious about driving these headphones needs to look into a proper headphone amplifier.

I said to myself that I should try to build a proper amp with parts that I already had lying around, so I looked at my stockpile. I realized that I had a brand new pair of AMB alpha20 class-a line amplifiers, rated at 3.0Wrms into 33Ω with a ±18VDC power supply.

AMB alpha20

The T50RP are 50Ω and can handle up to 3000mW, so I thought that they would be a good match.

Powering the alphas by my bench power supply, I connected them to my audio card’s line out (an Auzen X-Meridian) and hooked up the headphones. The result was impressive. The alphas delived sound that was clear, crisp, with very little distortion. Plus they managed to achieve SPLs well into eardrum-damaging territory. Good.

Next up was the power supply. I had a set of Salas BiB shunt regulators that were collecting dust, so they would do just fine.

Salas BiB 1.1 alpha20

I set them up for a CCS current of ~300mA.

The case would be a Modushop.biz Galaxy 1U aluminum one that I had bought for a project that never went beyond the design phase.

I was missing a transformer that would fit inside the 1U case, so I bought a 25VA unit from Mouser.

At last but not least, I needed a 1/4″ TRS jack that could be mounted on a 10mm thick aluminum face. Neutrik had the perfect part for the job:

nj3fp6c-bag

So, all I ended up purchasing was the trafo and the 1/4″ jack. Nice.

2015-11-29 17.57.20_resize

So now I had a functioning headphone amplifier.

The next step is to add a USB to I2S interface, a DAC, some sort of volume control, another power supply to power them and an Arduino & Screen to control them. Space might be an issue..

Comments

comments

3 thoughts on “Driving the Fostex T50RP MK3 headphones

  1. Will you test it straight out of DAM1021 raw output ? Could be a good “all discrete” match.

    For the dac, you may also go the jlsounds way: they sell compact, stackable & affordable usb + dac combination that may fit your case – that’ll depends on the size of your power supply. Salas BiB are not exactly for the size-constrained environments 😀

    • I originally built the alpha20s as output buffers for the Soekris’ raw outputs but they were not as transparent as I would have liked so I took them out.

      They do make excellent headphone amps though. 🙂

      For a DAC I already have an ES9023 based little board that I will use, plus I have plenty of USB to I2S interfaces laying around.

      The Salas BIB are indeed a tight fit, but when I listened to how they sounded (versus a classic LM317/LM337 regulator) I just couldn’t not use them. 😀

  2. Cool, im looking for a solution to power a set of AKG headphones with mobile devices too. Laptop and Ipad have trouble with them, my Oneplus (I see you have one too!) seems to drive them better than expected.
    This seems like a very good solution with excellent value for money.
    Nice projects, I Like!

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