OpAmp + MOSFET = a decent headphone amplifier

Headphone amplifier and Sennheiser HD-580[Читать на русском]

Flood in the Internet

These days search engines throw tons of schematics of "Class A headphone amplifiers" on us. There we can find anything: from monsters capable of driving big loudspeakers to humble OpAmps heavily loaded by 30 Ohms without any hope for help. The big wave of so-called hybrid headphone amps is scary: they look very high-end'ish, and I am the first one to advocate valves in sound reproduction, but mind you - using 15V at anode of ECC88 is insane.

Probably I would never be writing about this headphone amp schematic. But recently I observed one good man torturing a very decent vacuum tube with unreasonable low anode voltage and grid currents in a hopeless attempt to build what supposed to become his reference headphone amplifier. I tried to give him few hints here and there, but had to give up on that once he told me the model of the headphones he was building that monster for: that was some computer-grade thingy, "hi-fi model headphone intended for use with MP3 players and computer systems".

Rescue online

Today I am striving to avoid negative feedback loops in my audio designs. But this article is about a humble all-common-sense OpAmp-based headphone amplifier, the one that served me well for many years already since the last Century. It's easy and inexpensive to build, virtually does not require any tuning and yet can serve very sophisticated ears 😉

The ideas that this amp is build on have been found on the Net, obviously still in the last Century 🙂 Should you know the originator of this or similar layout - please drop me a note - I'd be happy to share the source with my readers too.

Let's take a quality operational amplifier and force its output stage into the real class "A" operation mode. For achieving that goal we simply need to load its output by a current source. The idea most probably belongs to Walt Jung and was first used in Philips DAC960 in nineties. As you know except the output stage all other OpAmp's guts already work in pure class "A". The effect of this trick that shifts OpAmp into the linear mode is illustrated in the article about the Wien bridge oscillator. There we used a budget OpAmp thus the effect was clearly visible.

Still the little OpAmp is not the best choice for driving 30 Ohms headphones. It cries for a current amplifier. Here we'll use easily available IR MOSFET's. Provided the complementary source followers idling at some 100 mA it will stay in pure class "A" while working on 30 Ohms or any other load with higher impedance.

The creation

Schematic diagram of the class "A" headphone amplifier

  • OpAmp = OPA2134
  • VD1 = red LED 1.7 V
  • VT1 = MPS2907A
  • VT2 = IRF610
  • VT3 = IRF9610
  • VD2, VD3 = 9...15 V Zeners
  • C1 = 1uF (film)
  • RP1 = 100 KOhm, log
  • R1, R3 = 1 KOhm
  • R2 = 100 KOhm
  • R4 = 2 KOhm
  • R5, R6 = 3 KOhm
  • R7 = 360 Ohm (*)
  • R8, R9 = 100 Ohm
  • R10, R11 = 3.3 Ohm
  • C2...C7 = 0.1 uF (film)
  • C8, C9 >= 4700 uF x 35 V
  • C10 = 47 uF x 16 V (aluminum)

The bias voltage on MOSFET's gates is developed over R5 with the same current that loads OpAmp and shifts it into the class "A". The only tuning that this layout might require would be setting the idle current through the source followers at around 100 mA. For tuning this idle current one may need to adjust value of R7.

A small film capacitor C11 of 100pF between OpAmp' output and its inverting input may proof useful in order to prevent oscillations when the amp is maxed out on output voltage (i.e. "clipping").

The result

That's how I arranged this headphone amp into the box some x-teen years ago. Should I be constructing this today I most probably would put the transformer in a separate box.
Headphone amplifier in its case, solder side view
Let me brag a bit about abilities of this humble creature 🙂 It once won a listening test against an all-tube amp. Both devices were loaded on a pair of reasonably low impedance and low sensitivity loudspeakers. By then it was the owner of the tube amp who admitted that my little silicone thing produced warmer and more honest sound and slightly better detailed stage.
Bottom line: if you need a decent headphone amplifier that is capable of driving not only 30 Ohm buds, but also audiophile quality 300 Ohm headphones and sometimes even sensitive loudspeakers - go ahead and build this one. It's cheap, simple, requires almost no tuning. Later on, when you are up to the serious vacuum tube based construction - this one will still serve you as a reference device to compare with 😉

Amplifier in the red-wood and aluminum case

The bonus

As I just mentioned above this device is capable of driving low impedance loudspeakers. In that case it might require slightly more meat in the power transformer and filtering capacitors. Still it won't be able to give out very high watts because the voltage swing is limited by the operational amplifier. Also please note that while working on the low impedance load the output stage will operate in a conventional class "AB" mode.
I guess the fact that you are reading this far might mean the topic was worth your time. I appreciate it a lot, honestly! It would be great if you drop me a note in the comments and also click those buttons below the text to let your friends taking a look at this simple yet very high quality headphone amplifier. Thank you!

12 thoughts on “OpAmp + MOSFET = a decent headphone amplifier

  1. Pingback: Усилитель для наушников : операционник и полевики | MyElectrons.ru

  2. Hi, my name is Rigel, I´m from Panama. I´ll like to do a final project using this configuration for my first course of electronics. I´ll like to know if using 24 ohms headphones will cause any trouble? Also, what are the voltages V+ and V- the you´re using? And this is a stupid question but i have the doubt, for the input and the output, I have to use 3.5mm output jacks on both ends?

    • Hi Rigel, I am terribly sorry for such a late reply!
      Sure you can load this amp with 24 Ohms headphones. It’s even capable of driving 2 Ohms loudspeakers, at the levels comfortable for home listening (but not to run a discotheque, of course 🙂 )

      V+ and V- can be derived from a 15 to 18 Volts dual-secondary transformer, followed by a rectifier bridge and ample filtering capacitance (let’s say 4700uF per polarity at least).

      Regarding jacks: 3.5mm ones at both ends would do just fine. To my personal taste two RCA jacks (left and right) for the input and a 6.25 for the output would look slightly more professional.

      Cheers,
      – Serge.

      • That’s great! When I can pull some money together I will have a go at it. The aluminium capacitor (47uf@16V) is pretty expensive, but I can get electrolytic for cheaper (I know, i know…)…would this make much difference? And would this be good for driving 40 ohm headphones, or would it likely blow them out at high gain?

        Thanks

        • Hi Dom,
          thank you for visiting MyElectrons.com and your comments!

          In the (very old) prototype I used aluminium electrolytic caps everywhere. These days alu-organic-poly are the best. You’ll have to chose 😉
          If you really want to save on this – you may still go with standard lytics, the topology is quite permissive on parts substitution.

          By the way right now I am preparing a new (MUCH BETTER) version of this amp to be sold as a kit. It should be out in +- 4 weeks. There will be a budget option of the kit too (still delivering an outstanding performance, poly caps as a gift for you 🙂 )

          I should be updating the article here asap. Meanwhile please take a glance at its Russian version (round color flag next to the photo) – it’s more up-to-date (parts etc.), and google translate or such may help you to pick up some news and discussions there.

          All the best!
          – Serge.

        • It will drive 40 Ohms phones peacefully – by default it does not provide much gain, hence it’s rather safe.

          The same time it will deliver a dozen of Watts RMS into 4 Ohms load, provided the power transformer and the gain are scaled accordingly.

          More to come in the new kit 😉

  3. where do we get information on the availability of your kit? i am very interested in purchasing one of these. are these op amps DIP, and does the board come with sockets for op amp rolling?
    thank you in advance
    mike

    • Hi Mike!
      Please take a quick look at some pics, hope they tell big part of the story: https://plus.google.com/u/0/photos/113221551053931279827/albums/6010415095736512737

      Sorry for not having yet a detailed article about the new JASTAmp-Pro here. It’s coming…

      The JASTAmp-Pro kit should be ready for shipment by early September. There will be three levels offered: “light” (only the HP-amp, IRF MOSFETs), “standard” (HP and Big Speakers capable + protection, Toshiba lateral MOSFETs) and “elite” (many high-end’ish improvements).

      All versions come with the OpAmps that are hard to beat 😉 But sockets are still included.

      Soon I should be able to take pre-orders. If interested – please drop me a note at: patrushin at gmail . com.

      • Hello! Id like to know if there’s any progress with the JASTAmp-Pro kit. I would really like to build one… But if not, I’ll just settle for the amp described in the article above.

        Have a great day!
        Andrei

        • Hi Andrei,
          JASTamp-Pro has been built, tested, fine-tuned, and quite a few kits prepared for selling. I am a happy user/listener of that trial version for now. Then… we moved from Europe to the USA, and with the all stress of the move I am lagging on answering requests like your.

          Let me reply to you via e-mail (and add the contact form to this site).
          Cheers!
          – Sergey

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