Old hobbies die hard. It’s been several years since I’ve actually done any audio related projects at home and have been quite happy with my living room listening setup. Somehow – I think mainly thanks belong to my ex-colleague who was constantly building some amps or speakers – I’ve found the spark again in audio reproduction and things related. Perhaps more on this side of home improvement later…
I recently ended up buying a new set of headphones, as my awful, old and (t)rusty Philips headset from 90’s was finally giving up and my wireless AKG set is way too noisy to really enjoy music with. I haven’t ever had anything that could be considered as a high-end headphones but now I do. I was already aiming for roughly 100 euros price range, but after trying out various models at local store, I ended up with Sennheiser HD-598. Both for how the set sounded and how it was built. Feels and looks like something that is meant to last for years to come. After listening through few records at home I also found out that none of my listening equipment had decent enough headphone output, so I had to build an amp to drive these.
So to the amplifier section. First and simplest choice is single-ended Class-A MOSFET design. These are all over the internet for many decades now, so nothing new invented with this wheel but I’ll share it anyway. For something that is constructed from handful of surplus parts in hand without too much effort for the design, result sounds very good. It has flaws (or points for improvement!) but I very much like the simplicity of this thing. Judged by listening, it is much better than what my amp or CD player has (not to even mention my computer audio card phones output.)
For the design, I had planar FQD1N80 MOSFETs laying around, so this was obvious choice. It’s a planar construction, so it’s well suited for linear operation mode. It has quite high on resistance so it seemed like a perfect and very balanced match for single-ended headphone amplification. For sake of simplicity and low part count (and perhaps lower overall noise!), this incarnation uses resistor instead of current sink.
Tested this through with signal generator and oscilloscope, everything seemed fine from 10Hz – 100kHz sweep. Not too much added harmonics, waveshapes were symmetrical and seemed to match the input amplitude for whole frequency range. Channel amplitudes were even with each other level-wise over whole frequency range without any additional fine-tuning.
Quickly drafted schematic of one channel and AC analysis result in LTSPice. FET model isn’t same as I couldn’t find any close(r) matches from the library. Despite the model, results agree with the quick measurements I made.

Obviously it can be improved in many areas, but this shall serve now as a reference to which I can compare when I try something different. Also a thing to note is that simulation is run into a nice resistive load so real situation is a bit more complicated and even more complicated when extended into SPL’s from those non-ideal headphones. But that’s a different story, Mr. Linkwitz has made some nice notes on modeling speakers. When I have time, I’ll try to craft a spice model of these headphones to play around.
To feed the amp, I’ve also acquired couple of DAC boards based on Asahi Kasei  AK4393 chip.  First complete amp setup was paired with 24bit/96kHz USB audio card for hassle-free digital out from any PC, AK4393 DAC board and this single-ended amp design. I really do enjoy how it sounds – It’s super silent noise wise and is more than capable of driving my headphones (SPL levels upto painful). Amount of new detail from my records with this setup is overwhelming. So improvement (or change atleast 🙂 is well noticeable.
What I’ve read from various sources, these DACs by Asahi Kasei should be superb for the price. I myself don’t yet have anything else on the matter other than initial listening impressions, but based on that, it’s very well worth the price! But look here for some serious stuff on the AK4393 DAC – Lampizator and Behringer Ultramatch. The site has also plenty of other interesting reading if you’re into Audio and things related, so take a look.
To continue this project, my next target is to build couple of different kind of headphone amp designs (perhaps one push-pull discrete and some nice tube amp design) to test those side by side and perhaps find some differences in how they sound and decide my favorite. One of the DAC boards will also get all unnecessary stuff removed from signal path, so that is also interesting to hear if changes are audible and can it be considered improvement or not.