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F5 HA   2015 - 2016


The F5 headphone amplifier is a downscaled version of the "legendary" F5 power amplifier by Nelson Pass designed for headphone duty. Patrick aka EUVL (XEN Audio) came up with the schematic more as a mental jogging exercise (as he puts it himself) but eventually realized it P2P with a very compact layout on simple veroboard. This is an all FET amplifier schematic based on the Toshiba 2SK170BL and 2SJ74BL complementary pair as front end, that, with proper device matching, requires no DC offset servo and will deliver very low distortion and very high bandwidth. Output devices are either the popular IRF9610/610 or better Fairchild FQP3P20/FQP3N30, or, when you want to stay all Toshiba, the 2SK2013 / 2SJ313.


I consider myself lucky that the prototype amplifier boards were given to me permanently for testing, and to eventually case them up with a proper power supply of my own choice.



schematic courtesy XENschematic courtesy XEN




During testing I tried various power supply topologies from a standard 7815/7915 bipolar supply, LM317-based regulators to Salas shunt regs that I had used before in a number of builds. I learned my lesson here that not everything has to be powered by shunt regs, especially since they failed in this case because of severe oscillations --> triggered by the high bandwidth.

My final choice came down to either a simple cap multplier (inspired by Rod Elliot's schematic but improved upon by Patrick) or the so-called Nazar-regulator, named after the smart russian fellow who drafted it. Both were very easy to build on vero board and working exceptionally well, both in terms of noise performance and sonic properties. To my ears both sounded decidedly better than a simple fixed regulator, so I finally settled down on the Nazar regulator since it carries on the theme of a 'simple' but very effective design.





Alternative Nazar regulatorAlternative Nazar regulator





For the case of the F5 HA I choose an all aluminum case that I designed and built in Autodesk Alias 3D-Software. At the time I had a rare opportunity to have the aluminum parts professionally milled on a CNC machine, and I happily accepted the offer. The case is based on readily available Fischer heat sinks and sports a solid 25mm front and a 10mm back panel. The divider that holds the Xfeed board and it's supply (Didden regs) was also cut on the milling machine, bottom and top panel were custom made at Schaeffer Apparatebau. Despite the smallish appearance this sure is my heaviest case. :)

Sonically this is a step up from the Dynahi, which comes down to the point of circuit design and personal preference (plus better device matching which results in lower distortion and low DC offset without the need for a DC servo!). The Dynahi having NFB does sound very dynamic but also kind of hard on critical instruments, whereas the F5 HA is a different kind of animal. Like all of the recent high bandwidth and zero-global-feedback amplifiers that I have heard the F5 HA sounds very natural and authentic... a tiny bit rounder than the DAO for instance but dynamic and powerful nevertheless. Definitely a keeper!


For those interested in the F5 headphone amplifier and more in-depth information please refer to the thread on DIYaudio 


I also uploaded a PDF document from EUVL containing all the vital information:




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