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DAO    2016


With the year 2016 came the DAO (literally meaning 'The Path' in chinese, but also referring to the philosophical concept of the 'Dao'  道 ), an all FET, Zero-Global-Feedback, Pure Class-A headphone amplifier. It is yet another creation out of the XEN stable and their most complex published design yet. 


Similar to the F5 HA the DAO headphone amplifier is originally based on Nelson Pass' tinkering  with circuit ideas, in this case about using a Power JFET from the (no longer existing) company Lovoltech, the LU1014D. Thinking of using them in a power amplifier he came up with a circuit describing how the triode characteristics of this particular Power JFET could be put to use with so called Cascode Modulation. Since I cannot lay out all the details and information in depth I kindly ask those interested more in the topic to have a look at the information posted on the DAO thread on Diyaudio or in the original articles in Nelson's ZEN Version 8 and 9 papers.

Patrick aka EUVL transformed the circuit idea to use it in a headphone amplifier by cleverly balancing cascode voltage and cascode modulation while keeping bias at 200mA. The cascoded LU1014D thus became an ideal candidate for a Source Follower Headphone Buffer circuit. To provide more driving power the original buffer was augmented with a Taylor Current Source (TCS), and to make the design more versatile with  headphones of any impedance (I am using headphones with 60Ohm and 350Ohm impedance) it employs an additional gain stage (HAGS) to provide a gain of 5, all without global feedback and in pure Class A. Those modules were the amplifier part. 

On the input side I had the choice of three different Crossfeed circuits (optional) --> the Meier circuit, the popular Linkwitz and a modified Danyuk version. I choose the passive Danyuk Crossfeed followed by a simple JFET buffer. Parts quality in general is the highest I ever used in a project with all semiconductors curve traced and matched to extremely low tolerances by the XEN team. Resistors are all 1% with the exception of the Crossfeed circuit that uses Vishay S102 foil resistors (0.1% tolerance) and tightly matched Wima film capacitors.





Source Follower and TCSSource Follower and TCS







The DAO headphone amplifier is working  with a rail voltage of +/- 24V that can be provided in many ways. The design is deliberately kept flexible by EUVL to adapt to the needs and wishes of the individual builder. Power supplies can range from a simple CRC supply to LiPo batteries, which were my first choice in the beginning but honestly I couldn't figure out how to house the needed 12 battery packs and their balanced charging circuit inside the given case... So instead I choose a CregC supply that consists of two capacitor boards with 80.000uF of capacitance and four Curl cap multipliers nested inside  to power the Source Follower, TCS and HAGS. The Danyuk Crossfeed requiring +/- 9V  is fed by a pair of very compact Didden regulators employing no less than 32 smd parts crammed on each board. Because of the tight layout a premium quality, magnetically shielded toroid is highly recommended. 




Trial fitTrial fit



Building took a little longer than expected, but then this is not something you are building in a hurry. It definitely helps to have a good understanding of circuit theory, and I learned my lessons here. Theory still is not my strong point. ;)  Also no less than 17 compact boards had to be populated and tested... but except for a silly wiring mistake on the headphone jack everything worked as intended during the first test run.


Now, is this finally audio nirvana?

When I am looking back to the year 2004 I think I have come a long way... I built a good dozen headphone amplifiers of various topologies and listened to all of them extensively while often comparing them side by side. I learnt a lot which always is a good thing to keep your head active.  With the DAO my personal and subjective quest for "best" sound has come to an end as it seems, unless I do a major system change (good to always leave a backdoor open ;-)). There is virtually no critizism possible with the DAO... while all the other headphone amplifiers had on or the other weak point or attribute I didn't like so much, even being really subtle, the DAO does everything right. It portrays  music in a way that to my ears leaves nothing to be desired, and the limitation is coming rather from the recording quality of a certain track or the headphones in use. It has vanishingly low distortion (also measured), a fullbodied and natural timbre and imaging as good as it gets with headphones on your head. There is nothing I would have to add.




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