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Classic C3G OTL  2004  


This is how it all began...


After building loudspeakers for almost 20 years I switched to headphones only in about 2004. At the time I owned a small commercial solid state headphone amplifier, an Amity HPA4. It sounded truly awful even over decent Sennheiser HD600 headphones. Harsh treble, unbearable midrange glare and a narrow soundstage plus assorted noises were not to my liking. So I searched the internet for amplifier schematics and eventually came across a tube amp schematic by a certain H. Siemens that was quite popular in german DIY circles at the time.





The schematic is very straightforward. It uses the german pentode C3G wired as triode and working as voltage amp (not as cathode follower like pretty much all other OTL schematics!),realized with minimal parts count. Due to the relatively high output impedance of the circuit (taking the output from the anode) you can get away with smaller values for the output capacitors. I used oil filled Siemens MKV 10uF capacitors in my build.

When I started to tackle this I basically had no clue about electronics (no training in EE) and had to learn a lot of things. But the beauty of tube headphone amplifiers is their simplicity (at least this one), so I kept reading and studying literature to eventually include my own ideas in the schematic. The power supply was converted from solid state devices and simple CRC filtering to a big tube rectifier and extensive LC filtering. Otherwise I kept close to the original schematic except for the choice of the parts. So I am using oil filled MKV capacitors as output and Oscon Polymer caps as cathode bypass, both with good results.


The amplifier sounded quite dull and veiled in the beginning and was rather disappointing... which only in part can be attributed to the high output impedance. But I soon found out that those MKV caps need considerable burn in, and sonics improved further with the use of bypassed Sanyo Oscon caps in the cathode and MKP fílm filtering caps in the psu. The resulting sound was warm, detailed and very pleasant to listen to. It was miles ahead of the little solid state amp and very satisfying as a first build! It is probably not the most neutral amp around but always fun to listen to on all kinds of music.