Hi all, as you may know it is difficult for me to stay idle, so I decided to review some of Sijosae's archives http://www.headphoneamp.co.kr/bbs/zboar ... asc&no=147
Sijosae is a Korean builder who has a knack for simplifying circuits to the bare essentials, following theory and testing his results. His builds are artistic in that he etches his own boards, but provides point to point perf board instructions with his schematics. The site is in Korean but can be easily translated with Google translate.
Now to the circuit. This is take off on Erno Borbely's hybrid mosfet amplifier http://www.borbelyaudio.com/pics/405borbely2506.pdf
The amp is for intermediate builders who are familier with various amp classes and topologies. The components are easy to find and rather inexpensive.
I started with a knockoff Blue Velvet that I bought from eBay, it is a Chinese version step attenuator, but for the price it fits the bill. I built a buffered virtual ground circuit using 1000uF, 220uF and a njm4556 opamp, this helps with crosstalk and keeps the rails rather tight. I also added a LM317T and a 75 ohm resistor for a trickle charger ( battery pack later down the road)
The theory ( I obtained from the Borbely website), basic Longpair/current mirror consisting of two 2sk117 Jfets (NTE 458) and 2n2907 PNP (Rat shack pn2907) a 2n2222 and led combo provide a ccs for the LTP. The two out of phase signals are combined via the mirror and sent to a P channel IRF9530 in a class A common source configuration. A 2sc2344 (NTE375) and 12R emitter resistor provide 190 mA of Q current. Shijosae's amp put out 120mA, but since the wattage is enough for the MOSFET and BJT to handle there is no heatsink. They do get hot though.
I cased it up in plexiglass, I only wish I had a band saw to make it a little neater, but that is down the road. It lights up rather nice.
The amp has a 9V caddy, but 9v is nowhere close to enough voltage to bias the amp. I kept it on the build because it has already allocated the real estate on the board, so I will use the connector for a future 12v AA battery pack.
The sound, oh! With the sound paused, it is completely invisible, it drives 32ohm closed head Sennheiser's with ease and the sound has punch. There is enough voltage to drive my 600 ohm cans as well.
I took my time with this one as it took about a week to build, a little at a time. Sijosae has the results for his amp on the bottom of the link webpage, the THD is nice, I used a 12v SMPS fora power source.