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 NEW  Bruce Heran outlines the details and construction of his simple DIY 6DJ8 (ECC88) Tube Hi-Fi Headphone Amplifier Project.

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PostPosted: 20 Jun 2009, 11:24 
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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.


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PostPosted: 20 Jun 2009, 12:08 
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Solid State Borbely (Sijosae version) Schematics
Attachment:
buffVG.PNG

Attachment:
poormansBorbelySijosae.PNG


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PostPosted: 21 Jun 2009, 12:35 
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Great work Rogers. What is going on with the enclosure? It looks semitransparent. What did you use?

So is the Japan labeled Alps Blue Velvet in the photos a knockoff?

There is a ton of great information on the Sijosae page. I'm sure some will find these CCS testing results and comparison interesting:
Attachment:
CCS-Comparison.png

Cheers, Gio


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PostPosted: 21 Jun 2009, 14:01 
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Hi Gio,
Yes according to many threads on other forums, Alps does not make a Blue Velvet step attenuator, and the length /spline shaft is a givaway. i have bought real Blue Velvets from HeadAmp (for a steal) http://www.headamp.com/order/index.htm and they have a smooth operation and a different shaft. The Chinese ones are found on eBay for $10-14 US and they do sound good for a simple cmoy or YAHA etc. The plexi is a smoked color for the face and brushed for the top and bottom. Really easy to work with, but the cuts are rough because all I have is a hack saw. Will have to hit the pawn shops for a band saw. Thanks for the CCS link, he has many nice little projects including a discrete BUF634.


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PostPosted: 05 Jul 2009, 07:53 
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Looks wonderful.
There is an interview with Erno in this years AudioExpress magazine. I forget which issue. If you get the magazine, or can, its a great read and gives a lot of insight as to some of his favorite components and why he loves them. If I recall Toshiba mosfets are his favorite and he also goes into which types of caps sound best in circuits.
Nice build man. I really like reading your posts.
Uriah

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PostPosted: 05 Jul 2009, 17:47 
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Thanks Uriah,
I pick up the AudioExpress once in a while when I get up Barnes and Noble. There are some great designs in there, I will have to search for that article.

Rogers


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PostPosted: 24 Feb 2011, 16:16 
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Hello

Do you have hybrid version ?

thanks


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