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The background

A DIY enthusiast from Korea

Hyun Joon Rho, DIYer from South Korea is thrilled. This gentleman has built 19(!) headphone amps but this one makes a statement in his collection of amps. He isn't so good (say he) in expressing himself in english but this amp is really, really good, according to him. I can only agree that it's a good amplifier topology and it works very good and is stable.

http://www.head-fi.org/forum/thread/34101/per-anders-qrv-03

Hi, everyone!!
I'm Hyun Joon Rho, DIYer in South Korea. It is my first post in Head-Fi. Several Weeks ago, I saw Per-Anders's QRV-03 at his homepage. I was very curious about its sound, So I had made up my mind to build it... and built. I just mention what I have modified and result shortly due to my English.
1. I used +/- 12V power (virtual configuration using TLE2426 and JRC4556 Op amp) and omitted Input DC coupling capacitors.
2. replaced R11,R12 with 1K trim pots to adjust current..
3. replaced BD139, BD140 with 2N2905A, 2N2219A. (I built another QRV-03 using BD139, BD140). Compared both QRV-03s, I think later is more stable and better sound.
4. replaced LT1115 with OP07CP Op Amp. (It is hard to get LT1115 in Korea)
5. set output-current(idling Current) with 30mA approximately. Adjusted value of R17(10R) and R18(10R) with 60R. (This value was set by increasing step by step, 10R~60R)
6. A measured value of the Headphone Terminal's DC is 0.2mV~0.6mV. It is comparatively difficult to adjust Headphone Terminal's DC, but It's possible to set minimum DC by balancing two R11, R12(1K trimpots).
7. - Listening QRV-03 - Although it is very subjective and hard to describe my experience in English, Sound is so clear and rich... In a single word, Excellent!!! This QRV-03 gives me the most satisfaction among what I have built 19 kinds of DIY Headphone amps.
Lastly, I am very grateful to Per-Anders.

-- QRV-03 Link (Korean Website, QRV-03 pics that I built)

http://www.headphoneamp.co.kr/bbs/view.php?id=others_total_gallery&page=1&sn1=&divpage=1&sn=off&ss=on&sc=on&keyword=qrv03&select_arrange=headnum&desc=asc&no=1038

It seems that the posts I mention aren't online anymore but if you want to take a look at his amp, please send me a message and I'll send a couple of pictures of his amp.