Introduction
In search for the very best
Once upon a time in a God-forsaken world of high-end audio, where corporate gods devoted their resources to brainwash every single audiophool into worshipping their dodgy overpriced hardware, corrupted reviewers into giving them 5 stars in magazines and where subjectivity and design-by-ear is all that matters, a person known as NwAvGuy was born, to give the World the One and only headphone amplifier - Objective2 or O2 as a breath of fresh air.
Worlds collided back then in May 2012.
Pure subjectivists, led by those-that-shall-not-be-named tried to come up with the best, or maybe the worst word to describe this Invention. Their slanderous call was not heard. The overpriced audiophool market took a great hit by this well-engineered $50 portable amp, which continues its legacy to present day. ---

Closer look
Objective2 is small portable headphone amplifier, designed from ground up to be purely "wire with gain" amplifier, in another words fully neutral, not altering the sound signature one bit. And that's basically what it is.
Powered by either 16-20 VAC or two 9V batteries, which can be upgraded to six li-ion cells for extra runtime.

Fig. 1 - Front panel

Nothing really fancy here - that's what makes it great.

• On-Off switch
• Headphone out
• Japanese ALPS volume pot
• Gain switch
• Input jack

When ordered through JDSLabs, you can customize the layout online. By default, the power jack is also present on the front panel, but I opted for moving it to the back for convenience.
You can also exchange 3.5mm headphone jack for 6.35mm or replace 3.5mm input with RCA sockets.
Some of these customizations can get quite expensive.


Fig. 2 - Rear panel

Don't mind the holes, those were drilled afterwards. Pretty much nothing to see here except for the power jack.


Fig. 3 - PCB

This is where stuff gets interesting.
O2's frontend is based on two JRC 4556AD, one for each channel. This op-amp is actually designed to amplify baseband audio, unlike those esoteric OPA627s used primarily for video.
For power, at least two battery packs are needed, to provide two negative terminals +9V, 0 and -9V. Regular DC won't work. My unit is slightly modded and utilize six li-ion packs which extended the 6-hour battery life to 30 hours.

Gain adjustment
By default O2 is shipped with gain set to 2.5x and 6.5x. For modern low impedance high sensitivity headphones, this is too much. Before finishing your order at JDSLabs, you can ask for lower gain settings. But you can of course mod the amp yourself to get the gain you want.
All you need is this formula:

R17 = 1500/([your gain]-1)

R17 represents one of the four resistors highlighted on the Fig. 3.
There are four resistors in total, one for each channel and one pair for single gain settings. Use the above formula to calculate the gain you need.
For 2x and 10x gain, those values would be 1.5kOhm and 166Ohm respectively. Then simply desolder or cut the ones inside and replace them with new ones. Bingo, gain is adjusted.


Sound assessment
For detailed technical measurements, I recommend visiting NwAvGuy's blog: http://nwavguy.blogspot.com/2011/07/o2-headphone-amp.html

I extensively put O2 into test, but it always comes out with flying colours. I could not detect any coloration anywhere in the spectrum. With line-in sources, it actually made the bass "tighter". On this note the amp is very revealing, and does need good source.
There's plenty of volume reserve for any kind of headphones on the market. I measured incredible 8 volts RMS on undistorted output. That's enough to reach pain levels in vintage 600-Ohm K240.

(in progress, continuing early tomorrow)