I have built several DIY headphone amps in the past few years and have become good friends with Martin over at AGDR audio.
He has a new site up and has lots of his projects he has done over the past few years.
He has several good projects and helpful information on how to improve on amps like the original O2.
Here you will find:
* O2 build mistakes to avoid
* Easy O2 headamp modifications
* O2 headamp parts
He has published his O2 Inverting headphone amp, his desktop ODA or what we think that the designer of the O2 might have done with a
desktop verison had he stuck around on the scene...I have built both of these amps and you can read my threads and thoughts and opinions on
their performance.
Their is also a O2 Booster board for those that want to play with your O2 amp:
The O2 Booster Board provides several benefits:
Nearly twice the current output capability on peaks, 250mA vs. 140mA per channel on a stock O2 Headphone Amplifier. Why this matters
Nearly zero (50 microvolt) DC output offset voltage, vs. typically 3000uV (60x more) for the stock O2.
Zero turn-on or turn-off thumps due to an included headphone relay circuit
Doubles the slew rate to 6V/uS, and 20V/uS if the NJM2068 gain chip is replaced by 2x LME49990 adapter
Power rail clamp diodes. This is an important O2 ommision - some versions of the 7912 voltage regulator have been known to fail to start up into some load conditions (in general, not just the O2) without a rail clamp diode.
Lower input-resistance induced distortion. Many op-amps have a little-known problem of distortion being caused by higher levels of series resistance on their inputs. The NJM4556A is believed to have this problem. The OPA827 and OPA140 FET-input op-amps used in the O2 Booster Board have been desgined by TI to specifically reduce this problem.
Adds 2 LEDs that show when the power management mosfets in the O2 are on. Helps if any O2 troubleshooting is ever required, to see immediately if both power supply rails are OK.
Allows for true zero ohm output vs. 0.5 ohm for the stock O2 Headphone Amplifier. Lower amplifier output resistance means a better damping factor.
THD and noise figures of the chips, per the datasheets, slightly beat (better) the O2's stock NJM4556A
Its a relatively simple build....
One of his new exciting low cost amps he is working on and hopes to complete in the hear future is his new "super" cMoy..
A mint-tin CMOY headphone amp like no other! Uses the brand new Texas Instruments OPA1688 and (optionally) OPA1622 headphone driver chips. Ultra low distortion with extremely low idle current for long battery life. Uses either throw-away 9V batteries or lithium rechargeable batteries.
Texas Instruments has just released a brand new generation of headphone chips, available only in surface-mount packages. These chips have many benefits over the older chips:
The lowest Total Harmonic Distortion and Noise (THD+N) of any headphone chip to date
Extremely low quiescent (idle) current for low power draw and long battery life. Up to 1/6 the draw of some older op-amps.
High output current levels. The OPA1688 is rated for a maximum of 75mA per channel, while the OPA1622 is good for 150mA per channel. Most older op-amps in CMOYs are only rated at 20-25mA.
High capacitive load drive with no output resistor. Most op-amps used in CMOYs were never intended to drive the levels of capacitance found in headphones and headphone cables, resulting in oscillations unless series output resistors are used tp isolate the load. The new TI chips are designed to drive headphone CLOADs directly, with no output resistor.
So if you ever had a penchant to muck with some DIY...take a look!
All the best
Alex
http://www.agdraudio.com/