The Objective2 (O2) Headphone Amp DIY Project

Alright. Now I just need to find a new gain switch. The problem is not finding one, the problem is finding a place that doesn't have 20€ shipping...

Without gain switch installed(with no solder bridge or pcb trace damage), gain is 1X. If you are stuck I can send it from India (4 switches for 1$ plus postage, everything in bubble wrap envelope- approx. 15 days delivery time). Sending by Indian postal service is very economical.
 
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Without gain switch installed(with no solder bridge or pcb trace damage), gain is 1X. If you are stuck I can send it from India (4 switches for 1$ plus postage, everything in bubble wrap envelope- approx. 15 days delivery time). Sending by Indian postal service is very economical.

Thanks for the offer!

But Stefan at Headnhifi is already sending me some.
 
um, Hi... Not sure if this is the right place to ask this, but since I'm about to just toss my o2 amp in a box in the closet and move on with my life I thought I would give it a try...

I built the amp nicely, not troubles (that I know of) went through the testing and it seemed fine. plugged in some cheap headphones and voila, Bob Marley...

But then I noticed something totally weird... I was getting what sounded like both left and right signals in both ears. I AB'd it with another amp, listened to what was coming from the dac, listened with different sources, different headphones, tested cables adapter and... yup, both ears had left and right (causing "phasey yukness") and destroyed the stereo field "soundstage" and left me with a really beautifully loud and amazing low end what seemed like an awesome but messed up amp...

That was the first problem... I went through the testing again and again and scoured all your forums (I say your because this is my first post here)... In the time I've spent trying to troubleshoot this thing I could have built 5 more amps and maybe one of them would be working by now lol...

Anyway in the process of trying to figure out what is wrong with this amp I must have shorted the U2 chip and now the thing is dead. no light, nothing.

Now I'm pretty sure I can find the fried part and order another chip and get it going again, but what was wrong with it in the first place. I would love to know what would make it play out of both channels like that before order another one and try again (which I plan to do because I didn't really get to hear how the amp sounded and I really want to check it out).

And it had very low distortion even a high levels, sounded pretty great and didn't have any missing elements, just no stereo field...

I screwed it up somehow, but I would really love to know what was wrong with it before I spend the rest of my life troubleshooting 30 parts...
 
I looked and tested and don't see any but you're probably right. I'm going to take them off and go desktop style at this point... Hopefully I can find the part I fried shorting the U2 chip and bypass the input and output jacks and it will function properly. Sure is a neat little amp... Thanks for your help ;)
 
Anyway in the process of trying to figure out what is wrong with this amp I must have shorted the U2 chip and now the thing is dead. no light, nothing.

You can use NE5532 as U2(RS suggested NE5532 can be used if JRC2903 is NA) or can even bypass PM circuit completely for testing purpose. My guess is you might have ruined the MOSFETs and not U2 as ICs are pretty robust.
That "phasey" sound is because your i/p or o/p ground connection is "floating" i.e. not making proper contact. You might have got faulty i/p or o/p jack.A few post back someone had mentioned the same problem.
And yes this is the right place to ask all O2 related issues.;)
 
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I don't know if anyone else has observed this, but I have channel imbalance problems with my O2 at low levels. It's fine at higher volumes, but I don't like to listen too high especially with IEMs, and at listening volumes, it actually is an issue at times.

I've narrowed the issue down from testing and logic to the potentiometer... it seems the Alps RK097 pot might just not be that great. Can someone tell me if it would be okay to swap in this pot to test if this solves the issue: PTD902-2015F-A103 Bourns | Mouser

In theory it should fit, but in theory and in practice are often to different issues. Thanks!
 
I don't know if anyone else has observed this, but I have channel imbalance problems with my O2 at low levels.

Yep, the problem is the pot, that issue has come up a lot. Any of the analog 9mm pots are only accurate to 20% or so between channels. It shows up most at low volume levels. The Bourns pot will work - I've built a few O2's with that pot - but will have the same channel imbalance issue. The Bourns pot solves another problem though. There is a problem with the left channel getting scratchy which seems to be a design problem with the Alps 9mm units.

To solve the channel imbalance problem you would have to go with an electronic pot, like JDS Labs did with their new C5:

JDS Labs - C5 Headphone Amplifier

You also may be able to adjust the gain up or down on one channel on the O2, using the gain setting resistors, to get the two channels to match up a little better.
 
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No wonder Rocket Sciensist disappeared.

Hello everyone,

I'm surprised that this discussion lasted as long as it did without Rocket Scientist blowing on the flames. Why did RS disappear if this design delivers such wonderful cost vs. performance value? I really appreciated RS time to document his/her design so dutifully. I wish more of us would do that. But is O2 as impressive as the following would indicate?

I dutifully built my O2, and liked it initially. But I have no other testing instruments than my ears. My "instruments" tell me that O2 is a good performer for $30 naked, but once wrapped up in a $120 package it is rather average for DIY.

Are you really hearing something I don't?
 
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I don't know if anyone else has observed this, but I have channel imbalance problems with my O2 at low levels. It's fine at higher volumes, but I don't like to listen too high especially with IEMs, and at listening volumes, it actually is an issue at times.

I've narrowed the issue down from testing and logic to the potentiometer... it seems the Alps RK097 pot might just not be that great. Can someone tell me if it would be okay to swap in this pot to test if this solves the issue: PTD902-2015F-A103 Bourns | Mouser

In theory it should fit, but in theory and in practice are often to different issues. Thanks!

It's been mentioned earlier in the thread......clip out R19 and R23. This will turn the high-gain position into a unity gain position and yield a better (higher and better balanced) position on the volume control.

Cheers,

Dave.
 
Hello everyone,

I'm surprised that this discussion lasted as long as it did without Rocket Scientist blowing on the flames. Why did RS disappear if this design delivers such wonderful cost vs. performance value? I really appreciated RS time to document his/her design so dutifully. I wish more of us would do that. But is O2 as impressive as the following would indicate?

I dutifully built my O2, and liked it initially. But I have no other testing instruments than my ears. My "instruments" tell me that O2 is a good performer for $30 naked, but once wrapped up in a $120 package it is rather average for DIY.

Are you really hearing something I don't?

That you have assigned a monetary value to it betrays your inherent expectation bias. I'm not afraid to say it here since people here seem intelligent rather than fanatical like certain other forums, but do listen blind for a while between this and your other amp (truly blind, that is) and see how you like it. Unless your other amp is very colored (ie tube) or way less accurate, I'll be impressed if you can tell a meaningful difference.

Many people dislike the O2, that's for sure. But what they're failing to realize isn't that the amp is bad -- rather, it's as *accurate* and in that sense, close to as perfect as can be -- but instead they just do not like or are not accustomed to the sound it produces. You often hear in subjective, and therefore, mostly meaningless reviews, how the O2 is "not warm enough" or "lacks bass punch". They're forgetting to affix the phrase, "compared to what I'm used to." In other words, they amps they like or routinely use are in fact, probably less accurate, less neutral, and more colored. They just happen to enjoy the coloration.

I'm one of those folks actually. I love the buttery smooth and warm tube sound. But I'm not fooled by one moment that my tube amp is actually more accurate or features less distortion. That's what it really is, but that's beautiful distortion to my ears.
 
Great, thanks for the information. Are there any electronic pots you can recommend for use with the O2?

IMHO it would be difficult (& expensive) for a novice to implement digital vol. pot in O2. Detailed discussions of analog v/s digital pot can be found here:-

JDS Labs Blog | Premium Headphone Amplifiers

agdr's method of reducing gain using voltage divider (if 1X is too much) is preferable. If space permits(bigger non standard enclosure) stepped attenuator can solve this issue for parts costing no more than 5$.
 
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I get a huge "pop" noise when switch ON the amp..

Connecting a 10K resistor between the "+" of C8 and "-" of C9 helps to eliminate the turn-off thump. There is a chance it would help with turn-on thump too, although I haven't tested it. If you want to experiment just connect a 1/8W or larger 10K resistor between the two points in the diagram below. Please post the results if you try it!

The resistor helps with turn-off thump because the only load after the mosfets that RocketScientist / NwAvGuy had connected is the ICs U1, U3, and U4. The ICs are not exactly a linear load and are unrated as to what happens below 2V. The resistor gives the mosfets a direct linear load which may improve switching time and help the power rails come up and go down in syncronization. The "thump" happens when one mosfet switches a fraction of a second before or after the other rather than at exactly the same time, and/or when one power supply rail comes up or drops faster than the other.
 

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Hi all! Long time lurker here, decided to post my success story here. My implementation of the O2+ODAC has RCA outs for the ODAC, in the B3-080 chassis. The best thing is, all the inputs, RCA, buttons and AC jack are at the back. The front has a very clean minimalist design with only the volume, LED and 1/4 jack. This means everything except the LED is remote mounted. Can't wait to show u guys the pics!
 
In my moment of noobness I had mistaken U5,U6 for a pair, which means I swapped their places and soldered them in. The voltage across the battery terminals measured only ~20V. Luckily the opamps and ICs were undamaged. LOL. They were a nightmare to desolder and remove, I ended up having to destroy them.