The Objective2 (O2) Headphone Amp DIY Project

Maybe someone could design a charger for it.

I think it can be done. :) I'm slowly working on something similar for this O2 knockoff in a narrower but longer BUD EX-4501 case.

What I'm doing there, and probably the easiest thing to do with the O2, is give up entirely on the O2's internal AC supply for charging the (now lithium polymer) batteries (but keep it for AC power usage of course) and instead cable them out to an external charger made specifically for those batteries. I'm planning on using these 720mAh "9V" batteries. Then make two adaptor cables that go from the two pairs of 9V clips in the charger to panel-mounted 2.1mm DC power jacks on the back of the O2.

To make it work R1 and R2 in the O2 would have to be removed to break the internal charging link. Removing those resistors next to the O2 rear panel makes room for the two new DC power jacks too. Then one end of D2 and D6 would have to be lifted and then wired in series with the internal switch on 2.1mm power jacks, like a panel mount version of the O2 jack, Mouser #502-RAPC722X. So when the adapter cable from the external battery charger is plugged into the jack it disconnects the O2 circuitry and puts the charger directly across the battery.

Once charged and the cable is pulled out of the power jack the battery is once again re-connected to the O2 via the jack's internal switch. Same thing applies for the other battery. It is important here to use panel mount DC 2.1mm power jacks that are fully insulated from the mounting panel. None of the 3 contacts on the new power jacks can contact the O2 rear panel.

Some benefits of doing things this way are not having to add a likely-SMD lithium poly charging chip(s) inside the O2. The charger used is exactly the once recommended by the battery manufacturer. That charger is a fast charger with an appropriate battery management chip inside rather than a trickle charger like the O2's circuit. The batteries will charge fast, in an hour or two, rather than taking all night as with the original O2 setup.

And the result would be... 720mAh/250mAh = 2.9x , nearly triple the run time. Plus as you say lower impedance looking back into the batteries (I didn't know about that actually until your post!). The voltages of the lithium polymer "9V" are slightly lower than the NiMH, so the resistor values in NwAvGuy's power management circuit would need to be adjusted slightly. I have the math on how to do that in my O2 modification thread here somewhere.

If anyone wants to try messing with the lithium polymer batteries in the link above, the company involved has since run out of the dual bank charger but has the same batteries with a single bank charger here. Those batteries are exactly the same physical size as the 9V NiMH of course, so they fit just fine in the existing O2 case. But - important! - as has been posted above do not try using these 9V lithium polymer batteries in the O2 with the existing O2 trickle charge circuit still enabled. Bad things will likely happen (as in fire and/or possible explosion). Don't do it!
 
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Apologies for a totally newbie question, but I'm looking at the ODAC board and trying to understand the 4 line-out pins on it: left, right, and two more. Are the other two the ground wires? IOW, if I were to wire up RCA jacks to the ODAC, I would run one wire from L to the tip of the L jack, one wire from the R to the tip of the R jack, and then wires from the two extra holes to the sleeves of the jacks?

I've never seen any of the O2 projects for real and so can't really say for sure what you are looking at but, if you think these two wires are grounds then you should be able to measure continuity of those to the appropriate grounds in the unit to confirm.
 
Apologies for a totally newbie question, but I'm looking at the ODAC board and trying to understand the 4 line-out pins on it: left, right, and two more. Are the other two the ground wires? IOW, if I were to wire up RCA jacks to the ODAC, I would run one wire from L to the tip of the L jack, one wire from the R to the tip of the R jack, and then wires from the two extra holes to the sleeves of the jacks?

Correct, the four pins are Left, Right and two Grounds. I would join the two RCA's ground tabs together, then run a single ground wire back to the board.
 
Sorry for the really naive question, but I ordered an o2 board from JDS, am assembling, and am having trouble with the power jack (J1). I want to panel mount J1 (and have an appropriate insulated jack), but the PCB holes are really big, too big for my screw terminals or hookup wire.

What's the best way to solder a panel mount jack to J1? How have people done this before? Is the solution just to get bigger gauge hookup wire (if so what gauge)? Or is there a better way to do this (preferably with the right adapter)?

Thanks!
 
You could solder the wire you have into the big holes. Nothing wrong with that. Or, you could bypass the holes and solder the hookup wire direct to the diode legs on the bottom side of the PCB. Or even the top side.

Awesome, thanks. Which diode would I solder to? D4? In which case I'd still solder ground to the hole in j1?

And if soldering to j1, should I just attach the wire to one side of the larger hole, and leave a gap on the other side? Or try center the wire and flow solder across any gaps?
 
I don't have the schematic handy so can't verify the diode designations.
What I would probably do is strip enough wire insulation so that I could bend a small 'V' on the end of the wire. Then set that V into the hole so it stays in place while soldering. Then trim the excess as usual.

Great! The v trick will definitely do it for me. Thanks a million.
 
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Right then......

I've never used or even seen an O2 for real so I can't say with certainty how much of an issue this really is. The noise occurs as the rails collapse, and specifically if they collapse perhaps a little unequally with one ahead of the other. The other problem is that when a certain minimum voltage is reached, the opamps cease to function correctly and so a DC voltage appears at the out pins.

What the O2 badly needs is a reliable muting circuit on the headphone output, but that is easier said than done, at least as a simple mod.

What you could try (and this is totally uncharted territory) is to alter the value C8 or C9. In other words make one or the other bigger say by fitting a 330 or 470 uf. You could experiment by tagging a 100 or 220uf cap across one or the other while in situ. If no improvement or its worse then try altering the other cap.

Beyond that I can't thing of any real solution... its why power amps have a switch on speaker delay and instant drop out on power off... to prevent noises. Headphones are so much more sensitive too.
 
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Can't see how that could cause a problem beyond the batteries leaking etc.

You need to fault find it by measuring the basics. Follow the guide linked to at the bottom of post #1 :) Check the all the supplies, the unregulated rails before the regs, the regulated after the regs and finally the switched regulated rails that go to the opamps. Easiest and safest to do those in reverse order.
 
Hi guys,
This is my first post, so be easy on me!

I just finished building my first O2 + Dac, i did it 100% to 'spec'. Now i want to build a desktop version with RCA inputs, rear mounted power, remove the batteries and the internal charger, and put a better connector (probably a 1/4" Neutrik headphone jack).

- I've found how to put the RCA inputs - basically solder them to where we would put the ODAC, (Correct me if i'm wrong)

The rear power is straight forward, but what size wires do you guys recommend? Cat5e copper wire is enough? seems too small...

Are there instructions on how to remove the battery trickle charger from the circuit? Remove R1, and R2, remove the 4 battery terminals, what else?

And last question, if i want to lower the gain of the basic kit, right now i have the volume knob at 1/4 and it's LOUD on my headphones. What do you guys recommend?
 
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If you will never ever use batteries then you can link out D1 and D5. There is no point removing any other parts because they are effectively isolated anyway without the batteries. If you wanted to remove it all though, then its the connectors and R1 and R2 and D2 and D6.

To lower the gain try removing R17 and R21. That will give unity gain for that switch position. If that seems OK (and you will never use the higher gain switch position) then I would also suggest linking out R16 and R22 (which will link the caps across them out too).

If the above is still to loud then you need to add an input attenuator by simply raising the value of R3 and R7. Try 10k initially.