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Old 8th May 2013, 08:40 PM   #3281
okim is offline okim  Poland
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Quote:
Originally Posted by agdr View Post
Yeah I typo'ed that. You have +/-7.5Vdc rails and 15Vdc rail-to-rail, which is what the mosfet gates see.

Here is an even better part that has +/-20Vgs. No resistive divider needed. You could use it in the O2 circuit just the way it is. Smaller Rds (on) too, around 0.1R. Fairchild FDS4559:

http://www.fairchildsemi.com/ds/FD/FDS4559.pdf (opens PDF)

Mouser #512-FDS4559, Farnell #1471052, RS Components #6710523.
I ask for that, because I will have dac section on positive voltage which will be discharging positive cells faster than negative.

Quote:
Originally Posted by Knusperkeks View Post
Hey, this sounds like a quite nice idea. I'm currently thinking about a SMD-O2 version powered by li-ion cells with an integrated dac (probably the odac). Because i only want to drive iems, the 2V rms from the odac would be totally fine so i can left out the gain part to same more room. Is there any easier way for the low voltage protection part than nwavguy's solution? None of the li-ion protection chips i found so far, are suited for a bipolar design. Greetings
Look for Nokia li-ion or li-ion for apple devices (all nokia cells have protection). Own protection will be problematic, I think.

P.S O2 is not nessesary for iem's, sabre will control iem's without any gain
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Old 8th May 2013, 10:09 PM   #3282
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The idea to use cell phone batteries is not bad, but i was thinking of another way for realizing the dual-rail undervoltage protection not the protection of the battery itself. And you're right, the dac could probably drive iems but as far as i know, it has a quite high output impedance. Greetings
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Old 9th May 2013, 04:57 PM   #3283
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Originally Posted by smudgeface View Post
So I tested this a bit more: the noise disappears completely with the DAC unpowered, confirming once more that the DAC does cause noise. I have not ye tried removing the ODAC from the case to see if the noise is reduced. I'll try that later tonight. If this turns out to be the case, I wonder if additional shielding might work to isolate the noise, assuming it is radiated and not getting in on the LOD.
I have built and tested 4 O2+ODACs so far, they all tested the same way: With the USB cable unplugged, 3.5mm analog source unplugged, the O2 is dead silent even at 6.5x max volume, using Monoprice 8320. With the USB cable plugged in, the O2 is dead silent at 2.5x. At 6.5x noise can be heard once the volume is turned past 2-3 o'clock. (If using HD650, it's totally silent in every scenario.) So it's either the ODAC, or the USB cable. My USB cable has ferrite bead on the O2 end.
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Old 10th May 2013, 03:42 AM   #3284
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I spent an hour today, playing with the ODAC in a variety of different configurations: internal vs external, shielded vs unshielded, pc power vs USB hub - all had the same results. The ODAC, as a source to the O2, generates noise on the input while powered up. I was not able to find any significant correlation between the amplitude of the noise and any combination of the above listed configurations. With my 27 ohm, 115db sens. IEM's, at 2.5X gain and 100% volume, the noise is a bit above my threshold of hearing - approx 20-25 db. At 6.5X it seems about 2-4 times louder (makes sense) so I would guess it to be about 30-35 db.
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Old 10th May 2013, 09:07 AM   #3285
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I finally signed up on this site, just to say thanks to the designer (if he's still watching) for an excellent design. I finished building and testing it last night and it works excellently- my non-scientific tests switching between ipod classic headphone socket and o2 output via the dock connector find the sound much clearer in the latter, with a noticeably wider soundstage. Not that nasty "warm" (harmonically distorted) sound you get with a lot of equipment. I'm driving some slightly knackered HD570s though, which have a few odd resonances due to the plastic coming apart (!), so when I get some new HD650s soon I hope the result will be significantly better! Thanks nwavguy!

Two things occurred when I was building/testing that you might be interested in. The first is that before installing batteries, I spent some time listening to the o2 with a 12VAC power supply attached. Everything worked fine- no voltages surprising. Then suddenly and unexpectedly, the LED blew (and of course the output stopped working). I have no idea why this happened (especially since it had worked fine, had normal brightness etc for 2hrs or so).
I spent last night carefully checking voltages etc, and again everything was in order EXCEPT the regulated output of the 7912- oddly, this measured at -12.55V - which seemed even outwith the quoted widest tolerances (the 7812 was outputting +11.85V at this point). I expected this would be still OK though, since I understand the opamps here can work without issues from slightly unbalanced supply rails.
I continued with voltage checks over the next 15 mins or so. To my surprise, on remeasuring the 7912 output, it had dropped to a consistent 12.05V. So my question is, is this a normal behaviour as it warms up, or something else?

Hopefully I can post some pictures as I finish the case design- I've managed to squeeze some RCA inputs into the back, along with the power input socket- but it was not easy! I just always prefer to have power through the rear when on my desktop. The RCA inputs will allow me to connect my M-Audio device easily.
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Old 11th May 2013, 03:43 PM   #3286
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Hey Guy's

I'm new here I have been reading a lot lately and I have a problem with my O2.

A couple of months back I build my O2, while voltage testing all looked good. but after connecting the headphone I came to a problem: I seem to have the following problem: LOUD TURN ON TRANSIENT as described in the FAQ of NwAvGuy. But then backwards: when I turn off the device I have a loud turn off transient and turning on is normal.

So what did I do already: I checked all the voltages again. Q1 and Q2 seemed to be a bit off so I have replaced them with new parts. After this I did the voltage check again, now all seems fine. But after connecting headphones the loud turn off transient is still there...

I can't find nobody in this thread who has the same problem and google isn't much help either. Is there somebody with a bit more experience that pleas can help me?
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Old 12th May 2013, 05:01 AM   #3287
agdr is offline agdr  United States
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Quote:
Originally Posted by Rollmop View Post
Then suddenly and unexpectedly, the LED blew
A few people have posted about that happening over the last year or so, some saying the LED actually exploded like it was hit with a huge amount of current.

I think there is a small design error in RocketScientist / NwAvGuy's power managment circuit. Pin 6, the inverting input on the second half of U2, gets pulled all the way up to the positive rail when the mosfets are off, which they are for the first second or so when the O2 is turned on to prevent thumps. But the input common mode voltage range of the NJM2903D comparator that he is using only goes to 1.5V below the positive rail. When that parameter is exceeded strange things can happen, like the other (non inverting) input which connects to the LED (pin 5) start pulling a lot of current and burn out the LED.

I don't have a solution for this one - would need a comparator with rail-to-rail inputs, open collector, and works with 24V - other than try a different NJM2903D chip. People don't report the problem very often, so apparently most of the NJM2903D chips can take the abuse.

Last edited by agdr; 12th May 2013 at 05:05 AM.
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Old 12th May 2013, 05:44 AM   #3288
agdr is offline agdr  United States
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...more info, looks like I exceeded the 30 minute edit time. From this discussion thread at TI

Comparator LM2901AV: Common-mode input-voltage range: How to calculate / define - /etc... Amplifiers & Other Linear Forum - /etc... Amplifiers & Other Linear - TI E2E Community

it sounds like the TI/National version of the part will allow one input to go all the way to the positive rail as long as the other input (LED in this case) stays within the common mode input voltage range.

So another thing to try would be the TI version of the part, LM2903P, Mouser #595-LM2903P.

Last edited by agdr; 12th May 2013 at 05:47 AM.
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Old 12th May 2013, 04:44 PM   #3289
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Hello DiyAudio!

I want to build a ODAC, could someone confirm these components allows me to build a complete ODAC:

Click the image to open in full size.
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Old 13th May 2013, 06:54 AM   #3290
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[QUOTE=agdr;3487670.

So another thing to try would be the TI version of the part, LM2903P, Mouser #595-LM2903P.[/QUOTE]

That is an interesting point you make- I hadn't examined the specs of the opamp, and probably wouldn't have concluded that if I had. It is possible that when I first tried with new batteries, which were probably/possibly not fully charged, the LED current surge occurred when the mosfets turned off for the first time- since it didn't happen on initial power-up. Subsequent battery discharges have lead to normal operation ("normal" in my case seems to be the dreaded on/off mosfet switching- but at least I know when to charge the batteries...)
Fingers crossed that the same problem won't happen again, otherwise it's going to get a bit expensive in LEDs.
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