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Old 16th August 2011, 03:21 PM   #121
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A big THANK YOU to Olli1324 for his Group Buy effort!

And thanks for your added comments Rembrant and the link. I agree about the run time. And if you don't want to compromise the audio performance with the low power op amps, bigger batteries would let you have your cake and eat it too.

Others might be interested in this topic as I agree batteries are important if someone mainly wants to use the O2 for portable use. Some might be tempted to modify the battery scheme.

LiFePO4 batteries might last longer but the verdict is still out on the sort of inexpensive cells used in the battery you linked. They haven't been generally available long enough to really know. I've had prism LiFePO4 cells fail unused shortly after purchase because they were poorly made. And there are many similar reports from others. I don't think it's reasonable to extrapolate from expensive industrial LiFePO4 cells like A123. We don't yet know how long the cheap Chinese consumer versions will last.

Conversely, I honestly have 4+ year old Tenergy 8.4 NiMh batteries still going strong in various applications with plenty of capacity left. I don't fully cycle any of them daily, but some have hundreds of cycles. The key is avoiding damaging cell reversal and the controller in the O2 addresses that.

I'm sure it's not a huge deal to some, but with shipping the LiFePO4 batteries come to $33 which is more than the cost of the entire O2 completed PC board and triple the shipped cost of the Tenergy NiMh batteries in the parts list. $33 buys 3 sets of the NiMh batteries and I would wager by the time the 3rd set dies a single set of Chinese LiFePO4s will have long since given up. But, for some, the extra run time could be worth it.

If you're really after the added run time, you could certainly adapt an O2 board into a bigger enclosure with the extra charging circuitry and those battery packs. Or perhaps you're already going to do that.
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Last edited by RocketScientist; 16th August 2011 at 03:23 PM.
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Old 16th August 2011, 05:39 PM   #122
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There's a couple errors on the mechanical drawing with respect to the hole diameters in the panel (the "d=xxx" numbers). Hopefully nobody has ordered, or made, any panels yet. And even if you have, you can just make a couple holes larger. But I wanted to alert everyone to the error ASAP. I'll be revising the PDF shortly.

I'll also be publishing the Front Panel Express CAD file although at this point it's untested (I haven't ordered a panel) and I'll make that clear. I have printed the panel to scale, cut out the openings, and checked it for fit. That's how I discovered the errors mentioned above.
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Old 16th August 2011, 08:42 PM   #123
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The new files are posted in the O2 Resources section, I've revised the THD vs Output plots in the first O2 article, and I've also added some input overload plots also shown below.

The first plot is running on batteries with about 80% charge showing 2.0V input, the second is on AC showing a very "hot" 2.9V input. In both cases the total distortion from both stages combined is still negligible (see the captions for more details):

Click the image to open in full size.

Click the image to open in full size.
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Old 18th August 2011, 07:47 AM   #124
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RocketScientist, I read your comments re: different gain settings and different op amps, where you mentioned the NE5532. I haven't given thought to those kinds of circuit mods. I have been thinking about what I want mine to be when finished, and longer portable run time would be nice. I've decided to get the extra parts needed for the low power version O2, so I can do some comparative listening between the two. I already have the OPA2277s. At one time they were in a +-6V Cmoy-style HP amp I had. A perfectly capable audio chip, IMO, but not really enough grunt to power my Grado SR80s directly. I'd be interested in any other info you might have in regards to these power/performance issues. Your blog has a "might be worth it" verdict for 32ohms 'phones like the Grados. Can I add some series load resistance? There's power to spare.
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Old 18th August 2011, 08:54 AM   #125
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@sofaspud, the main issue with the low power version is the TLE2062 in the output stage. There just are not many op amps that can drive low impedance headphones well that come in a DIP8 package, can handle a 24+ volt power supply, and have significantly lower quiescent current than the NJM4556. In all my research there's exactly one: The TLE2062

I think you're on the right track with listening to both versions with your headphones and you can decide if you hear a significant difference. To be honest, I doubt you will even with the Grados (unless it's subjective bias--i.e. your brain plays tricks on you). There are locations on the board if you want to try adding series resistance. There is power to spare, and that would ease the strain on the output stage, but it might also somewhat degrade the frequency response and perhaps damping factor. I would have to look up the impedance plot of your Grados and do some math to estimate by how much. Adding 4 ohms is certainly safe but doesn't buy you much. Adding more like 16-32 ohms would lower the distortion but I'm not sure at what "cost".

I'll be publishing my 2nd op amp article shortly and it shows the performance of the OPA2277 vs the NJM2068 (and NE5532). By itself it's not a big difference. The dominant issue is the TLE2062 into a 32 ohm load.

One other slightly more twisted idea... you'd have to hack up the PC board a bit, but you might be very happy with ONE NJM4556 with just one op amp per channel and still use your OPA2277 in the gain stage. That would roughly cut the idle current in half from the regular O2. And it would still perform better than Grado's own RA-1 $350 ($400?) headphone amp which uses a single op amp. The Grado headphones are relatively efficient so you won't come close to reaching the 70 mA data sheet (100 mA practical) limit of even a single 4556 unless you want your ears to bleed
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Old 18th August 2011, 10:14 AM   #126
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I appreciate the response and all your hard work (and to Olli1324 for GB). I may well give several of these options a try, but I also want to stay close to your "low cost "one size fits all" headphone amp" approach. That appears to point heavily towards the NJM4556. The run time could be addressed at the power supply end if need be (but no, I'll stay out of any cell technology debates, thank you). I may make mine wallwart-only. I haven't decided. Looking forward to the new article. Thanks.
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Old 18th August 2011, 03:14 PM   #127
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Could someone make a BOM for the Mouser upload tool? That would make it really easy to get everything we need with a much lower chance of error.
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Old 18th August 2011, 03:51 PM   #128
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https://docs.google.com/leaf?id=0B52...3Y2Iy&hl=en_US

Download that and copy and paste the purple highlighted cells into the Mouser upload tool.
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Old 18th August 2011, 04:14 PM   #129
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Im really loving the concept and idea behind this amp, the write ups and all the technical mumbo jumbo I dont quite fully understand. I can't wait to build one!

Do you think we could get some nicer pictures of the populated board? I would like to visualize it some more
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Old 18th August 2011, 04:17 PM   #130
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Thanks forsaken. More pics are on the list of things to do... The board I built up to test the V1.1 PCB is rather "ugly" and I want to build up a proper one for the pictures.
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