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.
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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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.
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.
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):
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):
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.
@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
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
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.
https://docs.google.com/leaf?id=0B5...ZDBmYi00YjA4LTk1NzgtZTMwNTAzMWQ3Y2Iy&hl=en_US
Download that and copy and paste the purple highlighted cells into the Mouser upload tool.
Download that and copy and paste the purple highlighted cells into the Mouser upload tool.
Thanks mauxigens. IMHO, it's better than "believing" in the usual snake oil with zero objective evidence to back 98% of it up. Hopefully once people get some O2s built up and spend some quality time listening to their music, instead of their amp, they will also become believers
I read through all the O2 posts and am slightly overwhelmed by all the information presented and have probably managed to mix up some of the information in my head. I do know that the O2 looks like a it'll be a useful and fun project to build.
some questions I have:
Does the output impedance of the source device (computer sound card or portable device in my case, both with unknown output impedance and voltage) have an effect of the output impedance of the O2?
I saw mentioned a modification of having multiple headphone jacks and adding resisters to them to drive higher impedance headphones, what is the advantage to that over just changing the gain multiplier? would it be useful to do that for the 250 ohm headphones I have (Beyer 770 pro)
some questions I have:
Does the output impedance of the source device (computer sound card or portable device in my case, both with unknown output impedance and voltage) have an effect of the output impedance of the O2?
I saw mentioned a modification of having multiple headphone jacks and adding resisters to them to drive higher impedance headphones, what is the advantage to that over just changing the gain multiplier? would it be useful to do that for the 250 ohm headphones I have (Beyer 770 pro)
The answer to your first question is no. As a "one size fits all" headphone amp, any headphone or line out source should be fine. There has been concern expressed over input clipping and that is a legitimate problem, but I see it as something best dealt with case-by-case. For the vast majority of builders I don't see a problem, ie don't overload the inputs! Pretty simple "fix." Most sources have their own volume control, and if you know yours has a "sweet spot" it may be worthwhile to make sure your O2 input will handle that level. Otherwise I see it as a non-issue.
As for multiple outputs, ideally you would want a separate output stage for each jack. That shouldn't be too difficult, and maybe someone will contribute that mod. I mentioned added series load resistance in the context of my 32 ohm Grado 'phones. With higher impedance headphones extra resistance shouldn't be necessary. My other pairs are 60 and 300 ohms - I'm not worried about driving them adequately. IMO you shouldn't worry about your 250 ohm Beyers. I believe the NJM4556 is spec'd for +-70mA into 150 ohms. The gain multiplier isn't the issue with multiple outs - it's the output. If you simply parallel jacks on the output the voltage is the same, but the load impedance and currents change, so series resistors may help there. But, to quote, "at what cost."
As for multiple outputs, ideally you would want a separate output stage for each jack. That shouldn't be too difficult, and maybe someone will contribute that mod. I mentioned added series load resistance in the context of my 32 ohm Grado 'phones. With higher impedance headphones extra resistance shouldn't be necessary. My other pairs are 60 and 300 ohms - I'm not worried about driving them adequately. IMO you shouldn't worry about your 250 ohm Beyers. I believe the NJM4556 is spec'd for +-70mA into 150 ohms. The gain multiplier isn't the issue with multiple outs - it's the output. If you simply parallel jacks on the output the voltage is the same, but the load impedance and currents change, so series resistors may help there. But, to quote, "at what cost."
Thanks sofaspud. Do many people actually listen to multiple headphones at once (i.e. 2 people with one amp)? I've done that on airplanes and such but that's about it.
The problem with two listeners is unless they're the same or very similar headphones, and both want the same volume, you pretty much need independent volume controls. Otherwise it's likely to be too loud for one person and not loud enough for the other. That could be done with the O2 by duplicating everything after the gain stage, but I'd suggest using the NE5532 in the gain stage to handle the extra load.
If they are similar headphones, and both listeners have similar volume tastes, just put the jacks in parallel like the Benchmark DAC1 Pre does. Because the O2 output impedance is so low the headphones won't significantly interact.
Even with two 16 ohm headphones in parallel, because most 16 ohm headphones require only a few hundred millivolts, the O2 can handle the 8 ohm load. In fact, the O2 can generate over 1 V RMS into 8 ohms fairly cleanly which is way more than most portable devices can put out into 16 ohms and would be earsplitting with most 16 ohm earbuds.
And with two 32+ ohm headphones it's entirely a non-issue to run them in parallel. The NJM4556 is a fairly amazing op amp. As configured in the O2 it's rated for 140 mA and produces 200 mA fairly cleanly.
The problem with two listeners is unless they're the same or very similar headphones, and both want the same volume, you pretty much need independent volume controls. Otherwise it's likely to be too loud for one person and not loud enough for the other. That could be done with the O2 by duplicating everything after the gain stage, but I'd suggest using the NE5532 in the gain stage to handle the extra load.
If they are similar headphones, and both listeners have similar volume tastes, just put the jacks in parallel like the Benchmark DAC1 Pre does. Because the O2 output impedance is so low the headphones won't significantly interact.
Even with two 16 ohm headphones in parallel, because most 16 ohm headphones require only a few hundred millivolts, the O2 can handle the 8 ohm load. In fact, the O2 can generate over 1 V RMS into 8 ohms fairly cleanly which is way more than most portable devices can put out into 16 ohms and would be earsplitting with most 16 ohm earbuds.
And with two 32+ ohm headphones it's entirely a non-issue to run them in parallel. The NJM4556 is a fairly amazing op amp. As configured in the O2 it's rated for 140 mA and produces 200 mA fairly cleanly.
Well, some of us don't have anything that can open that type of file properly. I would have done if I could have. If someone could just copy the mouser part# list in here with code tags that would help immensely. For the future, .txt files are compatible with anything. One could do this order on a cell phone if the BOM was a text file.
Well, some of us don't have anything that can open that type of file properly. I would have done if I could have. If someone could just copy the mouser part# list in here with code tags that would help immensely. For the future, .txt files are compatible with anything. One could do this order on a cell phone if the BOM was a text file.
All you need is a web browser. That's why I used Google Docs. The link to the Bill of Materials Spreadsheet can be found here:
O2 Resources
It should open a new browser window with the spreadsheet in it and you should be able to copy and paste directly from the 2 shaded columns right into Mouser. It works for me in Firefox, Chrome and IE.
Well, some of us don't have anything that can open that type of file properly. I would have done if I could have. If someone could just copy the mouser part# list in here with code tags that would help immensely. For the future, .txt files are compatible with anything. One could do this order on a cell phone if the BOM was a text file.
You can also download and install the open Office or Libre Office suites, they are free and open source. I use these myself instead of having to pay microsoft for their office suite.
OpenOffice.org - The Free and Open Productivity Suite
LibreOffice Productivity Suite Download LibreOffice