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Old 18th August 2011, 04:14 PM   #131
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I'm not an engineer and didn't understand 3/4 of your technical articles, but i want to believe! The alternative would be the LPA-2a, but that would be 170 for just the amp and power supply board... plus, the small enclosure of the O2 is the same one as my AMB Gamma2, so yeah.
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Old 18th August 2011, 06:59 PM   #132
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Quote:
Originally Posted by mauxigens View Post
I'm not an engineer and didn't understand 3/4 of your technical articles, but i want to believe!
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
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Old 18th August 2011, 08:00 PM   #133
sebash is offline sebash  United States
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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)
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Old 18th August 2011, 09:59 PM   #134
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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."
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Old 18th August 2011, 10:13 PM   #135
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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.
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Old 18th August 2011, 10:57 PM   #136
sebash is offline sebash  United States
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I have no plans whatsoever to drive multiple headphones from the amp, I was just curious why you would add extra resistance to the output. Thanks for the reply's. Has anyone verified the revised pcb?
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Old 19th August 2011, 01:25 AM   #137
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Quote:
Originally Posted by olli1324 View Post
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.


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.
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Old 19th August 2011, 01:41 AM   #138
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Originally Posted by Rembrant View Post
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.
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Old 19th August 2011, 02:00 AM   #139
sebash is offline sebash  United States
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Quote:
Originally Posted by Rembrant View Post
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
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Old 19th August 2011, 02:14 AM   #140
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I see, it doesn't allow you to right click copy. You have to use cntrl-c. should have tried that two days ago.
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