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Old 19th August 2011, 02:22 AM   #141
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Originally Posted by sebash View Post
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?
I missed this question earlier, sorry. Sofaspud was considering the low power version that uses low power op amps. That version isn't very happy with anything below about 50 ohms although it's still acceptable at 32 ohms. Sofaspud, I believe, discussed adding resistance to raise the impedance the low power op amps see to reduce the distortion. But it sounds like he's favoring the regular "high power" version now.
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Old 19th August 2011, 02:45 AM   #142
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HEADS UP! I was hoping I was just being paranoid, but my caution not to buy parts yet has already proven to be good advice.

Someone asked the question elsewhere if they could get rid of the power control circuit and just switch the AC power. In exploring that, I ended up doing more power on/off testing beyond what I had done before based on the simulations that agdr had run that didn't match my earlier results.

I uncovered a specific low battery condition where the existing design can put out a fairly ugly transient. It probably wouldn't hurt most headphones but I don't want to take any chances.

THE BEST FIX: The right way to fix the problem is I'm simply adding a new capacitor value to the BOM and changing one other value. C16 and C21 drop from 0.22 uf to 0.022 uF (which is the new line item). And C22 increases from 0.47 uF to 1.0 uF. I'll be updating the docs shortly. The net change in cost is +$0.12.

THE WORKAROUND: If some of you have already bought parts, don't panic. If you just leave out C16 and C21 completely, and keep C22 at 0.47 uF, it solves the low battery problem. The downside is the turn on "click" is significantly worse but not dangerously so (it's about 0.8 V RMS for half of a 400 hz cycle). You can also add the correct cap values later if you want next time you order parts for another project. Or use "junk box" parts as they're not critical capacitors. Any value from 0.01 uF to 0.027 uF would be OK at 50 volts or higher.

Sorry about this, but I'm glad I caught it now before hopefully most of you have ordered parts. I've been working on the O2 in my spare time and it's required some late hours, etc. Don't be too shocked if something else crops up. There's little point in ordering parts until the first batch of boards are ordered at (I think?) the end of the month. The boards will have the longer lead time. So I encourage everyone to please be patient in case other changes need to be made.
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Old 19th August 2011, 04:01 AM   #143
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I should add, if you're only going to use your O2 on AC, the previous cap values are just fine. The fix only has to do with running the batteries down to the shutdown threshold.
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Old 19th August 2011, 06:50 AM   #144
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sounds like he's favoring the regular "high power" version now
I'm guilty. It was I. And yes I'm going for the high power version. I'm not as decided about springing for the TLE2062s, but my current plan is to investigate just what kind of portable ps performance I can get with my headphones in my normal use. Then proceed from there. A little 5 or 12 volt DC-AC inverter for charging might be a handy accessory.
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Old 19th August 2011, 08:54 AM   #145
Andori is offline Andori  England
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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.
Playing around importing the BOM into Mouser, I see that 512-KA7912ATU_Q is now discontinued. Although a 512-KA7912ATU is the direct replacement, I'm guessing a cheaper 863-MC7912ACTG would be suitable (as you used the +ve version).
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Old 19th August 2011, 04:19 PM   #146
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Yea I noticed that the linear regs are non stock parts at mouser. I am just wondering what replacements would be suitable.
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Old 19th August 2011, 04:24 PM   #147
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LM7812ACT
LM7912ACT

Are the alternitives specified in the BOM
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Old 19th August 2011, 04:34 PM   #148
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Mouser USA has 500+ of the Fairchild regulator in stock but they've gone from 0.19 to 0.67. The $0.19 part was not RoHS compliant so that's why they were cheap and are discontinued. I was just trying save everyone $0.30

The best part is the $0.47 On Semi MC7912ACTG. The On Semi regulators have a lower noise spec and the "A" versions have tighter tolerances.

I'll revise the BOM when I'm fairly sure I don't have any other changes. As I said, please don't order parts yet. There's little reason when the boards are still many weeks from being delivered.
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Old 19th August 2011, 05:36 PM   #149
Andori is offline Andori  England
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Originally Posted by RocketScientist View Post
Mouser USA has 500+ of the Fairchild regulator in stock but they've gone from 0.19 to 0.67. The $0.19 part was not RoHS compliant so that's why they were cheap and are discontinued. I was just trying save everyone $0.30

The best part is the $0.47 On Semi MC7912ACTG. The On Semi regulators have a lower noise spec and the "A" versions have tighter tolerances.

I'll revise the BOM when I'm fairly sure I don't have any other changes. As I said, please don't order parts yet. There's little reason when the boards are still many weeks from being delivered.
I figured that was the case

I'm waiting until the boards are ordered before I order any parts. I was just playing to see what the approx cost of parts was in GBP. They come in about 19 for the basic parts, so would need to get 3 sets (or add other stuff) to get it over the 50 threshold for free shipping.
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Old 19th August 2011, 08:11 PM   #150
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^^ Same. Though I'm going to get a bunch of things I need for other projects at the same time. When you need to pad money to an order. Think of a tool you need or wish you had. Hmm, I think I need a rosin flux pen. See how easy that was!
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