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Old 5th November 2011, 07:20 AM   #641
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Hi there, just an all-around newbie here.
I've been soldering a O2 kit recently, and noticed I've got a pair of mismatched C13 C14 (coupling?) capacitors, in which one is a 2.2uf one and the other is a 3.3uf cap.
BOM states it could be ">=2.2uF >=50 V", but I'm wondering if installing a mismatched pair would deteriorate the signal, or is it just irrelevant/insignificant?
I know you're thinking "why ask the internet and not just go out and buy a new pair of caps", but I'm just thinking if it's worth 1 hour+ of time (no electronics shop around where I live unfortunately) to change a cap that may or may not cause any difference.
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Old 5th November 2011, 11:31 AM   #642
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You may be able to measure the difference, but definitely wouldn't hear it. Nothing worth any appreciable amount of effort for sure.
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Old 5th November 2011, 11:43 PM   #643
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Hi, finally finished my O2 and am very happy with it. Been using batteries this last week whilst I was trying to build a desktop enclosure, that all went a bit wrong today so bought a large Hammond case from my local Maplin(UK).

Using it with an old Pioneer DV-757Ai multi-format player & Sennheiser HD-650's. I've settled on low/high gains of 2.25 & 3.5... had no issues during the build and I really can't fault the O2.

Thanks to RocketScientist for designing such an amazing cheap headphone amp!
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Old 6th November 2011, 01:10 AM   #644
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Originally Posted by agdr View Post
Check around the gain switch and U1 for solder bridges to the PCB ground plane. The metal shell of the gain switch is connected to ground via the two pins at the corners and the two middle pins of the 6 gain switch pins are soldered to ground. You might have a bridge from one of the remaining 4 pins of the switch to one of those. A magnifying glass is highly recommended for this one! Some bridges can be very hard to see just by eye.

If the gain switch were just bad/open (no connection) you would get a gain of 1 in that position. Would still sound OK in that channel, just not as loud. But if the gain switch gets shorted to ground in one channel the inverting input of U1 gets grounded and effectively turns the op amp into a comparator. Your incoming music waveform would get converted into square waves at the same frequency which would sound very distorted. Below is a plot of what happens there.

You can also test for a solder bridge by unplugging the amp from the AC adaptor and pulling out the batteries to disconnect all power. Then measuring with an ohmmeter from pin 6 of U1 to ground (metal shell of the gain switch) and flip the gain switch in both positions. Then do the same with pin 2 of U1. If you measure a dead short to ground at either pin in one of the gain switch positions, there is your solder bridged trace.

The other thing to check is that the values of R17 match R21, and R19 matches R23, of course. With the amp powered down and disconnected as above you can just measure across those resistors with the ohmmeter. In one position of the gain switch you should get the right reading.

Good luck!
Thanks for the reply. Everything checked out okay checking the gain switch to the two pins on U1. I double checked both resistors and they matched up. I may try using the scopes at my campus lab and see if the output is coming out square like you described. I would be checking the output of U1 so pins 7 and 1 of it correct?

I did notice if I toy with the button on the switch, move it around a bit the sound appears to be normal, equal channels but then goes wrong if I let go or mess with it again.
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Old 6th November 2011, 02:07 AM   #645
agdr is offline agdr  United States
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That sounds like a bad switch (open connection). In that case your gain would just drop to 1 in that channel. It would not sound distorted, just not as loud as the other channel. The switch might have been defective from the start or may have been damaged by too much soldering heat.

A scope would tell you for certain! Yep, pins 1 and 7 of U1 for the output vs. pins 3 and 5 for the inputs. It would be a good idea to take a look at the power supply rails too with the scope.

Last edited by agdr; 6th November 2011 at 02:20 AM.
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Old 6th November 2011, 02:46 PM   #646
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Completed my first O2 build on Friday with only a few small self-induced issues along the way. Amazingly, it worked perfectly despite my best attempts at screwing it up. It sounds great! Drives my HD-595s to any volume I can tolerate without any problems. Listening to Radio Paradise now via my iPhone and their 128K AAC link. Sounds fabulous!

Thanks again to NWAVGuy, Ollie and everyone who made this happen. What a great way to start my DIY Audio "career!"
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Old 7th November 2011, 12:58 AM   #647
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I imported the bill of the materials into mouser and did a double check of everything just before ordering and I noticed that most of the gain resistors and a few other resistor's watt ratings are 1/2, 1/3, and 1/4 instead of the specified 1/8. I cross-checked the part and manufacture numbers and they're correct, but why are the power ratings getting changed? If order them will they not fit? This problem showed up in both versions of the BOM I've checked and I'm getting a little frustrated because I keep having to change my order and it's getting confusing to check whether or not I have the correct parts selected.

Last edited by dewasiuk; 7th November 2011 at 01:07 AM.
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Old 7th November 2011, 01:08 AM   #648
MrSlim is offline MrSlim  Canada
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Originally Posted by dewasiuk View Post
I imported the bill of the materials into mouser and did a double check of everything just before ordering and I noticed that most of the gain resistors and a few other resistor's watt ratings are 1/2, 1/3, and 1/4 instead of the specified 1/8. I cross-checked the part and manufacture numbers and they're correct, but why are the power ratings getting bigger? If order them will they not fit? This problem showed up in both versions of the BOM I've checked and I'm getting a little frustrated because I keep having to change my order and it's getting confusing to check whether or not I have the correct parts selected.
Send me a PM with your email and I'll share my Mouser project with you so you can compare yours with it..
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Old 7th November 2011, 03:14 AM   #649
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Originally Posted by MrSlim View Post
Send me a PM with your email and I'll share my Mouser project with you so you can compare yours with it..
I compared the two and everything seems good now. I didn't know that higher wattage rated resistors can get as small as 3.9 mm or less
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Old 8th November 2011, 12:02 AM   #650
sebash is offline sebash  United States
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I built an O2 on Saturday and it turned out well. Took me about 7 hours total with setting up and prep, soldering and finally testing.

Top of the O2
O2 amp top | Flickr - Photo Sharing!

Bottom of the O2, to show off my not so good soldering work.
O2 amp bottom | Flickr - Photo Sharing!
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