The Objective2 (O2) Headphone Amp DIY Project
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sofaspud
diyAudio Member

Join Date: Nov 2005
Location: San Antonio
Quote:
 Check the U2 pin 1 and pin 7 voltages" p1= -11.72, p7=8.5
That >3V difference doesn't seem right to me. I think that ideally they should be equal + and - voltages. I regrettably haven't kept as close attention here lately as I could have, but have you verified the "Install Only U2 Next" and "Check The Raw DC Voltages" portions of the O2 Initial Testing instructions?
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sgrossklass
diyAudio Member

Join Date: Nov 2004
Location: Germany
Quote:
 Originally Posted by jfrana "Check the U2 pin 1 and pin 7 voltages" p1= -11.72, p7=8.5
Pin 1 seems OK. Pin 7 is odd. Why should it not be able to swing almost all the way to +12? What's the voltage on pin 4?

agdr
diyAudio Member

Join Date: Sep 2010
Quote:
 Originally Posted by sgrossklass Pin 7 is odd. Why should it not be able to swing almost all the way to +12?
The R8 - R25 voltage divider. That +9.8V RocketScientist shows for pin 7 in his troubleshooting voltages never got updated - still for the old R24 = 2.4M. With 1.5M it works out to be around +8.65V.

Last edited by agdr; 17th February 2012 at 02:17 PM.

 18th February 2012, 08:58 AM #2124 Zsubbo   diyAudio Member   Join Date: Oct 2011 How can I calculate the gain resistors value to have 3.3~3.4x in my O2?, I can't seem to find it anywhere on NwAvGuy's blog.
 18th February 2012, 10:49 AM #2125 sgrossklass   diyAudio Member   Join Date: Nov 2004 Location: Germany That's opamps 101: Gv = 1 + Rf/Rg Solve for Rg. Gv is what you want, Rf is 1k5. Make sure you leave at least an extra 20..30%, preferably 30..40% of headroom above Gv*Vin,max when using sources like a CD player. Digital sources may have output amplitudes of up to +2dBFS with very "hot" CD material (actually the maximum I've seen was +2.0something dB). Output amplitude is normally given for 0 dBFS. For a CD player with exactly 2.0 Vrms (outputs tend to be slightly louder nowadays, 2.2..2.3 Vrms is not uncommon) that means a maximum permissible gain of about 2.6 when running on mains supply. That's why 2.5 and 2.0 are popular low gain options. People with very sensitive in-ears would even tend to go for unity gain (Rg removed) to get the volume pot into a range with better channel tracking etc. Last edited by sgrossklass; 18th February 2012 at 10:58 AM.
 18th February 2012, 11:03 AM #2126 Zsubbo   diyAudio Member   Join Date: Oct 2011 Thanx, I'll be using a Matrix mini-i from USB as a source, it has an output of 2,1 Vrms, so I think I'll just go with the gani of 3x, that way there should be enough headroom, and I don't have to buy resistors. As a bonus, heres a few pics of my build.
regal
diyAudio Member

Join Date: Jan 2004
Location: MD
Quote:
 Originally Posted by sgrossklass That's opamps 101: Gv = 1 + Rf/Rg Solve for Rg. Gv is what you want, Rf is 1k5. Make sure you leave at least an extra 20..30%, preferably 30..40% of headroom above Gv*Vin,max when using sources like a CD player. Digital sources may have output amplitudes of up to +2dBFS with very "hot" CD material (actually the maximum I've seen was +2.0something dB). Output amplitude is normally given for 0 dBFS. For a CD player with exactly 2.0 Vrms (outputs tend to be slightly louder nowadays, 2.2..2.3 Vrms is not uncommon) that means a maximum permissible gain of about 2.6 when running on mains supply. That's why 2.5 and 2.0 are popular low gain options. People with very sensitive in-ears would even tend to go for unity gain (Rg removed) to get the volume pot into a range with better channel tracking etc.

If you use headphones that don't even come close to needing 2VRMS, has anyone considered bypassing the gain stage. I mean a lot of us have the gain resistor jumpered running the input at unity, if our DAC is 2Vrms 600 ohm output, is that enough to hook directly to the pot ?

 18th February 2012, 07:57 PM #2128 KGee   diyAudio Member   Join Date: Feb 2012 Am I supposed to do anything with the middle back row pin of the two 3.5mm jacks? The lead barely goes into the hole (you can just see it in the picture), but do I still fill the hole with solder? Thanks.
 19th February 2012, 12:59 AM #2129 Fiola   diyAudio Member   Join Date: Oct 2006 Location: San Diego I upgraded the screws for the front panel from the ones they supply with the case to a nicer one from McMaster-Carr. I used their black hex socket head cap screw and I think it complements the black front panel really well. Thought I'd share it if anyone else is tired of the sliver screw looks These are the ones I got McMaster-Carr I used thread size 6-32 since i already had the tap for it.
 19th February 2012, 03:30 AM #2130 ethanolson   diyAudio Member   Join Date: Nov 2011 Location: http://binged.it/ukmr5Y definitely looks good with those hex screws and your volume knob

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