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?
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?
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.
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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.
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.
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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 ?
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.
These are the ones I got
McMaster-Carr
I used thread size 6-32 since i already had the tap for it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
O2 complete kits - UK only
If anyone is interested, I have 10 complete O2 kits available for immediate shipment - UK only - see http://www.diyaudio.com/forums/group-buys/207163-o2-headphone-amp-complete-kits-uk-only.html
If anyone is interested, I have 10 complete O2 kits available for immediate shipment - UK only - see http://www.diyaudio.com/forums/group-buys/207163-o2-headphone-amp-complete-kits-uk-only.html
Nothing in particular. You just need to keep the gain stage out of clipping by choosing your gain settings accordingly, as outlined before.
People with sensitive headphones/IEMs and loud sources tend to drop low gain to unity, but your Beyers are firmly on the other end of the sensitivity spectrum.
People with sensitive headphones/IEMs and loud sources tend to drop low gain to unity, but your Beyers are firmly on the other end of the sensitivity spectrum.
Thanks, but I'm a hopeless newb to this. Would I be going for a higher gain setting e.g x6.5 or a lower gain setting e.g. x1 for my Bd dt700 600 ohm head phones? Or am I missing something and is it the output voltage I should be concerned about? I plan to use the amp at my office always connected to AC power.
So why not build the amp as per the plans, and see whether you have any gain issues? The stock arrangement gives you a low gain of about 2.5x and a high gain of about 6.5x, selectable with a front-panel switch. If you are concerned order some extra resistors (values are on the BOM and schematic) and use the sockets, so you can try some different settings. Report your findings back here!
Hello,
eventually I will build the objective2, when PCB-boards are easily available in europe. It would be my first headphone amp then. I'd like to experiment on a perfboard first, having a little experience building a few guitar-stompboxes in the past. Just for proof-of-concept I'm thinking of building the amp-part of the schematic first and the power-supply and power-management later.
Here my questions: Would it be possible to use temporarily two isolated 9V DC outputs of my guitar-effects power-supply for positve and negative rails of the o2 amp section?
Or would it destroy something? (that the technical specs will suffer I'm aware).
What would happen, if one of the both 9V-outputs is disconnected temporarily? Would my 300 Ohm headphone or the circuit be fried?
Please be patient, because I'm writing not in my language, thank you.
eventually I will build the objective2, when PCB-boards are easily available in europe. It would be my first headphone amp then. I'd like to experiment on a perfboard first, having a little experience building a few guitar-stompboxes in the past. Just for proof-of-concept I'm thinking of building the amp-part of the schematic first and the power-supply and power-management later.
Here my questions: Would it be possible to use temporarily two isolated 9V DC outputs of my guitar-effects power-supply for positve and negative rails of the o2 amp section?
Or would it destroy something? (that the technical specs will suffer I'm aware).
What would happen, if one of the both 9V-outputs is disconnected temporarily? Would my 300 Ohm headphone or the circuit be fried?
Please be patient, because I'm writing not in my language, thank you.
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If by isolated they mean each output is on its own transformer secondary (I did some searching but couldn't find a schematic for the unit on the net), it should work fine. Just hook two of the 9V outputs in where the batteries would normally be connected, being careful to get the polarities correct. If one supply disconnects that would be no different than one battery disconnecting - or shorting itself out, which happens - and that is what the O2 power management circuit takes care of. So no problem.
You will find out soon enough about the isolation. If they are not truely transformer-isolated you will get a nice spark when you hook the wires onto the center two battery terminal locations, that ties the "-" of one channel to the "+" of another. Won't hurt the O2 but the power supply may not be happy.
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Thanks, agdr,
but what I meant was to omit the O2 power management also and just to build the main amplification first and to get some first impressions. As far as I know the DCT 200 power-supply ist truely transformer-isolated and short-crcuit-protected. Under these circumstances - how big is the risk to damage the headphones by temporarily using just the 2 x 9V outputs on the rails of the amp-section?
but what I meant was to omit the O2 power management also and just to build the main amplification first and to get some first impressions. As far as I know the DCT 200 power-supply ist truely transformer-isolated and short-crcuit-protected. Under these circumstances - how big is the risk to damage the headphones by temporarily using just the 2 x 9V outputs on the rails of the amp-section?