The Objective2 (O2) Headphone Amp DIY Project

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Laptop and phone adapters will be DC rather than AC. The O2 is the oddball in specifying an AC adapter.

Your meter is set to AC volts in the picture, which is correct for the 02 but not for a laptop charger.

If you get no readings then check the internal fuse in the meter.
 
Well...either that or I am the problem, check e.g. that picture. Am I doing something wrong with the DMM? I tried checking other AC and DC adapters, e.g. from my laptop, phone charger etc. and I never get any value?

8RhKPwr.jpg

Looks like your DMM is the culprit
 
Laptop and phone adapters will be DC rather than AC. The O2 is the oddball in specifying an AC adapter.

Your meter is set to AC volts in the picture, which is correct for the 02 but not for a laptop charger.

If you get no readings then check the internal fuse in the meter.

I tried DC and AC setting on the laptop and phone adapters (just to make sure) and it always reads zero. I get readings for the resistors though...Damn, so close yet so far :D
 
Typo in Objective2 resistance test schematic for R14 and R20

I've just completed my first O2 build and it sounds awesome through the ATH M50Xs. A bit nerve-wracking connecting and powering the amp up for the first time, even with sacrificial headphones, but so far it's working perfectly.

But one problem during the build that others on this forum seem to have encountered also was during the testing phase, which took hours to track down and figure out.

On NwAvGuy's "O2 Details" blog page here, in the 'Initial DIY Testing' section, it states "... If you have a DMM, with only U2 in its socket (U1, U3 and U4 sockets should be empty), measure across each of the resistors shown in the diagram to the right ...". The 'diagram to the right' is the 'o2_resistances_no_power.png' schematic here, which clearly states for R7, R14, R3, and R20 "All four: 100 - 300". But as I measured and others have pointed out also, R14 and R20 are in fact 10K resistors, which was really confusing when cross-referring to the testing schematic.

Clearly from the instructions the resistors are supposed to be measured in circuit, and I'm pretty sure they're meant to be 10K: the BOM here lists them as such, and the circuit diagram here, and in the 'Circuit Description (geek stuff)' section of the blog also: "... the Johnson Noise of the 10K input resistors (R14 and R20) ..." and "R14 and R20 set the input impedance to 10K ..."

My apologies if this has already been pointed out elsewhere on this forum but I looked and looked and couldn't find anything, and if I'm wrong then please let me know, but I spent hours trying to work out why the readings were different til digging back through the documentation.

This post is meant to be constructive - hopefully it will help anyone else building an O2.
 
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You have to take the circuit at face value...

R14/20 set a 10k impedance only if R3 and R7 are zero ohm links. Anything other than a link for those resistors will alter the 10k impedance seen at the input.

If the source is DC coupled then the value of R3 and R7 then also play a part in DC offsets... complicated isn't it :)

Finally the 220pF caps across the 10k depend on the value fitted for R3 and R7 with regard to what these caps do. They form a low pass filter (RF rejection) and so the lower the value of R3/7 and the less effective they become.

Fitting higher values for R3/7 also forms an attenuator in combination with the 10k's.

The simple answer is just fit something 'reasonable' like a 1k.
 
Hi there, I'm just getting back into a bit of DIY and wondering if this is still a good project to have a bash at? I've been through all the info on the blog but noticed it's not been updated in a while so I headed onto here...

Are folk still building as the 2011 BOM or have there been any changes/updates with part spec improvements or availability changes?

Also, happy to hear other suggestions of builds to look at.

MAny thanks and happy building!
 
Hello MGeeOne.

I build an O2 a few years ago. I've enjoyed it quite a bit and do think it's still a great project to build.

Not sure about updates to the BOM parts, but I'm sure you can compare the list to what's available now and make a choice. As always, there are many folks here willing to help if you come up with a specific component selection question.

As for other builds to consider: you'd have to share your goals and preferences for anyone to give you anything meaningful to look at.

All the best!
 
Thanks Fidget, I'll have a proper look through the BOM in the morning...need to switch off a bit and stop being distracted by what's going on in the real world...although our lockdown over here gives me a bit of time to have a play...no work plus kids at school makes me want to build stuff!

As for goals...I have a nice Beresford Caiman DAC that I used as my daily with my K701s and SR225's and would like a battery powered amp for use out and about. Nothing too fancy, but able to drive these cans....oh and maybe not be too shabby for my Shure 425 IEMs.

But like the count in the US...no huge rush ;)

Cheers