Building O2 amp - High DC offset

  1. thighmaster69
    Hey guys, this is a crosspost from diyAudio, was wondering if you guys had anything to add

    recently have started making an o2 amp. While checking through all voltages on the board, using NwAvGuy's troubleshooting guide. I noticed that the DC offset at P2 measured 36 mV and 16 mV, which is a lot higher than it should be. According to NwAvGuy, it should be less than 8 mV. I measured the voltage at the op amps but again it was the same values and swapped around, they presented the same value, so I don't think it's the op amps.

    The values seemed to change a bit, especially battery power vs AC power.

    I checked all the solder connections under the DIP sockets at U3/U4, reheated them to check for cold joints, and the resistors R12 and R13 both measure 37k ohm.

    I then checked pins 1 and 7 on U1 as according to this post and the voltages came up fine.

    I'm wondering what I should check at this point or fix, or whether 16 and 36 mV is fine?

    I plugged in some beater earphones and when turning on the amp, there is an immediate loud click.

    It seems to play music, but I got an odd issue where there would be repeated loud clicks. I haven't been able to reproduce this specific case.
    Last edited: Jun 8, 2017
  2. Speedskater
    Why not direct your question to the other forum? There are a lot of technical people over there.
  3. Rroff
    16 and 36mV themselves aren't an issue but they do tend to indicate something not right - my own DIY self build version of the O2 measured 3.3 and 3.6mV.

    Are you using parts exactly as per the bill of materials or substitute parts? especially interested in what you are using for the 2.2uf film caps and the op amps in the gain and buffer positions. Are you using 1% metal film resistors? R12/13 should really be pretty much dead on the nose of 40K give or take if you are (unless you are using actual lower values) - if you are using 5% tolerance and/or even worse carbon resistors then that might be why you are seeing reduced performance.

    Repeated loud clicks might be the power management circuit tripping for some reason.
    Last edited: Jun 12, 2017
  4. thighmaster69
    I got the DIY kit from head 'n' hifi and going by the BOM all the parts are correct. I've replaced all the ICs multiple times.
    At this point I'm nowhere close to getting this thing working properly, I'm thinking I should probably cut my losses now and buy an amp.

    I can get an O2 or a magni at the same price. For HE400is, which would be better? Also, which one scales better with better headphones?
  5. Rroff
    Honestly I really like the O2 from an engineering perspective but I'm critical of it from a subjective perspective - personally I don't rate the sound signature of the by the BoM build at all so I'd have to say the Magni though aside from some vague ideas as to the requirements of driving planars I'm not that familiar with what works best with the HE400 headphones.

    I built a variant of the O2 with LM317/337 pair regulators, uprated some of the capacitors to "audio grade" polypropylenes and changed some of the others around, changed the op amps out for OPA1612 (and a couple of different variants for the buffers) amongst other changes and think it sounds far superior to the original design.
  6. thighmaster69
    Have you personally tried the schiit magni?
    And were you using headphones with a sound sig when testing out your O2 somewhat close to the sound of th HE400i? (Bass extension, bright/warm, dry)?

    One of the things I wish I could do is up the bass a little, and I'm afraid of the magni because I heard it has less bass.
  7. Rroff
    Most of my headphones/experience have been the Sennheiser HD series, various Denon, Beyerdynamic DT range, Sony, etc. don't have much experience with planars. The Magni (and other Schiit discrete based amps) sound a touch bright to me but I've never sat down and A/B tested them against anything.

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