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"Millett Hybrid" redux: NuHybrid headphone amp, using the Korg Nutube

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  1. docentore
    Another day another mod. I have replaced the C13 and C14 with film 10uF (WIMA MKS2) caps and C20+C21 with MKS2 2.2uF.
    Testing this with 2 AD825's I have received today - liking so far. Still waiting for OPA627 to come for tests (bought some on ebay but I theink they are too cheap to be genuine, not going to bother).
    gaz2613 likes this.
  2. ShizzyGuo
    Can I leave it on 24/7? Any downsides besides life expectancy of tubes which I heard were rated pretty high
  3. pmillett Contributor
    Sure. Korg quotes the lifetime at 30,000 hours. That's 3.42 years if you leave it on all the time.

    As far as I can tell, this is based on the lifetime of a vacuum fluorescent display as quoted by Noritake. The Nutube may actually last significantly longer, since the plate current used is lower (because the brightness of the phosphor doesn't matter much). The failure will be a very gradual degradation of characteristics, not a sudden failure...

    I am not aware of any failures yet...

  4. gaz2613
    If anyone knows anyone who is looking to move their NuHybrid Headphone amp on please let me know as I would like another one. I am based in the UK. Love this little amp! Got some Burson V5i's on the way for my current one to try out. :)
  5. gaz2613
    I have been listening to mine and notice a distortion in high notes such as instruments and even on high vocals that sound almost like a crackle when I turn the volume up. When listening at quite a low volume it's crystal clear. Would it be worth adjusting the bias voltage higher than 11v?

    It was doing this with the original OPA551 opamps as well as the Burson V5i ones.

    Please help
  6. pmillett Contributor
    One way to find out - try it! Try moving the bias up, and also down, maybe 2V at a time, and see what happens. You can't hurt anything...

    gaz2613 likes this.
  7. Varoudis
    Hi all,
    I need help with troubleshooting! I think I have burned my head and I can see..probably a silly mistake!
    I have attached photos and measurements. Back photo is misty as I sprayed the flux to see the joints better.
    - With Pete's advice I have re-flow all pins (nothing changed).
    - Voltage on TPs is very unstable, goes up/down 1V...
    - RCA-Out have AC+DC voltage .. if you plug phones you get super cool noise...amplified... (see measurements)
    - I have double check the resistor values with an online calculator.. they seem ok. Can you spot something?? Pete's photo online has 10k R3/4 .. BOM has 1k (I have 1k).
    - I get DC.. all over the place.. even with the DC coupled Outs.
    - GND on PS side has 50mV difference from TP-GND (the board side)???

    Something is leaking like hell?? IC dead? Diode?
    How should I approach it? I have test equipment (scope, power supply, good DMM etc) and its not my first project, I have build a number of HAM projects, transmitters etc...

    Any ideas?

    EDIT: most voltages are referenced to TP3. I have some PS-GND referenced measurements.



    Last edited: Mar 16, 2019
  8. gaz2613
    I have noticed the bias on mine seems to fluctuate. When I first set mine it was around 5 mins for the bias to settle before I set it to 11v. A few weeks later when I checked them it was at 10.35v and settle at around 10.65v after 5 mins so I reset back to 11v. I have distortion on mine too on high notes and vocals when volume is turned up so not sure if mine has an issue or whether the bias needs adjusting as I have not gotten round to testing further yet.
  9. Varoudis
    Settling like this sound okish. Mine is just amplifying noise and cracks like full volume... lol
    There is something wrong with leak on the ground side or something..
  10. docentore
    I'd agree with Varoudis on this. There has to be leak somewhere. Wash the board with some good PCB cleaner.
    Mine does not fluctuate at all, I've set is @11V and it stays like that.
    gaz2613 likes this.
  11. gaz2613
    Mine starts off a little under 11v and then gradually comes up to the 11v I set it to. My distortion problem turned out to be my Beyerdynamic DT990 Pro headphones. I tried some Denon D2000's and the distortion was gone so the Beyerdynamic ones will be going back to Amazon.
  12. Varoudis
    False alarm guys!
    My build was soooooo "broken" that I tested everything with any sort of test equipment that usually you don't need for a simple build like this... that I couldn't see in front of me...
    I had forgot a super static pudding all the time when I was testing the amp.

    The amp is very nice! Voltage is super stable to 1mV... (I ended up tuning to 10.000V!!) Very clean sound and minimal hiss with even unity gain (I haven't test it with my sensitive Klipsch speakers though).
    Ill use it as a tube unity gain preamp (tube distortion in essence :) )

    One observation that I didn't expect is that the amp reverses the waveform. I though that the tube stage with flip it once and then the OpAmp with flip it again :S
  13. docentore
    What was wrong?
  14. Varoudis
    Sorry I didnt say it clearly.
    I usually have a desk mat for my electronics work (in my home office/ mancave).
    As I didn’t, I kept one of Mouser’s black pudding.... uber school boy error.... this pud is there you let any discharge go through easily... so uder currents flew all over my board every time I was testing...

    Heheheh memes about my stupidity welcome!!

    Now on a positive note. What opamps have you used with this amp?
    Any measurements for the highend or lowend extention?

  15. docentore
    I have tested good few: OPA627, OPA604, AD825, AD8065, NE5534, OPA134, AD797, ADA4610. Can't hear much difference tbh and I have no hw (and knowledge) do perform any measurments. Went back to OPA551 as it gives more current than other ones and I'm running the amp with T50RP-Argons.
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