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

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  1. HiGHFLYiN9
    Very nice! Can you share with members what model Hammond you used? I like it much better than the Serpac, but the Serpac is cheap and good for people on a limited budget.

    I finished mine over the weekend and have been letting it break in. Initial listening sessions have been very positive. I shielded the wooden box I made with copper tape electrically connected to the front and back aluminum panels.







    Not completely done, but close :) if you're wondering, the brown boxes are closely matched Russian Silver Mica coupling caps. They're huge for 0.1uF, but sound mighty good.

    A shout out to TomB for the initial build and parts updates :D
    Last edited: Jun 1, 2017
    JamieMcC, hav2uo4 and Heffa like this.
  2. Heffa
    Wow, that looks awesome - I love the retro radio feeling of your build! Well done indeed! :beerchug:

    My enclosure is a Hammond 1455N1601

    In the picture you can see my power supply (AMB Sigma11) in two separate 1455N1201BK boxes, one for the transformer and one for the actual Sigma11.
    It has four 24V outputs so it can drive several amps at once, even though it have only driven my PPA amp (in the black 1455N1601BK enclosure) for the last 6 years or so :)

    I need to change the resistor for the LED, it's way too bright at the moment :L3000:
    hav2uo4 likes this.
  3. HiGHFLYiN9
    Nice! Lookin gewd! :) Based on your mention I'm going to put together an AMB Sigma11 myself to hang with the NuHybrid :L3000: Are you running 100VA or so to handle 4 outputs? I just ordered a 50VA (up from the 25VA in the BOM) from Antek for $23 shipped, such a good deal. It should fit in the 2200 series case, but just barely. Build Build Build.
  4. Evgeni
    I have built mine a month ago. It's a wonderful amp for my HD-650 and I love it.

    I have a slight problem though. If I touch the pot (it's the upgraded TKD pot as recommended by Pete) I can hear a buzzing noise. This somehow disappears if I crank it up to max. What could be the problem? Maybe I should ground the pot body or something?

    Also I can hear the pot generating noises when close to zero which I thought would not be a problem with a pot this price...
  5. HiGHFLYiN9
    It does sound like a grounding issue. Grounding the metal part of the pot could help. I assume the knob you are using is metal and not plastic?
  6. hav2uo4
    HiGHFLYiN9 is correct, I can confirm it's a grounding issue. I also have the TKD pot (with a large plastic, pseudo metallic knob). It used to buzz very loudly when I touched it. Also picked up A LOT of noise from nearby devices. The noise was so bad the amp was unusable. Soldered a wire onto a ground connection and wrapped it around the knob's threads and clamped it down with the supplied nut. Fixed a lot of noise problems. The circuit still seems prone to picking up noise from other devices (even with volume knob removed), but careful amp placement and wire routing helps a great deal.

    Another noise observation from my amp. I had random noise coming from the right channel. It'd usually get triggered by input signals. Figured I had a cold joint somewhere, so I reflowed every right channel connection. Didn't fix the problem. So I only used my least sensitive headphones. After quite a bit of use, problem has mostly gone away. It's still audible on my highest efficiency headphones, but it's mostly tolerable. Wondering if it's coming from the nuhybrid tube itself and it's burned itself in.

    But with my higher efficiency phones, it was nearly impossible to get a good volume setting. So I bought an attenuation cable and it gave me better volume control and made the noise floor completely inaudible for even my highest efficiency headphones. Wish I would have had one sooner. Below is a link to the one I have (hope it's not against the forum rules - no affiliation, just like it).

    Some of you guys have made some really nice looking cases (I went with the plastic half-case in the BOM). I imagine the shielding helps with many of the noise problems I described, can anyone confirm?
  7. HiGHFLYiN9
    Other than the pings here and there with startup I haven't noticed anything (I haven't tested it much yet). The whole thing from front to back is shielded, so that may be helping. I'd imagine a device like the NuTube would pick up noise from cellular signals, so if your phone is close by, you'd likely hear it. I had a Manley that would pick up cell phone noise and it took a while to figure out where it was coming from.
  8. silverfishla
    Hey guys, I'm going to build this amp soon as I am receiving some of the parts that I need. I had a question about the Op amps (and their function in the unit). Are the Opamps used singles or duals, and what is the function of each? Is one for right and left channel or is one a buffer? I'm a newbie, so I apologize for asking such basic questions. I've read over and looked over P. Milletts schematics and information on his DIY page. Most of the explanations he gives are about constructing the unit but not what all of the separate pieces do ( I guess that's just assumed that we should know, but I don't). Can anyone draw out a schematic and explain what each part actually does (that is so much to ask, so I don't expect that it is a very reasonable request, but I'm giving it a shot anyways :) ). Your amps all look great and are an inspiration.
  9. vixr
    OPA551PA is a buffer, if you look at the first post in this thread, you will see the link that Pete supplied, all your questions will be answered there...
  10. trevmex
    This looks really fun to make. I have a noobie question about it, though. Will this amp work with low impedance headphones
  11. hav2uo4
    This amp is able to drive all my low and high impedance headphones with ease. But if they're highly efficient you may experience some background hiss/noise and have trouble getting volume settings that aren't too loud. But an attenuation cable can be built easily enough or purchased relatively cheaply and solves both problems. It's a neat little amp. Can be a microphonic little devil, but I'd recommend without hesitation. Hats off to Pete!
  12. trevmex
  13. ken_chuang
    I just gotten the PCB and Nutube too. I realized there were reports of microphonic noise, so I ordered a regular pot rather than a stepper. Then I stumbled on this Nutube Guide ( http://korgnutube.com/en/guide/ ), where there is a section on "To prevent microphonic noise".

    For my build, I am planning to try the following to address microphonic noise.
    - Wrap Nutube over with thread seal tape (aka. PTFE tape or plumber's tape), similar to tube damping rings.
    - Acrostic damping material inside chassis.
  14. HiGHFLYiN9
    This one might be easiest "Placing a heavy metal plate (e.g. lead) on the surface of a Nutube can also reduce noise." Trimming a piece of steel and putting it on top with double sided tape wouldn't require much effort. Also, if you put some footers on your chassis (I used EAR, they're less than $1 each, http://www.partsconnexion.com/isolation_ear.html ) should help.

    I don't think it's really possible to decouple the tube from the PCB it's connected to due to all the solder connections. The best bet would be to give it its own PCB off board or move the potentiometer off the board.

    It's fun to think about. This is a new audio device that not many people have used yet, so best practices haven't been established.
  15. trevmex
    OK, just got the big bag o' stuff from Mouser, and the tube and board. Just waiting on the upgraded pot and footers from PartsConnexion and I'll get started!
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