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

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  1. Alkolor
    I think I'm decided on the spacing for the large caps now. I'll be avoiding Ø18 as those have been deemed a negligible change, and I believe this thread has picture confirmation of Ø16 caps fitting in the normal spacing.

    I'd still appreciate hearing anyone's thoughts on audio grade vs. high endurance type electrolytics in the coupling (C3, C6, C13, C14) vs decoupling (the rest) positions, if I've understood the circuit correctly, as well as output resistor choices (R8, R9) that people have tried. This is all very interesting, but still quite mysterious.

    It's still a pretty tight space on the board since I'd be working in the lineup of the RCA outs and a couple of the film caps and large electrolytics. I'm sure I could figure something out if I really wanted to use those Ø18 caps, but after messaging Pete and getting his feedback, I think I'll be working with Ø16 or less since the 1000uF up-size would have negligible effect on my headphones. I think I'll avoid any lead acrobatics for now, but thank-you for your input.

    I think the default board has multiple pot spacing options, at least enough for the one I will be using.

    These are interesting, but I'm sticking with the default board for this build. I'll look at custom boards or designing my own if I get deeper into this stuff.
  2. docentore
    Not compatible with the Blue Alps RK27. I will be mounting mine on daughter board off the board. This will also help with mocrophonics when changing volume.
    NuClear235 likes this.
  3. NuClear235
    Here is pisture of better universal PCB design easy to use axial or radial capacitors. (under the orange ones)

  4. tomb
    I can offer some opinions based on experience, but they're still only opinions. C13 and C14 might benefit with a film cap, but you'd have to use something like a Wima film or similar, because very little else will fit. As for using FG electrolytic caps all over, why not? They're audio grade, but with the switcher power supply, they may do as much benefit in the power decoupling role as in the signal path. It was probably simpler to just spec and buy all FG caps and be done with it. The ones in the signal path are bypassed with Wima film caps, anyway. Probably in this situation, upsizing the caps may have as much benefit as improving the quality.

    I would not recommend the use of power caps in the signal path, if you can keep from it. There are instances where that's done, but reasonably good audio-grade electrolytics (Nichicon ES, FG, KZ, or Elna RFS Silmic) are more available now than in the past, so there's not really any reason not to use them. Power quality caps do not sound very good. The bypass Wimas will temper this to a large degree, but they will still sound worse. The audio-grade electrolytics are inexpensive enough anyway.

    EDIT: If you are looking for more specifics, you will get more bass with Nichicon ES caps, but you need to bypass them with films. They have tremendous bass, but are not very good with highs. Nichicon KZ are excellent all-around, but are often detailed to the point of sounding etch-like. Nichicon FG are good all-around, but sort of a compromise between the other two.

    Elna Silmic RFS II are as good as the old Black Gate NZ caps, IMHO. However, they are HUGE for their ratings. You may have to compromise on rating or figure out some ingenious mounting methods.
    Last edited: Feb 12, 2019
    Alkolor and endia like this.
  5. Alkolor
    That's an interesting idea to consider, but quickly looking over the ones available in 10uf, I would have to get into more spacing acrobatics than I'd like to for this build. Do you know what using a smaller capacitance one would do? The 4.7uF ones look small enough to fit.

    I'm in the "why not?" category here as to making things complicated by diversifying the BOM and looking at different brands, as well as the mixed opinions on how much audio grade really matters. Cost isn't much of a factor for only a few caps. It's been a fun learning experience to question these things. I guess it would be better to phrase my question "is there any downside to using power caps in the positions that are not coupling the signal path?" I plan to keep C3, C6, C13, and C14 as audio grade (being the coupling caps in the signal path), but could save some space and get higher endurance with some power caps for the rest. Seeing your use of the UPW for C5 and C11 earlier in the thread sort of sparked this question.

    I have these 3 types lined up as my possible choices for the 4 signal path caps, or all of them if I decide not to go with the power caps elsewhere. Elna's are currently the front runner in the "why not?" upgrade due to their recommendations in many posts. The 470uF ones should fit as they are Ø16, which I see fitting in earlier pictures in this thread, but the 1000uF ones are out (only FG comes in Ø16 and they're out of stock right now - I'd prefer to keep the 35V rating). For the 10uF caps, I'm unsure if I could squeeze in Ø8 for the 50V ones to match stock ratings, but the 35V ones match the stock FG size and are noted as "upgrade" on Pete's BOM.

    For the power caps, if I go that route, I am looking at UPW's. I like the higher stats on the Panasonic FR's, but for some reason the 10uF ones are listed as $56 on Mouser right now and are out of stock. I could get the B version and deal with wide lead spacing though.

    Thanks for your response.
  6. tomb
    Go down in smaller capacitance, and you'll lose bass response. That said, Pete probably has a safety factor built in, so 4.7uf ones might work. However, Wima makes MKS film caps with lead spacing of only 5mm. It will take a bit of acrobatic lead bending (if you don't want to just squish them in there), but they'll probably fit easily enough. Almost ANY film cap is going to be better than an electrolytic in the signal path. C13 and C14 AC-couple the FETs to the NuTube, so there might be quite an improvement.

    No, there's no downside with using good quality power caps in power positions. I would argue, though, that Panasonic FM and Nichicon UHE are better than both of the ones you mentioned.

    Here's the thing: without trying to be critical, the NuTube Millett Hybrid uses a switching AC adapter. It's a good quality adapter, to be sure, with only about 150mV of ripple. However, you can hear improvements in ripple down into the single digits of mV in some headphone amps. A high quality linear-regulated adapter (Jameco has some nice, inexpensive ones) might be down around 50mV of ripple, or at least three times quieter than the supply Pete spec'd for the NuTube. Maybe the lower one makes a noticeable difference in sound quality, I don't know. We'd have to ask Pete to find out. Switching adapters very easily adjust themselves to a wide range of line power (120 or 240 without a conversion plug). Linear-regulated adapters can't do that. So, the switching power supply ensures that the NuTube MH can be built all over the world without hassle and great results. It's just an educated guess on my part why the switching power supply was selected. Again, we'd have to ask Pete to know for sure.

    However, if you can hear the effects of the ripple (not the ripple itself, necessarily), then that means the power supply is partially heard through the amplifier circuit. In that case, it might be more beneficial to use audio-quality electrolytics for the power supply, too. IOW, the fact that they may sound "good" may be more important than the fact that they have more ESR and less ripple current rating than a good power cap.

    A further note from the above answer on film caps - many film caps' leads are quite easily adjusted to different spaced pads on the PCB; electrolytics, not so much.

    That $56 is probably a Mouser error. I've seen that happen before on other parts. You might just call them if you really want that cap. While not quite as good, the Panasonic FC is perfectly fine.

    You might just have to read up on the spec sheet, though - or compare with DigiKey to make certain, because this one: EEU-FR1H100B is only 33 cents. The only problem is that Mouser says the lead spacing is 5mm, but the diameter is 5mm, too - that's impossible. So there may be a good chance that the lead spacing is actually 2mm on that cap. It's worth investigating by studying the datasheet on the Mouser page for that cap.

    If you want a great power cap, look at both the ESR and ripple current rating. I think actually that ESR (less is better) is more important in digital circuits (DACs), whereas the ripple current (more is better) is better for power applications.

    One thing is probably certain above all else - this is way too much analysis/thinking for the NuTube. It's inexpensive enough to buy some of all of these parts and then try them out on your own to see what differences you may notice, if any. :wink:
    Last edited: Feb 12, 2019
  7. pmillett Contributor
    What Tomb says :)

    You might be able to fit in the 10uF Wima MKS part. I used it on the class D hybrid. For a 10uF film cap it's pretty small.

    One goal with this design was to make it as affordable as possible. That's why I used the electrolytic coupling caps there and also the switching adapter. Not to mention the international issues. That's also why I used the same caps in various positions - to simplify the BOM. That said, I don't think there is a huge difference in this circuit between audio grade caps and good low ESR caps...

    It can't hurt to use a linear adapter, assuming it has reasonable filtering and/or is regulated so that it doesn't have a lot of 120Hz output ripple. 120Hz ripple will probably cause more audible artifacts than the switching regulator ripple. C1 and C5 are the most important to help reduce the ripple, so using a switching adapter, low ESR is important for these two. For some reason I tend to use Nichicon electrolytics... UPW are OK, but Tomb is right, UHE is better. UPW's have been around for many years - UHE are newer and in general have lower ESR.

  8. Alkolor
    I won't deny that, but half of my fun is the research and discussion around component selection. As a designer (in another field), I get the value of simplicity in design and the BOM, but also like to see what I can customize in a 1-off build. When you know simplicity is a design factor, you wonder what you can do if you throw it out. I also don't often get the chance to trial with different components for only a few bucks, so I'll heed your advice and give that a go. I have some time before Pete can ship the NuTube and board, so right now, I'm cruising the forums and datasheets.

    I'll definitely look at getting film caps for C13 and C14. My spacing concerns for any of the caps aren't due to the leads but the body footprint. Bending the leads in won't be a problem, but fitting an 11x7.5 film cap next to the tube, resistors, and other caps will take some doing. Plus, for under $1, I can have a pair of electrolytics in case I can't manage the film cap. It seems like a good candidate for under board mounting since the film cap won't be like sticking a 25mm+ electrolytic on the bottom, and they'd be much easier to lie on their side (or just under-mount those 4 resistors for space up top). I could do the same with C4 or C7 to make room for the larger 1000uF caps too. So many options. I'll also be looking into the linear power adapter.

    As for all the types, I made a spreadsheet of the specs (part of my enjoyment). Across mentions from various forums, I've compared UFG. UKZ, and RFS for audio caps, and UPW, UHE, FR, FM, and FC for power caps. That probably stirred the fire of "why audio caps?" since they don't list all the specs and rate lower in most areas compared to power caps. So they will fall into my "why not" category wherever I decide to use them. The UHE vs UPW was unclear in this one since UPW has higher ripple and lower ESR for the 10uF's, but the reverse is the case in the 470uF's, but UHE's have a higher life rating both ways. The frequent mentions of UPW caps and the datasheet noting them for switching power supplies made them seem like a good reference point, but UHE being a newer series makes sense. The Panasonic caps have a similar flip-flop in ratings between FR and FM, and FR may similarly be the newer series, but they also beat the Nichicons across the board for specs.

    I looked at the EEU-FR1H100B and it is tapped style. I bet the leads are just bent out to make that spacing, which isn't an issue. Thanks for pointing that one out. Oddly, the FM series doesn't have any 10uF options.

    Thanks for entertaining my rambles
    Last edited: Feb 13, 2019
  9. tomb
    Lying on their side underneath the PCB is perfectly fine. You would be surprised at how many film caps are shoe-horned in to odd places in an electronic project.

    Having been at this now for a number of years, I can give you some insight on cap history - at least from a headphone DIY perspective. Traditionally, Nichicon had UPW and Panasonic had FC. Those were the quality power caps that everyone used and recommended. They are still just fine for almost any electrolytic application but in the audio signal path. Both FC and UPW were considered high-performance and still probably have the widest ranges of sizes and ratings of "quality" electrolytic caps. The difference was that Nichicon UPW were available at Mouser, but not Panasonic. DigiKey carried the Panasonic, but not Nichicon (at least I think they didn't have them in the early days). Along the way (early 2000's, I think), Panasonic hit the market with the FM caps and everyone swooned. That lasted for quite a while. I think Nichicon came out with UHEs sometime later, but they never gained the reputation of the Panasonic FMs (for some unknown reason). The Panasonic FRs are a recent addition. There are many, many other series of caps made by both Nichicon and Panasonic with better ratings than the UPW/FC, UHE/FM, but not with as large an offering. Anyway, that's the background: from the perspective of dealing with both Mouser and DigiKey through the years. It may be that Nichicon and Panasonic always had these caps, or had them years before, but the key to us DIY-ers was when Mouser and DigiKey started offering them.

    As a for instance, neither one had the audio-grade Nichicons or Elnas in the early days (1990s to mid 2000s). One had to order them from Handmade Electronics, PartsConnexion, etc.

    Last edited: Feb 13, 2019
  10. docentore
    I'm still testing other op-amp options. How to confirm that amp is oscilatting or not? By looking for the sound artifacts? By checking the heat coming from IC?
  11. ShizzyGuo
    Does this amp power low impedance planar headphones well? Or should that be avoided because of damping factor issues
  12. pukkita
    Have a pair of HE-560v3 on the way, will post back...
  13. CatalinM
    I'm using it with a pair of Hifiman Ananda and I haven't noticed any issues.
  14. orderingrabbits
    I'm having an odd sort of issue. I bought a stepped attenuator and wired them to the volume pot pads, and it works great. However, the first couple steps of volume and the last step are really noisy, and when the attenuator is set to these positions I can pick up radio (like, it's clear enough I can actually recognize the songs and what people are saying) by touching the RCA ground with one hand and moving around my other hand. Any idea what might be going on? Other people say that noise increases at the extremes of the normal volume pot, is this just the same issue? I get enough volume throughout the clean section so it's not a huge problem but I'm just checking to make sure this isn't a symptom of a larger problem.
  15. Alkolor
    Just ordered the rest of my components. Now to play the waiting game..

    After much deliberation, I decided on all the cap changes I wanted to try. Since they were so cheap, I also picked up a full set worth of Elna's and Pan FR's for the 10uF caps in case I feel like changing things up. List of my changes and initial build plan below.
    • 10uF WIMA caps at C13 and C14 for an improvement on the input signal, and a backup one I may try at C1 for ripple smoothing
    • 22uF Pan FR's at C2 and C10 for added muting time and regulator output overhead
    • 10uF Elna's for the remaining spots
    • 470uF Pan FR's at C5 and C11 for low ESR in these power spots
    • 1000uF Elna's at C3 and C6 for possible bass improvement with lower impedance headphones
    • 80mV ripple AC adapter (nearly half)
    • TKD pot and a fancy knob
    There will be some lead wiring acrobatics and under board mounting to fit the larger components, but the challenge will be fun. Hopefully these don't turn out to be a bad idea.

    Thanks tomb and Pete for the feedback, and this thread for all the info.

    Can't wait to build!
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