Elekit TU-8900 2A3\300B Single Ended DHT Tube Amp - Build, Impressions, and Review

Discussion in 'Headphone Amplifiers and Combo (DAC/Amp) Units' started by Maven86, Oct 19, 2021.

  1. Maven86

    Maven86 Almost "Made"

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    I actually downloaded your PDF over on HF earlier. Really awesome write up, the mod with the transformer wiring sounds very tempting to do...

    I think there's a lot of truth to this. From my experience, I look at it this way. If you give 3 winders 3 of the same iron cores, you'll likely have 3 different sounding transformers. Winding methods play an even bigger part than core material in the final sound, it's almost like a secret family recipe of sort.

    That being said, it's also been my experience that if you gave that same winder another type of core to work on, say permalloy (Ni), the resulting transformer will also sound different than the iron but still have traces of their "house sound". It's like the audio equivalent of an artists signature on two different kinds of paintings.

    +1 on the Monolith Summits. Ward, and Yves have also been excellent to deal with.
     
    Last edited: Oct 22, 2021
  2. Whitigir

    Whitigir New

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    Having balanced input with transformers is fun and actually quiet enjoyable, or should I say worth an upgrades. Even though I had some strange issues here and there, I have solved it, and now I am enjoying it tremendously. Most of my stuff are all true balanced, so in comparison between RCA vs modified balanced inputs, there are clear upgrades in perceived performances.

    I will try to use a few more brands of transformers to tweak the input a bit and see if the experiences would go anywhere more positive ? Between tubes rolling and modifications experiments, my wallet cries :(
     
  3. Ryu

    Ryu Friend

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    Absolutely gorgeous build! Eagerly waiting for those sound impressions! I've been looking at upgrading from my Modded Crack for a while, and have heard great things about these kits.
     
  4. Maven86

    Maven86 Almost "Made"

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    Thanks. Things have been busy but I'll do some final comparisons this weekend and have the impressions up by Monday night.

    I've liked the Crack+Senn combo but I'd say the Elekits are voiced very differently. If possible, I'd recommend having a listen before splurging on a kit.
     
  5. EagleWings

    EagleWings Friend

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    Elekit TU8900

    I’ve been wishing to join the DHT game for a very long time and was finally able to do so with the Elekit TU8900. I got mine with the Lundahl Amorphous transformers. With literally zero soldering experience, building the amp was NOT exactly a walk in the park. Thanks to @famish99 , @Jh4db536 and @baseonmars , who provided helpful tips along the project. Just to be safe, I would not recommend this kit as your first project. Some things I would like to share with folks, who are interested in building this amp, but are new to soldering and DIY.

    Soldering and Building:
    Meticulous planning and taking it slow will ensure avoiding any errors. A good work bench, a PCB holder and appropriate lighting will go a long way. I used the WBT leaded solder, which has a bit of silver content. I learnt recently that, it may be best to avoid solders with silver content for PCB sodlering. Had I known, I’d have gone with the Kester 63-37 leaded solder.

    Headphone Out:
    As this is primarily a speaker amp, it has a resistor divider network for its headphone out, to bring the gain down to more useable level with headphones and also to present a load that is safe for the OPT. In case you want to bypass this resistor network, you can plug the headphones straight to the speaker terminals. If you don’t plan on driving speakers, then connecting the headphones directly to the speaker taps would be the easy route. I intend on using speakers with the amp and I want to retain the feature that mutes the speaker-outs when you plug in a headphone into the headphone jack. So I modded the resistor network of the headphone board to not have any resistors. All I had to do was skip all the resistors and use a wire to jump at the R125/R225 positions. You can see the Red and Blue wire on the headphone board in one of the pics.

    Please Note: This amp has a ton of gain that, if you bypass the resistor network, you might not have enough headroom to adjust volume. Even with a 300 Ohm Sennheiser and a DAC with 2 VRMS line out, you can hardly get past 8 o’ clock position. A DAC with a volume attenuation or putting an attenuator between the DAC and the amp would help. For the purest and shortest route for headphones, you can wire straight to the secondaries from the OPTs. You would need a couple of 4 pin VH connectors and a separate 6.3mm female socket.

    TU8900 - 2.jpeg

    Coupling Caps:
    Victor offers the V-Cap CuTF or the ODAMs as the standard upgrade options. From what I gathered, V-Caps can be a bit hi-fi sounding. As the Wavelight is already a bit hi-fi sounding, I wanted to push the tone towards the organic/natural side. After vetting my options, I ended up going with with the Audyn Copper Max. Even the Audyns Cu Max are slightly longer than the provided hole for the cap. I had to file away a bit on the short edges to make the hole longer to fit the Audyns. Victor advises not to use caps with wax, so I couldn’t use Jupiters. Duelunds are too wide for the hole and you might have to do something like what @Maven86 is doing. Audionote, Mundorf and Jensens were some options which seemed like the appropriate size for the cap hole in the PCB.

    Tubes:
    For the input/driver tubes, Victor offers the Sylvania 12BH7A and the Brimar CV4003 (12AU7). I’d recommend doing research on the input/driver tubes to find out if Victor’s offerings will fit your bill. Else, I’d recommend picking up the tubes of your choice from elsewhere. Based on the 12BH7 an 12AU7 I have on hand, I feel a 12BH7 is required to take full advantage of this amps dynamics and tactility. But it could be that the Brimar 12AU7 I have on hand are dynamically gimped.

    Victor offers good prices on the Linlai (Cossor) tubes. I picked up a pair of Cossor WE2A3 and a pair of Cossor Black Plate 300B (I wish I had gone for the Cossor WE300B instead). @yotacowboy has written some nice comparisons between the Linlai Elite 300B and Linlai WE300B clones. I would think the same would apply to the 2A3 counterparts. The long and short of it is, the WE clones have a bit of a tubey character, while the Elites are tilted towards clarity and neutrality.

    TU8900 - 3.jpeg

    Sound:
    I hear the tubes, caps and transformers require 50-100hrs of run-in. So I will hold-off from sharing any sound impressions for now.

    TU8900 - 1.jpeg
     
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    Last edited: Nov 5, 2021
  6. Maven86

    Maven86 Almost "Made"

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    Thanks for the impressions. I've had a similar issue with gain in non-NFB mode and almost feel like a step down transformer might be useful depending your source's output voltage. Do you still have this problem when turning NFB on?


    Also, sound impressions are now up.

    Elekit TU-8900 2A3\300B Single Ended DHT Tube Amp - Build, Impressions, and Review
     
  7. Tachikoma

    Tachikoma Almost "Made"

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    Somewhat OT, but did you commission the Otomon 801A specifically for headphones? How much did that cost?
     
  8. EagleWings

    EagleWings Friend

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    Thanks man. That was a great write up with comparisons. I am yet to test the NFB mode. @Jh4db536 reported back pretty much what you wrote about the NFB mode. So I have not been keen to try it. I’ve been busy and sick this week, so haven’t had a chance to listen to the amp this week at all. But hoping to give it a shot sooner or later.
     
    Last edited: Nov 12, 2021
  9. Joskus

    Joskus New

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    The 8900 is on its way, and I can't wait!
    The Lundahl's amorphous, Cossor WE2A3, Sylvain 12BH7A are also on their way.

    Thank you EagleWings for sharing your solution for modding the headphone out!
    Still, there are some things I am unsure of. (Mainly the safety of it)

    Let me sum it up, the way I understand it:

    The possibility of using it to drive both speakers and demanding headphones was one of the main appeals for me. Most amps can, but I was super excited about the addition of a headphone out that was essentially the same amp as for the speakers.

    The headphone output was also attractive as it would make switching between the two easy. Still, as you already know, the headphone output is not ideal to drive the likes of my he-6, which would make more sense to drive out of the speaker taps.

    Now I do understand that driving 50-ohm headphones out of the speaker taps can be dangerous, primarily to the amp itself.
    A speaker amp to headphone converter box is the most straightforward solution as I understand it. I would still have to unplug and plug the banana speaker cables every time I wanted to switch from headphones to speakers and vice versa.

    As I understand it, the headphone out on the front of the 8900 works essentially like a converter box. Or have I misunderstood something?

    Is there any way of making the headphone out more powerful and direct WITHOUT disabling the security aspects of having such a resistor network?

    Also, is the headphone out really that inferior to the speaker taps, just because of the resistor network?

    This is my first DIY amp build and first post on superbestaudiofriends. Please let me know if I have misunderstood anything.

    :)
     
    Last edited: Dec 22, 2021
  10. EagleWings

    EagleWings Friend

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    You can put in the R123 and R223 resistors, which would provide an ideal load to the output transformers. Skip all the other resistors and hardwire/jump the R125 and R225 positions. You can see a picture in my post above, where I have a blue and red wire running in place of a resistor.
    P.S: Do not skip the J17 and J18.

    I am not too familiar with the HE6, but I am aware it is a low sensitive headphone and craves power. Are you sure 2A3 tubes would be sufficient to drive the HE6?
     
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  11. Joskus

    Joskus New

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    I don't know yet. This is the HE6se v2, which are a bit easier to drive than the OG HE6, I believe. My thought was that if 8900 is powerful enough to drive high-efficiency speakers, they should drive the HE6 alright. If not I will get 300b, as I know of others that drive the HE6 with 300b with great success. Let's see!

    Anyhow, do you believe the speaker taps will provide noticeably a better sound than from the headphone out, even after your suggested mod?

    How are you finding the 8900 headphones out experience with headphones so far?
     
  12. Joskus

    Joskus New

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    I think this is the perfect solution for me. Thanks again!
     
  13. EagleWings

    EagleWings Friend

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    If you look up the sensitivity rating of the speakers that people drive with a 2A3 amp, it usually would be in the order of >97dB. But the sensitivity of the HE6SE V2 is 83dB. That's equivalent to driving a pair of low-sensitive bookshelf speakers. But yea, you can always switch up to the 300Bs, if you are not happy with the result with the 2A3. Let us know once you try it.

    Right now, I have literally no resistors installed in the headphone resistor network. So the headphone out in my 8900 is essentially equivalent to speaker taps. I am not an expert in the subject, but from what I have gathered so far, connecting a headphone load (30 - 300Ohm) to an OPT spec'ed for a speaker load (6 - 15Ohm), will make the OPT present a very high impedance on the primary to the output tube (2A3/300B), which can result in a very tight and dampened sound. Installing the parallel resistors R123/223 in the headphone network will make the OPT see a speaker load and will result in different sonics. I plan on installing the R123/223 resistors sometime sooner or later, to see what the effects are. So do try it with and without the R123/223 resistors to see which you prefer.
     
    Last edited: Dec 23, 2021
  14. Joskus

    Joskus New

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    Makes alot of sense. I will start with the R123/223 first and see how it goes.

    May I ask, are you not afraid of damaging the amp when bypassing the resistor network completely?
     
  15. EagleWings

    EagleWings Friend

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    Based on my understanding, connecting a higher impedance load to a speaker OPT shouldn't be an issue. But having the parallel resistor seems to be the more ideal way. What is not safe for the OPT is connecting a lower impedance load (4 Ohm) to an OPT spec'ed for 8 Ohm loads.

    8900 has a ton of gain and might not have enough headroom to adjust the volume for headphones, if the resistor network is skipped. That might not be a problem in your case, given HE6's sensitivity.
     
  16. Joskus

    Joskus New

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    Again, thanks for your insight. I will keep you updated on my results. My 8900 is currently traveling the world, so it might take a bit.
     
  17. Beefy

    Beefy Friend

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    You sure the units are comparable here? Most speakers express sensitivity in dB per Watt, and most headphones including the Hifiman use dB per milliwatt.
     
  18. EagleWings

    EagleWings Friend

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    Whoops, my bad. Yea just by numbers, 2A3 should be more than enough.
     
  19. Tachikoma

    Tachikoma Almost "Made"

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    Only if you're running it at maximum volume for an extended period of time. If my reading comprehension hasn't failed me, the higher reflected load can put voltages higher than B+ on the primary and cause a short. This is more of a problem for guitar amps.
     
  20. Joskus

    Joskus New

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    Okay, got you.

    I am starting to suspect that the stock headphone out with the original resistor network should be enough for the HE6.

    Does the resistor network degrade soundquality notably, or is it mainly about output power? I understand quality and power is connected.

    Still, If the headphones happen to have enough gain with the full resistor network installed, would you loose out on the potential of the amp by much?

    Steve Guttenberg reported in a review on the Elekit 8600 that the headphone out kind of sucked. If there was enough gain, I wonder if the resistor network is what is limiting the 8600?
     

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