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Little Dot MK8SE / MK6 Super Mods (All verified mods are on first page)

Discussion in 'Headphone Amps (full-size)' started by redge78, Sep 26, 2015.
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  1. SonicTrance
    Only 6SL7 with the CCS installed. ECC35 and 6C8G (with adapter) would also work, but 6SN7 would not be happy at 1.2mA of bias current.

    It's good to hear that the CCS implement was a success.
    Maxx134 likes this.
  2. baronbeehive
    Yes of course!

    I believe Maxx has the 6c8g.
  3. MrCurwen
    No, I mean the actual technical guide. It's in my posts on this page: https://www.head-fi.org/threads/lit...e-on-first-page.782183/page-199#post-14089563

    I understand you are talking about gNFB here, not gain. The amp has plenty of gain, that's not a problem. Please use precise language for ease of understanding.

    The built-in problem with gNFB in this amp is that lowering it would be good for the input stage (leave room to build clean open-loop structures) but at the same time it would destroy the performance of the output stage.

    The output stage needs the gNFB.

    In one aspect, yes. The CCS improves the function of the input stage in two ways:

    1) it forces balance, improving the THD of the signal amplification

    2) it makes the input stage current draw almost completely constant, thus taking out any PSU problems there might be.

    Very efficient decoupling can make point 2 appear less important. No amount of decoupling can balance the signal though.

    Bass performance bottleneck is the output impedance of the amp. Lower gNFB, and you destroy the output impedance. Higher gNFB will tighten bass performance.

    What you call soundstage is probably hindered by gNFB. Most people think open loop amps sound more 'airy' and '3D'. I've gone over the harmonics stuff a few times here, big loops cause higher order harmonis, which mask spatial information because it is mostly in the higher register.
    Maxx134 likes this.
  4. baronbeehive
    I've tried the TS 6SL7's and again, fantastic clean sound, I think these work better than the GE's because they have that slight mellow edge which I like without any loss of detail.
  5. baronbeehive
    It's getting mighty crowded... but here's a possible space for the filter MrCurwen! It shouldn't take up much space if and when I decide to do it.

    upload_2018-9-11_14-21-23.png upload_2018-9-11_14-31-28.png

    On the right is the CCS under the coupling caps.
  6. MrCurwen
    baronbeehive I recommend you construct the B- filter outside the chassis, connect via long wires (yes it's a possible stability risk but sometimes you take risks).

    If you find zero sound quality improvements, remove filter. If there is some, then find a place for it inside chassis.

    Reason for this is that there is only a low probability for sound quality improvements. There could be some though, not impossible.

    No reason to construct experiments inside very crammed chassis.
  7. baronbeehive
    Right, good idea!

    I've had time to fully appreciate the upgrades now, and I must say this is stunning!

    I did the impedance mod at the same time as putting in the CCS....., and I felt that possibly there was yet another improvement..... but I haven't controlled the volume like I did when I was doing the tests. So with that proviso I would just say that after the sum total of the mods to date the amp now has a power and authority and fullness of sound, noticeable with drums now, for example, and bass has a real heft.

    I might have been nitpicking before because I think that the real limitation is probably my headphones which I know aren't the most resolved of headphones, and doesn't have the best soundstage or tight bass, so I can only say we are at a top level now for me with this amp.

    Thanks to MrCurwen and Sonic, I couldn't have done it without their help.
    Maxx134 likes this.
  8. Maxx134
    Yeah I spent alot of money on tubes way back before all the mods, I went thru alot of 12pin types and then moved onto the 6pin tubes to settle on the Tung-sol 6c8g which you all know is equivalent of 6SL7.

    Yeah I have heard a few great amps at around the $4k price setting from good friends, so yeah I like other amps as well.

    I am almost done relocating to start back on this hobby, and was content with my bias settings, but now after seeing this I also want to try!

    I think we should try reduce the NFB after you settle on your top tube choices and also the impedance mod would help offset the output impedance changes, but may need some hard calculations unless you wanna just try experiments.

    That is a big change in the driver section I noticed from dismal dull stock setting.

    I believe they were a bit more neutral than the 6SL7 type for me.
    The Tung-sol was most resolving for me, while other great tubes I tried (different type) where actually adding more tube sound which I felt was a bit too euphoric.
    I wanted a clean sound.

    For greater resolve I am looking at tinkering one last time at the output stage. Specifically those WCF caps.

    Also, its sad these "traditional" designs are so component dependant.
    The market is almost all traditional designed, just like some top tube amps I played with.
    No real advancement in the tube amp market.

    So I am going to try the CCS, then swap out the output tube cathode caps with some others (Muse caps), and then WCF caps I have (are to romantic) for more clear ones .

    Then give up lol.

    I would say the output tube bias change is also what made a huge difference for me.
    I no longer use stock output tube type there either.

    So this thread is still not finished , but Baronbehive did a HUGe push with testing here.

    Also thanks MrCurwen for your knowledge and input.
    I really think its great and valuable .
    I think Sonic amp will be great because of this.
    Last edited: Sep 15, 2018
  9. MrCurwen
    It's best to do one major mod at a time, to know where the possible sound quality differences come from.

    You're welcome. Have you tried the filter yet?

    With superb output impedance your current headphones could have top quality tight bass response.

    TV special tubes?

    Bass will become more boomy and overall distortion will rise.

    Thank you.

    He's on his fourth scratch build amp already I think!
  10. SonicTrance
    No problem baron!

    You're right! Started my forth build just recently.
  11. Maxx134
    12at7 family variants.

    Do you still have tube type preferences?

    I gotta get rid of some tubes but there is no section for tubes in this site.
  12. baronbeehive
    Your bias settings were good, as verified by me LOL! You could possibly have gone down to under 500 ohms for cathode setting. I found that the CCS setting that Sonic chose was good, it preserved the liveliness of the cathode setting I had previously liked best but added a touch of good clean sound which together with some really linear tubes could further enhance SQ.

    Could try experiments, I'm a bit skeptical after what MrCurwen said. The soundstage is already excellent with high gain and I wouldn't want to jeopardise the bass quality.

    My top tube is still the GE 12SL7, it has the most clean sound.

    You should get that with the CCS, as clean as we can get with a closed loop design.

    I feel we are more or less at the finish now because of the NFB problem, interestingly the other amp is also closed loop and has superb bass response.

    I was looking again at some tube reviews and they have mentioned that the soundstage is largest with GE and Mazda tubes, I wondered if you have tried them yet, I know you are interested in soundstage. If you are interested I can give you the links. They mention 12ax7, which I believe you use, and 6V6 which you use on the other amp.

    I may do this too, after the improvement they've made to the other amp I think those Kaiseis must be something special.

    Yes, I found that some of this might be due to increased volume, that's why I tried to be careful to keep the volume constant.

    Yes, I did the CCS before the impedance mod, but then did the impedance mod when I soldered in the CCS in it's permanent position.

    I felt that the CCS kept the liveliness we now have after the other changes, and the impedance mod added a touch of fullness to the sound and bass heft. But as I said I didn't keep the volume the same for this so I can't be absolutely sure.

    No I'm looking at the connections but no further than that atm.

    Oh, great. Just that the reviews said at the time I bought it, that the HE500 was designed primarily to be musical, not the last word in resolve. For example, they mentioned that the bass was of good quantity if not the most articulate such as was available with the HE6's.

    ….. ???????
  13. SonicTrance
    Now I choose tubes based on linearity, gain and looks. I don’t build amps for tube rolling.
  14. SonicTrance
  15. MrCurwen
    Are there 12 pin 12AT7 variants? I guess there could be, why not. 12 pin tubes are pretty much all either TV special purpose tubes or radar tubes.

    If there were space, and if the loop didn't become unstable, you could add proper plate loads. That would increase linearity very drastically. It's not a subtle change.


    If I understand you at all I think your other amp has an OT. That OT is the primary determinant in output impedance, the gNFB simply helps it a bit. In this LD amp the gNFB is the primary motor in providing a decent output impedance.

    If you drive the PRIMARY of an OT with a decently lowish output impedance (some hundreds or tens of ohms is overkill level sufficient), then the OT secondary will have an output impedance lower than 1 ohm. Talk about tight bass then, let me tell you.

    Yes, that basically means that the coil inside the element has a tricky impedance curve. Flood it with current, and it's as good as any. Of course there are other overall limiting factors in the quality of the headphone, but you would be surprised at what can be dug out of those cans.

    You originally wrote in the future tense. Sonic has already built a number of amps based on my design principles.
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