Building A Tube Preamp: The Aikido

Discussion in 'Audio Hardware' started by fully_articulated, Jun 13, 2019.

  1. fully_articulated

    fully_articulated Forum Resident Thread Starter

    Hi guys. I hoped members here might enjoy seeing the build process for what will hopefully soon be a fully constructed tube preamp.

    After having some success last year putting together an ANK DAC kit and a Nelson Pass amplifier design, I was looking for a preamplifier kit for use in between them. I was missing the tube preamp section from my previous integrated amplifier, so was inclined in that direction for a separate piece. Quickly I landed on the VTA SP preamps, especially their top SP14 models at http://www.tubes4hifi.com/SP14.htm, which looked to have everything I wanted - including a high quality remote volume control option.

    While researching that as a project, I found out these VTA preamps were based on a circuit topology by John Broskie called Aikido. Mr. Broskie has a blog at Tube CAD Journal where he writes highly technical articles about many kinds of circuits and their design. 95% of his writing sailed completely over my head, but in the posts I could understand I found his writing interesting, entertaining, and refreshingly pragmatic. He also has a store GlassWare Audio Design that sells circuit boards of his creations.

    When building an Aikido based preamplifier a selection of tube types can be implemented for the same board, which was very appealing. I had a small collection of 6DJ8/6922 tubes already as they were utilised in both my integrated and DAC, but there was no 6DJ8 option in the VTA range. Also, by going with the Aikido option I thought it would enable [force] me to learn more about the fundamentals of what I was doing than the purchase of a full kit would. That turned out to be quite true.

    I ordered the boards for the Aikido Noval (9-pin) line stage and complimentary PS-21 power supply, both of which come with components and instruction manual. I decided that the project itself would be modelled on the VTA style, as there was not much sense to me to reinvent the wheel. This build is nearing completion (about six months after it began - much longer than I had wanted) and thought I might share the process here. I always like seeing build threads, and hopefully there's some of you here that are the same.

    Here it is in its current form. The front panel is taking longer to arrive than I'd wished, so I'm testing with a quickly hacked together MDF sheet.

    [​IMG]

    Just a warning before continuing: Although this thread will hopefully be informational, I am not an expert of any description, so I would refrain from considering this a how-to guide. I cannot guarantee anyone duplicating what they see in this thread won't be maimed, killed, or thoroughly disappointed by the finished article. In fact, I'd be very appreciative of any critiques of my theory/method by anyone more experienced. There was probably more guessing involved at some points than there should have been.

    To avoid text overload at the beginning, I'll try to bounce back and forth between the theory and it's actual application, but let's start with some pictures first!
     
    Kray, beowulf, jmpsmash and 12 others like this.
  2. fully_articulated

    fully_articulated Forum Resident Thread Starter

    In line with the VTA preamps, I decided to use the same remote control Khozmo stepped attenuator they offer as an upgrade, along with the same PSU board.

    [​IMG]

    This simple power supply (an Audiowind A230) as configured uses a L7805CV regulator to supply the 5VDC required by the Khozmo unit. The board comes with a couple of circuit diagrams showing you the parts needed. This was the first piece I put together, to test my soldering was still up-to-scratch after a long break. The LED I left off as I thought I may use it as the front panel power indicator later on.

    [​IMG]

    Curiously, as seen above, in pictures which show this PSU being used in the VTA preamps the configuration appears slightly different, with the diode positions also populated. However the schematics show diodes only in use when the board is configured for variable output. Can anyone with a VTA confirm?

    Sorry about the size of the images - maybe I should have scaled them smaller.
     
    DyersEve726, bluemooze, JNTEX and 3 others like this.
  3. fully_articulated

    fully_articulated Forum Resident Thread Starter

    PREPARATION
    Unlike my two previous builds, which were a case of a) Get Parts, then b) Start Building, this one required far more preparation and understanding of basic concepts - concepts that I had largely ignored in the past.

    Before populating any of the boards or ordering any parts for this project there were a number of variables to be taken into consideration as the Aikido circuit is very configurable. In order to begin, I had to work out the following:

    - What tubes to use. Everything else stems from this choice.
    - Tube heater voltage [configured via resistor on the PS-21 PSU board]
    - B+ voltage [configured both via resistors on the PS-21 PSU board, and resistor R12 on the Aikido board]
    - Tube idle current (bias) [configured via cathode resistors on the Aikido board]
    - Transformer(s) required.

    In addition ideas for power supply protection would need researching.

    Some of this proved a bit advanced for my limited skill-set and I ended up needing some help from diyAudio
     
    struttincool, timind and waterclocker like this.
  4. fully_articulated

    fully_articulated Forum Resident Thread Starter

    The PS-21 is really three power supplies on one board. On each side is a separate low voltage supply which can be configured at 6.3, 12, or 12.6VDC operation for tube heaters, and in the centre is a high voltage (B+) supply capable of delivering up to 300VDC depending on your requirements. This board comes with all necessary parts, but I did swap out the resistors for Dales for a couple of non-compelling reasons.

    [​IMG]

    The output voltages are set by the values of some of the resistors in this picture. The two orange 3W resistors control the PSU's B+ output voltage. There is a table in the instruction manual which gives the resulting output voltage for a variety of resistances.

    The other resistor of note is on the far right-hand side whose value controls the voltage output to the tube heaters. This is duplicated on the other side of the board for the second heater supply.

    [​IMG]

    The MUR410G diodes that make up the bridge rectifier for the heater supplies have very thick leads which were surprisingly difficult to bend. Here the rectifier consists of discrete diodes, but they can also be an integrated component like the disc shaped piece on the Audiowind 5V board.
     
    timind, JNTEX, 33na3rd and 1 other person like this.
  5. StuJM84

    StuJM84 Forum Resident

    Location:
    Kent, UK
    I know and understand exactly 0 about making electrical stuff, least of all pre-amps and HiFi bits, it would be like someone from China to approach me and speak Cantonese or Mandarin.

    However! i do find it brilliantly fascinating and i think part of me needs or wants to understand about boards and the bits & bobs on it, and so whilst i don't understand i will be following this build and enjoy it! And who knows, maybe i'll learn something too.

    So keep the updates coming! and best of luck with it of course!
     
    JNTEX and fully_articulated like this.
  6. ivor

    ivor Forum Resident

    Location:
    USA
    Great thread, congrats on your build! I would love to do this some day but I feel like I would have to read a textbook first. How did you learn the basics?
     
  7. Davey

    Davey I carry the moon inside a silver bag

    Location:
    SF Bay Area, USA
    Nice thread idea and fun to see all the pictures as it comes together. They do make some nice kits available through Tube CAD, I mentioned in another recent thread about one of the low cost modular phono preamp options they have available in the Tetra ... First full tube preamp, Yaqin MS-12B? ... Steve Graham at the https://wallofsound.ca/audioreviews/analog/diy-tube-phono-stage-build-needing-a-sponsor/ site put one together in a 4-part series last year, another fun read.

    Good luck with your project!
     
  8. TarnishedEars

    TarnishedEars Forum Resident

    Location:
    Seattle, WA
    Does this kit come with a schematic?
     
  9. Davey

    Davey I carry the moon inside a silver bag

    Location:
    SF Bay Area, USA
    The individual boards come with a user manual that contains all the info you need including ... Introduction/Overview/Schematics/Recommended Configurations/Tube Lists/Assembly Instructions. I don't know if they offer any complete kits, though the manuals do cover assembly options. As the OP implied, these aren't really kits for the novice builder, a site like the tubes4hifi home page he mentioned may be a better option there. For instance, this is the user manual for the phono preamp board I linked ... https://www.tubecad.com/2012/02/07/Tetra Sans PS.pdf
     
  10. waterclocker

    waterclocker Forum Resident

    Location:
    NW Indiana
    I've been working on a SP14 build, maybe I can help. Where did you order your Audiowind board from? I bought one from Parts Express and it came populated with the adjustment pot. I set it to 5v and it seems to
    be working well, maybe the occasional flicker of the display, I've been meaning to check how clean my 5v is. SP14 turned out nice, but I've been trying to track down a soft hum in my audio :/
     
  11. fully_articulated

    fully_articulated Forum Resident Thread Starter

    Little by little. I knew almost nothing when I started the ANK DAC last year, but you can piece it together slowly. Read a bit, watch a few videos, and some basic knowledge starts to take root.

    If I did it all again, I'd start with learning electrical safety first. 300 volts DC in the ANK, and I was largely unaware of its danger or how to protect myself.
     
    HiFi Guy likes this.
  12. fully_articulated

    fully_articulated Forum Resident Thread Starter

    Thanks! That Wall Of Sound project informed my own build quite a lot, especially the sections on chassis preparation.
     
  13. fully_articulated

    fully_articulated Forum Resident Thread Starter

    I purchased the bare board from 'that auction site'. I populated it according to the second schematic on this PDF: http://www.audiowind.com/pdf/PCB-A230.pdf

    Out of curiosity, is your Audiowind board also grounded to the main board, like in that photo?
     
  14. fully_articulated

    fully_articulated Forum Resident Thread Starter

    Thanks for messages everyone - they were nice to wake up to. Onwards!

    TUBE SELECTION
    The tube selection options for the Aikido Noval are extensive, including the 6CG7, 6DJ8, 6H30, 6N1P, 12AU7, and 12AX7. You can also use differing kinds for the first & second stages.

    Ignoring any other factors, the choice of tube could rest solely on how much gain is needed from the preamp, and chances are that it’s not very much. However I chose the 6DJ8/6922 tube type as I already had a small NOS stock of them and was familiar with their sonic attributes.

    But I would also attempt to find a configuration that could allow the replacement of the input tubes with the 6CG7 without having to change out any other components. 6CG7 tubes have a lower gain than the 6DJ8 and are also reputed to be excellent sounding, being a 9-pin version of the respected 6SN7 octal.
     
  15. fully_articulated

    fully_articulated Forum Resident Thread Starter

    The populating of the PS-21 board continues with diodes and capacitors.

    [​IMG]

    Adding a combination of electrolytic and film capacitors. The electrolytics are the snap-in variety, but I found it quite difficult to do just that and took more force than I would have liked. In front of them are the HEXFRED ultrafast diodes John sells as an upgrade option for the B+ supply.

    [​IMG]

    The first of the voltage regulators. The board is labelled LD1085, but LD1084 parts were supplied. A quick look at the datasheets for each suggested that the only difference was a slightly higher current capability for the LD1084 so I continued on, also adding a heatsink that wasn't specified - but why not right?
     
    JNTEX and Kyhl like this.
  16. blc__

    blc__ Forum Resident

    Location:
    New York City
    This is great. I've been looking at building one. I look forward to your listening impressions.
     
  17. waterclocker

    waterclocker Forum Resident

    Location:
    NW Indiana
    Yup, I've got it grounded in the same spot, but I wonder if I have a ground loop. The Khozmo audio ground is connected to the other side where the inputs are, I need to check if the grounds are connected.
     
  18. fully_articulated

    fully_articulated Forum Resident Thread Starter

    Yeah, I was looking for a way to ground the PSU in the same fashion, but when I connected it to the Khozmo it became grounded through the audio circuit. As you say, I'd be concerned about a ground loop if I now connected it to the board.
     
  19. fully_articulated

    fully_articulated Forum Resident Thread Starter

    HEATER VOLTAGE
    Selecting the tubes gave me my heater voltage; the 6DJ8 and 6CG7 both require 6.3V according to their datasheets.

    Mr. Broskie encourages using 12.6V and wiring the heaters in series for 6.3V tubes, but in that use case the heaters all have to have the same current draw - so not possible with a 6CG7/6DJ8 combo. There is even a current difference among the 6DJ8 'family' of compatible tubes, with the 6922 heater rated at 300mA, the Russian 6N23P variant at 310mA, and the premium 7308 at 330mA. I have no idea whether these variations from the 6DJ8s 365mA rating is significant.

    For 6DJ8 tubes the heater current total would be 730mA each side (2x365) and replacing one with a 6CG7 increases this to 965mA (365+600), so a transformer that could deliver over 1A per side would be needed.

    Unknown to me at the time, the easy part was now over...
     
    JNTEX likes this.
  20. Al Gator

    Al Gator You can call me Al

    This looks interesting. But I thought it was related to my favorite martial art. :D
     
    fully_articulated likes this.
  21. fully_articulated

    fully_articulated Forum Resident Thread Starter

    [​IMG]

    Although not really necessary, in order to ensure the large heatsinks for the other regulators would be well isolated from the board I took the additional step of cutting some mica squares for them to sit on.

    [​IMG]

    And here they are, attached. The values (or existence) of the three circular capacitors here are not mentioned in the instruction manual at all that I could find, but did come in bags labelled for their respective positions, so that was good enough for me and I bunged them in.

    [​IMG]

    The nut securing the heatsink to the front regulator comes very close to the mounting hardware for the one behind it, which is why I used nylon fastenings for it.
     
    Kyhl likes this.
  22. PopularChuck

    PopularChuck Forum Resident

    Location:
    Bay Area
    I love DIY projects of all kinds, and always enjoy WIP threads. Keep the updates coming, please.

    Eager to hear your listening impressions
     
    fully_articulated likes this.
  23. fully_articulated

    fully_articulated Forum Resident Thread Starter

    I'm getting a bit desperate to get this into my system and start listening, but USPS seem to be sending the front panel I ordered to an alternate reality or something. Without it I can't finish the final work needed.

    It has been hooked up to some test speakers on the workbench though, and it does make sounds, so +1 point for that I suppose.
     
    blc__ likes this.
  24. fully_articulated

    fully_articulated Forum Resident Thread Starter

    [​IMG]

    B+ VOLTAGE AND IDLE CURRENT (BIAS)
    This was the point I became stuck. The Aikido instruction manual contains a large chart of tube types and example operating points with their corresponding parts values. There is also a separate table of recommended parts values for certain tube combinations. Unfortunately I couldn’t reconcile the two as they did not seem to correlate, and my lack of knowledge halted progress.

    Most mysterious were the cathode resistor values that control the tube bias. Particularly, the examples given in the manual showed the input and output tubes being given differing values, with the input tubes being run at a lower current than the output. I'd also come across this in posts from JB's Tube CAD Journal, but couldn't find an explanation of why this might be so.

    Asking about it on the diyaudio.com forum yielded some generous help from the members there, but even so I had a hard time making sense of their info. They did confirm for me that the first stage tubes in the Aikido circuit could best be run at a lower current than the second. From what I could gather the input tubes do gain increase (amplification of the incoming signal voltage) and don't need pushing hard to do this.

    I had luckily also come across this PDF The Valve Wizard which, although geared towards guitar amps, contained a great primer for obtaining meaningful values from the tube datasheets directly. This seemed the best way to make sense of all the info presented in the manual.
     
  25. fully_articulated

    fully_articulated Forum Resident Thread Starter

    [​IMG]

    The PS-21 power supply board is completed. As you can see, it is solid-state for all supplies. I did consider using the PS-20 tube rectifier PSU that Broskie sells, but decided against it in favour of space and simplicity. It was easier to manage one board than separate ones for B+ and heaters. Maybe one day he'll produce a board like this one but with a tube rectifier in the centre.

    [​IMG]

    At the bottom of this photo, you can just see where this board will be connected to the chassis ground. It will be the ground termination point for the whole circuit - both this board and the Aikido board.

    [​IMG]

    After completing a board I'll clean all the splattered flux off with isopropyl alcohol so it looks nice, and to make it easier to check for bad solder joints. I takes some work to get it all off.
     
    BayouTiger, Kyhl and struttincool like this.

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