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

    You should post some progress pictures.:righton:
     
  2. TJF

    TJF New Member

    Location:
    Munich - Germany
    I would like to do a banaced setup, that is with 2 Aikido boards. Therefore I need 2 x PS-21 and a total of 4 of the transformers used here? Or is there a less material intensive recommendation?
     
  3. fully_articulated

    fully_articulated Forum Resident Thread Starter

    I don't know anything about balanced topology sorry. You don't mean dual-mono do you?
     
  4. TJF

    TJF New Member

    Location:
    Munich - Germany
    No. I mean balanced in- and output. Broskie describes it. You need 2 Aikidos and connect them both in symmetry. Very simple. So I assume that both boards need the appropriate power supply...
     
  5. fully_articulated

    fully_articulated Forum Resident Thread Starter

    Broskie built the PH-2 phono stage (which is essentially two Aikidos on one board) with one power supply, so it's likely you could do the same with your project. PH-2 and LP Playback

    I wonder if the even the phono board itself could be repurposed for it?
     
  6. TJF

    TJF New Member

    Location:
    Munich - Germany
    Broskie wrote on his site (your link... at the bottom): "Thus, one board is all that is needed for stereo unbalanced use or two boards for balanced preamplification." I would like to go with the Octal Aikido :
    "Each board holds two Aikido linestage amplifiers; so, one board for stereo unbalanced or one board for one channel of balanced amplification."

    Then it's only about half the area. And here is my question again... :D
     
  7. TJF

    TJF New Member

    Location:
    Munich - Germany
  8. fully_articulated

    fully_articulated Forum Resident Thread Starter

    I'm afraid I'm not really capable enough to answer your question. My thought was that if the PH2 can be powered by two transformers plus one PS-21, then your project probably could too. I imagine you'd have to keep an eye on the current requirements and spec the transformers appropriately, hopefully not exceeding the capabilities of the PS-21.
     
  9. TJF

    TJF New Member

    Location:
    Munich - Germany
    Thank you!
     
  10. TJF

    TJF New Member

    Location:
    Munich - Germany
    You wrote:

    and in an earlier entry:

    I think my plan could work with one PS-21 and two transformers. But probably only if I would use the type 266PA14 instead of the 266L14. Or, in general, one should use 266PA14 - in your sense.
     
  11. fully_articulated

    fully_articulated Forum Resident Thread Starter

    I think you're probably right. If you use the octal Aikido board and (for example) 6SN7 tubes, that will add up to 2.4A heater current per board, but there may be other tubes which are even higher.

    Note: The PS-21 board silkscreen and manual specify the regulator as the LD1085, which is only a 3A part. However the kit itself came with the 5A LD1084. It'd be worth checking your kit is the same.
     
    TJF likes this.
  12. samurai

    samurai Step right up! See the glory, of the royal scam.

    Location:
    MINNESOTA
    I've never undertaken such a project and probably never will but
    I appreciate your posts showing what is involved.
    Nice photos too!
     
  13. TJF

    TJF New Member

    Location:
    Munich - Germany
    I'm afraid there might be a problem with the dual operation. Broskie writes about the PS-21: "The PS-21 can deliver up to 50mA of high voltage...". The Broskie table with the tube values with my tubes of choice can be seen (150 Volt): The input=6SN7 (Ik=3.0 mA), output=6H30 (because of the necessary low Z=124 Ohm) has an Ik=30.0 mA! Total gain is enough: 9.2 (19.25 dB). What do you think?
     
  14. TJF

    TJF New Member

    Location:
    Munich - Germany
    I just see that the 6H30 is a 9 pin tube ... However, to get close to the ideal Z=100 Ohm, a higher current is applied. Maybe I have to calculate this with the Noval version.
     
  15. fully_articulated

    fully_articulated Forum Resident Thread Starter

    What's the input impedance of your amplifier? Low enough to warrant such a low Zo?
     
  16. TJF

    TJF New Member

    Location:
    Munich - Germany
    I don't have the value on hand right now. But of course yes: Ideally maximum 100 Ohm output impedance of the Aikido. I think I have to change the plan to Noval Aikido to get the value approximately. I cannot find a suitable Octal tube. Matching 9-pin tubes would be the 6H30 or 5687 at 150Volt. But always a high Ik...
     
  17. TJF

    TJF New Member

    Location:
    Munich - Germany
    Whereby I would generally prefer the Octal version. In John's list I see the 6AS7 (8-Pin) at 100 Volt (!) with Z=95 Ohm, which you would probably have to mount diagonally due to space reasons. This would be ideal for the output. Has an Ik=100mA. But as far as I know, the input would have to be operated with 100 volts(?). I can't find a suitable tube for that, which provides halfway gain. The 6SN7 only really starts at 150 volts...
     
  18. Giorgi

    Giorgi Member

    Location:
    Georgia
    [​IMG]Very nice build!

    BTW: there is one error on PS-21 PCB, in particular on one low voltage regulator (see attached picture).
    On the bottom side of PCB cut trace between C14 positive and C15 and solder wire from the C15 to negative of C14. Right now C15 is shortened and make no sense.

    [​IMG]
     
    fully_articulated likes this.
  19. Giorgi

    Giorgi Member

    Location:
    Georgia
  20. fully_articulated

    fully_articulated Forum Resident Thread Starter

    Funny you should mention that. I noticed the same thing only a few weeks ago, and send an email to John.

    He was surprised no one had brought it to his attention before. Your fix is the same as the one he provided, but because it's not detremental to operation I've just left it.

    Thanks for reminding me!
     
  21. Giorgi

    Giorgi Member

    Location:
    Georgia
    actually, recently I receive my PS-21 and PH-2. And this wire was already fixed on PS-21
     
  22. TJF

    TJF New Member

    Location:
    Munich - Germany
    Just for your information, if someone has the same intention: It won't work with PS21. I'm in the process of building my own power supply (one trafo) that can deliver a B+ of 300V and about 250mA. The current results from the low output impedance. I will also build two separate own heating power supplies.
     
    fully_articulated likes this.
  23. fully_articulated

    fully_articulated Forum Resident Thread Starter

    A little update after some tinkering around...

    I'd been having increased issues with the volume control for a few weeks, it either turning off randomly, or not even starting up at turn-on. Sometimes the LED display would go blank but the volume adjustment would still be operational.

    So it was time to put it back under the soldering gun. My assumption has always been that the likely culprit was the power supply, with the old-school 7805 regulator being starved of voltage, fed only by a 6.3VAC transformer winding. To produce the 5 volts required by the volume controller the 7805 needs at least 7VDC going into it or it quits. I wasn't able to measure this when the fault did occur, but it was my best guess as to what was wrong.

    The Audiowind power supply board is quite versatile, capable of being built for either the 780x or LM317 series regulators, but I instead opted to utilise a 5V LM1085 hoping its lower dropout voltage would be enough to sustain constant operation. The pin configuration and circuits for the LM317 & LM1085 are pretty much the same anyway.

    [​IMG]

    Two years ago in post #2 I asked aloud what the diode seen on some of these boards were for. Answer: It bridges the input & output pins to protect the output from capacitor discharge in case a fault. Not strictly necessary, if there's space for it on the PCB then why not?

    The other change made was replacing the input & output capacitors.

    [​IMG]

    The smoothing cap on the input was increased from 1000uF to 4700uF, and the output cap reduced to 470uF. I didn't spend a lot of time on working out these values. They were just approximations of similar values I'd seen used for the LM1085.

    After the board was installed back into the case and powered up all seemed well. The measured DC voltage after the rectifier and smoothing cap was 7.3V and I can imagine that was a bit borderline for the 7805.

    -----------------

    Since I was messing around inside the Aikido anyway it was an opportune time to try something I'd recently been considering - turning the Aikido into a tube buffer. I wanted to experiment and see if I could be happy running without any gain.

    The first tube in each channel of the Aikido performs the amplification and my recollection was that this stage could be bypassed. I also thought I remembered reading about how to do this, but when I came to look it up again I wasn't able to google up any details.

    John Broskie, designer of the Aikido circuit, essentially did this very thing with the ACF (Aikido Cathode Follower) and it was in that manual that I found the answer. A couple of resistors and a capacitor each side was all that was going to be required.

    [​IMG]

    Configuring the Aikido this way does put that extra capacitor in the signal path however. I suspect that with a capacitor coupled source component like my ANK DAC it wouldn't be needed, but I doubt many other sources are.

    I'm looking forward to seeing if I find myself wanting for more volume or not. Luckily this alteration is quite easily reversed.
     
  24. bluezee3228

    bluezee3228 Forum Resident

    Location:
    United States
    Hi there! I am curious where you got the brackets you are using for the Miflex caps. I am pursuing an Aikido Noval preamp build as well. I love the brackets. Great build also.

     
  25. fully_articulated

    fully_articulated Forum Resident Thread Starter

    Aliexpress for those. This isn't the same seller I purchased from, but there's plenty around for sale like this: 9.2US $ 20% OFF|10sets 12mm/ 16mm/ 20mm/ 22mm/ 25mm CNC Aluminium Tube Clamp Motor Mount Fixture Clip Holder for Multi axis Fixed wing Aircraft|Parts & Accessories| - AliExpress

    I have just recently added some 0.68uF Mundorf Silver Oil caps which fit on the board just fine, and seem to be just as good as the Miflex. They are only very slightly different from each other, if at all.
     

Share This Page

molar-endocrine