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Millett "Starving Student" hybrid amp

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  1. Fred_fred2004 Contributor
    I'm too impatient, I just wack in some cheap headphones and listen [​IMG]
  2. ArtemF
    in principle, nothing bad should happen given that you've put the output caps. To be on a safe side
    you can start from the output: measure the DC voltage on the output jack. If there's no DC component,
    switch to AC measurement mode and check if the amp doesn't generate by itself when the volume knob
    is in the left (=min) position. If there's no AC component, you can plug (cheap) headphones and give it a try.
    If it doesn't work, check the voltages in key points of the circuit and start investigating the problem.
  3. revolink24
    A couple of questions. Would 63V 220uF Nichicon Electrolytic caps (Mouser number 647-UHE1J221MPD work for C1, C3, C5, and C6? And is there a decent alternative (or maybe Im missing it) to C7/C8 on Mouser?
  4. baka1969 Contributor
    Originally Posted by the_equalizer  

    " The output stage has enough muscle, so I think it'd do perfectly with the 8 ohms cans. Though the lowest impedance I've tried with the amp has been some 16 ohm earbuds; it had no problems with them.

    Thanks! :)
  5. FallenAngel Contributor
    C1 and C6, I'd use considerably higher than 220uF, maybe even 1000uF.  For C3 and C5, I would really recommend 470uF Muse ES, not HE.

  6. revolink24
    Thanks for the response. What thermal material do you use for the MOSFETs (if any)? Also, does anyone know of a DPDT switch that could be used as a gain switch?
    Also, these would be correct for C7/C8, correct? http://www.mouser.com/ProductDetail/Nichicon/UES1C221MPM/?qs=kArNe9LFxXkkfHBfMYU2Pw%3d%3d
    More edit: What do you think of this DPDT switch for a gain selector? http://www.mouser.com/ProductDetail/Mountain-Switch/108-1MD2T2B3M1QE-EVX/?qs=sGAEpiMZZMvudeGI7i40XJQ7caDfyBlLkZYiLn5EiQs=
    As far as I know it would go each RCA to 2 and 65, and then 50k resistors or so at 1 and 4 and 100k at 3 and 6. (if you look at the data sheet on Mouser)
    I suppose this NKK would be higher quality, but any DPDT ON (ON) toggle should work, correct? http://www.mouser.com/ProductDetail/NKK-Switches/S335T-RO/?qs=V2oqQAr0jIiS08sISfWzXQ%3d%3d
    The only difference I can see between the two is 2A vs 15A, but there shouldn't be more than 2A on R16/R17, should there?
  7. the_equalizer


    That's definitely an option  :)

    - Measure  from ground to the tube's pin 3 or 4 (the one that is NOT grounded, naturally)
    - Measure from the ground lug to the the left, then the right lug (be sure to measure with the volume at min, and at mid or max)
    - Measure from the ground lug to the 'hot' lug.
    As always, measuring a 'live' circuit requires care and a steady hand so as not to short out things.
    As someone else posted, the risk of DC at the output jack is minimal, due to the capacitors. Still, you could get a defective cap or have some serious miswiring that puts DC at the output, so it's still a good idea to measure and then use some cheapos for the first listen.
  8. the_equalizer


    The highest impedance cans I've tried with my SSMH have been Sennheiser HD-600's and I found the volume output by the amp more than enough to drive those cans to nice volume levels.
    Still, I've never heard 600 ohm Beyers so it could be different. Anyway, you can easily build a 12AU7 version and, if you find the volume insufficient, just swap the tubes for 12AX7's (drop-in replacement no rewiring needed at all) which have noticeably more gain (I have tried them in my SSMH).
  9. ArtemF
    2 revolink24:
    thermal material is __necessary__ to insulate your mosfets from the ground.
    Otherwise you'll have to insulate the radiators that might be tricky.
    Check the list of items provided by Peter Millet on the first page.
    I recall there were two mounting sets that included thin insulating pads.
  10. revolink24
    Well some insulator is necessary, although whether these MOSFETs need thermal conductive material seems to be up for debate. But Beezar is still selling the mounting kits! Yay TomB! I totally didn't realize that.
  11. Beftus


    If you forget to add the 2K resistors right at the output (R6,R12) you get ~12.5 volt DC at the output. Been there, done that...
  12. the_equalizer


    Yeah. That would qualify as a serious miswiring !    So how is your SSMH doing Beftus? Tried different tubes? Has it burned-in nicely?
  13. Ikarios
    two quick questions - if I'm using a gain switch to switch between 50K and 150K ohm resistors (using a 50K ohm pot), is it better to put the resistors on the input of the gain switch (i.e. split the line from the input and connect the resistor leads to the poles of the switch) or is it better to connect the input to the main pole of the switch and connect the resistors to the switching poles (i.e. split the resistors at the "pot side")?
    also, what do I do about input ground in this situation? Do I need to connect it to the switch? In other words, can I use a DPDT switch for gain or do I have to use a 3PDT? I was planning on using a DPDT but I seem to have confused myself with all this wiring and switches (currently have five planned... [​IMG])
  14. the_equalizer
    I'm not sure about the first question. That is, I'm not sure if one way is better than the other. Maybe some others might chime in, or you can simply do both and see which one works better.
    About the ground switching, no, you do not need to switch ground lines, as ground is a common reference point to all lines; a DPDT switch should do nicely.
  15. Beftus

    It's burned in very, very nice. Haven't tried other tubes as it sounds so schweet already!
    I have a second SSMH still in parts (gonna use Elna caps), waiting to be built into a functioning amp. Haven't gotten around to building it yet, I just came back from my holiday in France. Will report back here with the results of the second build.
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