Millett "Starving Student" hybrid amp

  1. Sodacose
    As long as your source can drive a 5k impedance (worst case input impedance scenario with a 20k pot), the concern with using a different pot value is the input capacitance of the 19J6 and whether it will create high frequency roll-off.
     
    Let's do the math!
     
    Cmiller = Cg-k + Cg-p * (stage gain + 1)​

     
    From the 19J6 datasheet, Cg-k is 2.6pf and Cg-p is 1.5pf. Let's call stage gain Mu (though it's much lower in reality).  So Cmiller is about 60pf.  But we have two 19J6s in parallel for each channel so double this to 120pf.
     
    f = 1 / (2 * pi * Cmiller * Z)​

     
    We'll use the full 20k impedance from the pot as Z and remember to express Cmiller in farads, not micromicrofarads (uuf or pf). 
     
    1 / (2 * pi * 20,000 * 120 x 10^-12) = a little over 60khz
     
    The 50k pot PM spec'd gives a roll off at about 26khz using the same Cmiller as above. You'll actually have a little more high frequency bandwidth. I don't think we have to worry about oscillation here (60khz is still pretty low in the grand scheme of things), but you could add a 300-1k grid stopper if it worries you.
     
  2. Gofspar
    holy crap, thanks for the awesome reply and explanation. :))
     
  3. Sodacose
    You are welcome!
     
  4. Sodacose
    Here's a Starving Student variation that I finished recently:
     
    schematic.png
     
    It's a different tube, but same basic overall topology (tube voltage gain feeding MOSFET source follower) and same power supply. The tube is resistor loaded single-ended and it's direct coupled to the MOSFET, which is CCS loaded. All Class A, of course. The 7370 tubes are pretty cheap on the 'bay. The rest of the parts are very inexpensive and easy to source. If you are having trouble finding 19J6s or otherwise want to try something other than 12AU7, I highly recommend this amp.
     
    Note both the MOSFET and LM317 should be heatsinked. I used one heatsink at about 3" x 5" x 1" and bolted all the chips to it with insulating mounting kits (it only gets a little warm). The amp dissipates about 5W per channel in the TO-220 devices (48V @ 100mA per channel) so it doesn't need a ton of heatsink, but you're better off with too much than not enough.
     
  5. 93EXCivic
    I am ordering parts to do a Starving Student build using Fred_fred PCB. I want to use a Hammond 1455 enclosure. How tall does the enclosure need to be fit this setup?
     
  6. rmhuntley
    I really like the 1455nhd.  I"ve not seen board specs yet, but I hope one fits in that enclosure
     
  7. rikk009
    First timer here too. Decided to give it a go. Just one question for starting, should I go with the stock 19J6 tubes or a different pair for Hd650?
     
    Terribly sorry if it has been answered before, feeling lazy to search 450+ odd pages.
     
  8. CarlosUnchained
    I want to build my own tube amp and I'm looking for a design that can drive hungry LCD-X.

    Can anybody help?
     
  9. tomb
    It's really a personal decision. If you want the classic Starving Student, then go with the 19J6. Every part will be bog standard. The 19J6 is a bit expensive now, but still not as much as an audiophile-version of the 12AU7. Try to get a matched pair, if you can.
     
    rikk009 likes this.
  10. tomb
    Just my personal opinion, but I would look for something more than an amp with $35 worth of parts (orig. Pete Millett SSMH) to power a $1500 headphone. If you view it as the beginning of an ever-increasing path of capability, maybe it would be OK.

    Keep in mind that the Starving Student in its basic form incorporates no headphone protection measures. If you have a headphone plugged in when powering up the amp, the headphone can be subjected to some pretty severe DC spikes. If you always remember to only plug headphones in after the amp is powering up, then things will be perfectly safe. Just a point, however, to show that there's more to consider than just amplifier quality if pairing with a very expensive pair of headphones.

    The unique power supply used to power a Starving Student also has been know to fry certain DACs. Similar to a lack of headphone protections with an inexpensive DIY amp, inexpensive commercial and DIY DACs may have a complete lack of output protections. Connecting one to a Starving Student can subject a DAC to a static charge sufficient to burn out the DAC chip.

    Building something yourself also contains risks simply from the building process itself. Those of us who have DIY-ed for a while know to use a pair of $2 headphones after building an amplifier. You need to ensure that everything works and that sound tests are done with something that's not going to break the bank if it gets damaged from a mistake in building the amp.

    IOW, DIY-ing an inexpensive amplifier is not necessarily a method to save money. I don't mean to scare you too much, I would rather encourage the DIY community at every opportunity. However, I get worried when I see questions such as these - someone buys a $1000+ pair of headphones and then maybe wants a shortcut to an amplifier.
     
    Last edited: Jun 9, 2017
    CarlosUnchained likes this.
  11. Andy Mac
    Built my amp, ran good on start up.
    The music sounds to my ears good but vocals are stronger in the left channel with the vocals in the right channel quieter and with a distinct echo....

    Any ideas?

    I've switched the tubes with out success as the problem stays the same.

    Otherwise it's a very quiet amp with virtually no hum or background noise.

    I notice it doesn't like the iPhone as the source, it hums a lot.....
     
  12. Andy Mac
    Stand down, I figured it out.
    I had the earth and one of the input signal wires switched.
    After testing and retesting everything it's now perfect.
    No hiss, no hum, just clean tube music!
    It drives my Grados beautifully as well as my Yamaha HP-1's

    Thank you to all involved in this project. Simply awesome!

    Now it finish the case and post a pic I guess...

    One last thing, anyone know how hot these heatsinks get up to and will Perspex withstand the temperature?
     
  13. Andy Mac
    IMG_0955.JPG IMG_0956.JPG IMG_0957.JPG IMG_0958.JPG Done. Sounds awesome. I'm more than happy for a first time build...
     
    Last edited: Jun 27, 2017
    rikk009 likes this.
  14. rikk009
    Cool build and nice touch with glass top. Ever built a BH crack?
     
  15. Andy Mac
    No, this is my first ever build.
    What's a BH crack???
     

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