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Millett "Starving Student" hybrid amp

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  1. tomb
    I'm not sure the 33VDC supply will be enough. The resistors and other parts of the circuit are designed for the 48VDC power supply. The power supply is easily available on ebay. Just search for "cisco psa18u-480" on ebay and you'll get many economical choices.

    As for the hum, I suggest you find out what you did wrong with the CMoy before you try building another amp. Maybe thousands of CMoys have been built without any hum and noise. The CMoy design uses batteries. It's sort of difficult to get hum from a battery.
    mcmillant likes this.
  2. mcmillant
    oh yeah i use a 12v ac dc converter power so get the hum, i testing it with 16850 batteries is ok, the hum very low.
    And about irf510, can i repalce it with irf610 or irf520 ?
    Last edited: Mar 24, 2019
  3. onesojourner
    Could some one take a look at my mess and help me troubleshoot it? When I plug in some cheap ear buds all I get is a light hum. I am not sure where to begin the troubleshooting process. The pot does not appear to have any effect.



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    Last edited: May 18, 2019
  4. tomb
    Print out the Starving Student schematic. Look at each leg and connection in the schematic, match it up with what you've built, and mark both as checked and valid. Eventually, you'll find the issue.

    Just an FYI, but I've tried a couple of times to follow your wiring in those photos and I can't because almost all of the wiring is that same red color. I suggest you get out a magic marker or some colored tape and color code some of that wiring. You might find the issue a bit easier when you do that. Good luck and sorry you haven't had more input, but that may be why.
  5. DrivenKeys
    Wow, I love the use of an old pc cooler, wish I had thought of that. I haven't seen that type of potentiometer before, are the extra 2 prongs for on/off? Also, my tubes don't glow that bright, I wonder if I need to fix something? I built the 12au7 version.
  6. onesojourner
    Mine is not working so don't use it as an example. I have no idea what I have done wrong. I have gone over the diagram several times and from what I can tell it is all correct. I am not sure what those other two posts are for on the pot.
  7. DrivenKeys
    I'm sorry to hear it's still not working, I've looked at the pix and it's beyond me. That's unfortunate, I really like this build.

    It turns out my tubes aren't as bright because they're the type with heat spreaders covering the elements.
  8. tomb
    You might entirely re-wire that pot, or better yet: get a different one. If your tubes are on and glowing and other things seem OK, that volume pot may be your entire problem. You should have six connect points on a stereo volume pot, plus one to ground - usually just to the metal somewhere on the pot to keep the amp from humming. Some of your wiring/soldering looks pretty dodgy around that pot. I also don't recognize that pot, so have no idea how to help with the connections. (Not that I'm some sort of volume pot expert, but I have seen quite a few over the last ten years or so - never saw that one from what I remember.)
  9. onesojourner
    Thanks. I will get a different one ordered and see what happens.
  10. DrivenKeys
    Come to think of it, I ruined my cheap Alpha pot with sloppy soldering, I believe my issue was applying too much heat and allowing flux to run into it. Mine was full of static, with one sweet spot, ground hum, etc. If I had done much more, I may have had the same issue you've experienced.

    I had allowed my soldering tips to go black, and couldn't re-tin them with normal solder. So I was seriously overheating components before the tip finally melted the solder. I also rely heavily on a helping hands when soldering wires onto pcb pins. The pins leave very little room for error, and clamping the pot upside down on a table is how I let flux into it.

    I really like this build, I hope the new pot fixes it.
  11. onesojourner
    I will keep you posted. I don't think I over heated anything but who knows.
  12. ThermalAlchemy
    A good cheap Chinese knock off adjustable T – 12 soldering tip , iron Station. I have bought three of these it was so good one went to my son to for me one in case I need it as a back up but so far I have not needed it because it has never broken down yet after two years of use. I also purchased one of death cheap three and one soldering stations that had a built-in volt amp meter with a hot air station gun. Do your research as example here is a YouTube video on soldering stations very cheap and affordable will definitely improve your soldering skills because you can adjust the heat and on larger pads and components the heat to self adjust keeping the temperature.
    Also the addition if your having a really hard time possibly dirty components a small dispensable container of liquid flux meant for electronics in very good quality lead bearing solder With flux core again cannot stress how important it is to stay away from leadfree solder. Especially for beginners. For beginners for video tutorial on soldering search YouTube for Mr. Carlson labs and then seek his solder tutorial. There is also excellent old black-and-white videos from the 40s 50s and 60s produced by the manufacturers of electronics companies and solder production companies. Usually after watching these videos you will no longer be a beginner. With a adjustable self heat correcting soldering iron.
    DrivenKeys likes this.
  13. tomb
    Just to be clear, it's not the quality of the solder joints I was commenting on. Rather, it's the wiring technique used to connect to the pins and whether the pins are correct. At first glance, I thought two strands of wire were used to connect to the pot's signal ground. Upon a closer look, I guess those are resistor leads, so perhaps not so bad. I suspect the back two pins are an integral on/off switch attached to the back of the pot, which is fairly common.

    That said, I still suspect the pot: whether it works, whether the connections are correct instead of reversed, etc. You should be getting some symptoms in the audio - hum static, etc. If not, it implies that the signal is completely disconnected from the circuit. Where is the first place the signal goes after the RCA jacks? The volume pot.
  14. DrivenKeys
    Since we're on this subject, I wanted to ask about the pot/resistor combo. I searched and scoured the results, I think I know what to do, but wanted to ask before more surgery:

    12au7 version on a Fred pcb. I currently have a 100k Alps blue pot combined with 47k resistors at input. The amp sounds perfect, except I really have to boost my source to drive my 250 ohm Beyers. My computer and monitor can't output enough, I switch to lower impedance cans for movies.

    If I remove the resistors and just use the 100k pot, will that raise my input volume enough? It's a premium pot, so I'm not worried about staying in the lower range with easier phones. I'm fine with getting a new pot or resistors, but only want to dig in again once, if I can help it.

    Also, I had a balace issue that was fixed when I wiggled the c7 cap. Power was on and tester phones plugged in, I could hear the pop as I wiggled the cap and it came back online. Does that sound like an issue with the cap itself, or its connection? Things are cramped. Whenever I remove the board, these caps get bent around. I'm wondering if I should replace them when I pull out the resistors.
    Last edited: Jul 22, 2019
  15. tomb
    Short answer: Yes.

    If you're having to rack the volume pot to maximum all the time, then you need to remove the input resistors.
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