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Millett "Starving Student" hybrid amp

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  1. jlav10
    I was wondering the same thing
     
  2. tomb
    Just an advisement while you're asking for free stuff - the thing that's proprietary/intellectual property is not the circuit. It's the PCB layout. I pay royalties to the designer for every single PCB that I sell, regardless what it is.

    There are some who share the Eagle files for some things (like Pete Millett - but he doesn't share everything), but they're probably the exception, not the rule.

    If I'm not mistaken, the SSMH PCB designs (dsavitsk and Fred) are small enough to fit into the Eagle free version size constraints (12.4 sq in). That free Eagle download is here:
    https://www.autodesk.com/products/eagle/free-download

    Here's one of the best beginner tutorial series for Eagle:
    https://learn.sparkfun.com/tutorials/using-eagle-schematic/all

    You can get esoteric parts libraries (like PCB tube sockets) from Pete Millett's free Eagle library (wheat.lbr) here: http://pmillett.com/file_downloads/wheat.zip

    Finally, I won't try to talk you out of etching your own board. However, for others looking for a simpler path, I recommend OSHPark: https://oshpark.com/. You can check the correctness of your PCB design directly on their web site. Their prices are also very reasonable and the purple PCBs look pretty cool.

    Good luck!
     
  3. prof.utonium
    Or you could download and use without any limitations KiCad: http://kicad-pcb.org totally free and will import all your eagle projects.

    Here is a comparison review: https://www.build-electronic-circuits.com/kicad-vs-eagle-2018-comparison/
     
  4. alup17
    I am trying to build my own, and I have a few questions. (I am fairly new to DIY things in general) I am also building the 12au7 version

    1. I have been looking around, and people have mentioned grounding the heat sinks. Does this mean connecting them to the ground on the power socket, or to the case? If it is connecting it to the power socket, do I need to do the same thing for the case?
    2. I already have a 10k pot, and I am using low impedance headphones (32 Ohm). What size resistor should I use for R16/17?
    3. I am having a hard time finding a switch rated for 48 VDC. The switch that is recommended is not rated for 48v. Is this important?
    4. As long as I am using Panasonic/nichicon capacitors, does it matter which Panasonic/nichicon series I am using?
    5. Are there any other modifications that I should make that differ from the original BOM?
    Thank you!
     
  5. Wiljen
    1.) most times you connect the heatsinks to the case and you hook something on the case to the earth ground on the power socket.
    2.) Sorry, I'm gonna have to pass on this one as I need to go back and look before I answer.
    3.) I had this problem too and ended up finding some at both Mouser and Digikey.
    4.) Maybe in the sense that tolerances are tighter on some series than others. Making sure they are matched closely helps so you don't have channel imbalances.
    5.) for a first build I'd stick with the build as-is so you have a good reference point. The further you go away from the original bom the harder it will be for others to help you without seeing it.

    Hope that helps.
     
  6. tomb
    1. It actually helps if you don't ground the heat sinks. The reason being is that the MOSFET tabs are carrying current that's not the ground. I suspect the comments you've been seeing are instances where someone has grounded the MOSFET tab to the heat sink. That will cause a short and while damage may take longer than you think (more than a couple of seconds), the amp will certainly not operate properly. It's been a common issue with many builds.

    2. As for the pot and headphone impedances, neither mean anything in rating R16/R17. It's the headphone sensitivity that's the concern. The pot spec'd for the Starving Student (Alpha) is not very good at channel matching at the low end of its turn. So if your headphones are very efficient, you'll find yourself going crazy with very little adjustment on the volume and poor channel matching. The intent of the resistors are to artificially depress the gain so that it throws the "common range" of volume listening to somewhere in the middle of the pot's turning - 12 o'clock instead of 7 or 8, if you imagine looking at a clock. Even then, it's a trial and error thing. It's probably best to start with 50K for R16/R17 and see if you end up with most of your listening with the pot's turn at 12 o'clock or somewhere close.

    While we're talking about the pot - I would choose 50K if you have an opportunity. The impedance of the pot has little to do with the gain conversation in the paragraph above. However, the higher impedance you use, the less chance you'll end up cutting out the bass on some sources. The pot is the first thing in the circuit after the RCA jacks. So, if your source has output capacitors (many do, especially portables), they will form an RC circuit with the impedance of the Starving Student's pot. 10K is low enough that it may cut out the bass if those output capacitors on the DAC/DAP/whatever are not very big. There are three volume pot impedances in common use for headphone amplifiers - 10K, 50K, and 100K. With 100K, you will never, ever have to worry about cutting the bass and have a huge safety factor to boot, too. However, 100K is noisy on very good amps. 50K seems to be the Goldilocks choice for most headphone amps. It's why you see AMB and myself (Tangent once, too) selling 50K Alps RK27s. 10Ks are really used mostly in portables. There the low bass below 50Hz is seldom heard anyway.

    3. If I'm not mistaken, you're finding switches that are rated at 110V or 250V. There is no issue with using a higher-rated switch.

    4. Yes, it matters what type or series Panasonic or Nichicon capacitors you're using. Pete spec'd the basic excellent power-quality capacitors such as Panasonic FM or FC and Nichicon UPW or UHE. However, the music signal goes directly through C3 and C5. At the same time, good power-quality capacitors sound like cr*p. So, it's beneficial to use something of audio-quality for C3 and C5, not power quality. I actually spec'd Nichicon UKW in those positions in the SSMH kits. They're not very expensive and Nichicon developed them for audio applications in small portable players. If you really want to splurge, look for Elna Silmic RFS capacitors (the best right now), or Nichicon ES, FG, or KZ capacitors.

    5. The use of bypass capacitors can help, but the only ones that are dependable are Wima MKP or FKP capacitors (others will be an experiment you may not want to fool with). You can find details on the SSMH website.

    EDIT: if you are using at least 50K resistors for R16/R17, please neglect my comments on the pot impedance. It won't matter - 10K might even be better. :)
     
    Last edited: Jan 12, 2019
  7. CarRamrod
    Hi All... noob question here...
    I just built the kit from Beezar.com and have 1 issue. Occasionally it throws a spark when I flip the 'on' switch. Otherwise it functions perfectly and sounds great. Where would I start to troubleshoot this issue? It's only upon startup and only 1 or 2 quick spark(s). The 1st time I started it outside the case and before heatshrink I watched an arc jump from the 2 posts on the back of the power switch. Any and all advice is appreciated. Thanks!
     
  8. tomb
    I'm sorry to hear that this is happening. In my defense, however, I've never heard of something like this - especially at the voltages we run with the Starving Student.

    One might suspect the switch itself, but I wonder if it's a grounding issue. One other customer contacted me with a different issue and we solved it by ensuring grounding through the case. If you don't have a good ground, it's possible that a charge can develop between two contacts on a switch and cause what you are witnessing. This is especially true with the Starving Student, because the CISCO power supply - as best we can determine - is not grounded through the wall power connection.

    In the new kits, I substituted a nylon standoff for the case lid attachment to the PCB. That removed the issue of shorting with the LEDs resistor or its PCB pad. However, it also cut down on the grounding if there are no other contiguous connections from the PCB to the case. Inspect your RCA connections on the back plate. You should have scraped away the anodizing on the inside surface of the backplates around the RCA jack holes. In addition, do not use insulating washers on the inside for the RCA jacks. Combined with the self-tapping case screws, this should ensure that the PCB is grounded through all parts of the case.

    Just a guess, but I'd give it a try.
     
  9. antdroid
    This happened to me too, but I checked how i was grounding the rca inputs and washer was in the wrong order causing it to spark.
     
  10. tomb
    OK - that's twice now that we've heard of this. So CarRamrod - check this out!
     
  11. CarRamrod
    I suspected the nylon standoff had something to do with it since the PCB is kind of 'floating' in there. I'm listening to it right now and it didn't do it when I turned it on this time. The anodizing is all scraped off and I didn't install the led's bc I didnt have this enough gauge w
     
  12. CarRamrod
    Still occasionally doing it, I'm going to get a metal standoff and see if that helps.
     
  13. antdroid
    I'd take a photo for you but my SSHT is at my other work office location and I'm too lazy to go over there today. If I do end up going over later in the day, I'll try to find a screwdriver and snap a photo.
     
  14. tomb
    Admittedly, I was guessing at this, but there have been quite a few built already with the nylon standoff. No one has ever reported this issue. So, it may be Antdroid's suggestion or another problem entirely.
     
  15. antdroid
    The one I built 5 months ago from @tomb had the nylon standoff. It was not the source of my problem. I dont remember if I put the washer between the chassis and the metal tab of the RCA or vice-versa to fix the sparking issue.
     
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