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Millett "Starving Student" hybrid amp

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  1. Ferrari
    Yes, pay attention to the total current draw when you planning to use 6-8 LEDs in parallel and check that against the current ability of the used PS.

    Assuming you are using LED as indicator on the front pannel and if you use super bright LED (at least 3000 mcd), then ~2mA per LED is more than sufficient, that is what I usual do with my builds. 6-8 indicator LEDs would draw 12 - 16mA totally. But when you use a LED to light up the tube, then you need more current, 8-10mA is what I usual do (don't go up to 20mA -even that's on the LED's spec- that will shorten the life of the LED). For the later purpose, you can also look at much brighter LEDs @ 12.000 mcd or higher, but these are also more expensive.
     
  2. rds
    Someone asked earlier about the LPF. Increasing the capacitance will actually lower the cutoff frequency.
    Since your headphones are in parallel with the output resistor, the R is the RC equation is pretty much set by the headphones being used.
    In the original design 32 ohm headphones will cause a cutoff frequency of about 66 Hz, and 600 ohm headphones will make it about 2.3 Hz.
    So if you're using low Z headphones use a bigger cap for sure. 1000 uF or higher would be best (probably best in general). I think the reason Pete chose 150 uF is because they're cheaper.

    I think I'll go with 1500 uF Nichicon PW unless someone can give me a good reason not to, other than that I'll have to steal my lunch from the cafeteria.

    I can't think of a good reason to use a bigger C1 and C6, can anyone else?
     
  3. J.D.N
    Quote:

    Originally Posted by Ferrari /img/forum/go_quote.gif
    Yes, pay attention to the total current draw when you planning to use 6-8 LEDs in parallel and check that against the current ability of the used PS.

    Assuming you are using LED as indicator on the front pannel and if you use super bright LED (at least 3000 mcd), then ~2mA per LED is more than sufficient, that is what I usual do with my builds. 6-8 indicator LEDs would draw 12 - 16mA totally. But when you use a LED to light up the tube, then you need more current, 8-10mA is what I usual do (don't go up to 20mA -even that's on the LED's spec- that will shorten the life of the LED). For the later purpose, you can also look at much brighter LEDs @ 12.000 mcd or higher, but these are also more expensive.




    Wonderful. Your help is much appreciated. Im only looking at about 3000-4000mcd and might limit it to 6. It's just an idea i've got. Going to do a bit of planning and then get the ball rolling on a BOM.

    Quick resistor question: Is there anything wrong with using 2w 5% Carbon Film resistors?
     
  4. n_maher Contributor
    Quote:

    Originally Posted by rds /img/forum/go_quote.gif
    Someone asked earlier about the LPF. Increasing the capacitance will actually lower the cutoff frequency.
    Since your headphones are in parallel with the output resistor, the R is the RC equation is pretty much set by the headphones being used.
    In the original design 32 ohm headphones will cause a cutoff frequency of about 66 Hz, and 600 ohm headphones will make it about 2.3 Hz.
    So if you're using low Z headphones use a bigger cap for sure. 1000 uF or higher would be best (probably best in general). I think the reason Pete chose 150 uF is because they're cheaper.




    It's worth noting that a fair number of headphones start to roll off beyond -3db below 40 or 50 Hz, so it's not like you're losing much information that you'd hear normally anyway. Also, there are a lot of headphones that don't have linear impedance characteristics (although Grados do) which means that the equation is more complex than just assuming half the stated impedance. Have a poke around Headroom's graphs, of note is the fact that the HD650 are actually 400ohm at 40Hz and rise to a peak of nearly 500ohm at ~80Hz. That may be an extreme example but it just goes to show you that there are other factors in play.

    Quote:

    I think I'll go with 1500 uF Nichicon PW unless someone can give me a good reason not to, other than that I'll have to steal my lunch from the cafeteria.



    That might be a bit extreme and it will take that cap quite a while to charge, enough so that I personally wouldn't leave headphones connected when you turn the amp on. Moving to a 470uF coupling capacitor will take the -3db point down to a reasonable ~21Hz. I'm not one of those folks who thinks that you need your -3db to be in the basement to get good frequency response so bear that in mind. [​IMG]
     
  5. tomb
    Quote:

    Originally Posted by n_maher /img/forum/go_quote.gif
    <snip>That might be a bit extreme and it will take that cap quite a while to charge, enough so that I personally wouldn't leave headphones connected when you turn the amp on. Moving to a 470uF coupling capacitor will take the -3db point down to a reasonable ~21Hz. I'm not one of those folks who thinks that you need your -3db to be in the basement to get good frequency response so bear that in mind. [​IMG]



    I apologize if I started something by suggesting 470uf output caps. It's a common size for many headphone amps and has been traditionally used on the output of Milletts for years. No one has every complained from lack of bass with Grados on Milletts, either.

    That said, Pete has made several references to the PS sort of hiccuping as it starts up on the Starving Student as designed. So, increasing inrush current is probably not something to take lightly. 470uf might be a reasonable increase - it might not. We'll have to wait until one of us tests it first and reports back.

    Under that scenario, Nate is correct: 1500uf would be extreme. It might trip the safeties on the PS and never start - for very little benefit in return.[​IMG]
     
  6. n_maher Contributor
    Quote:

    Originally Posted by tomb /img/forum/go_quote.gif
    I apologize if I started something by suggesting 470uf output caps. It's a common size for many headphone amps and has been traditionally used on the output of Milletts for years. No one has every complained from lack of bass with Grados on Milletts, either.



    Bah, it's a healthy debate to have - I agree that moving to 470uF is not a bad idea. I just ran some test tones through my SSH (starving student hybrid) and there's definitely a drop off below 70Hz. Of course my listening levels in the office are such that I don't even notice it.
    Quote:

    That said, Pete has made several references to the PS sort of hiccuping as it starts up on the Starving Student as designed. So, increasing inrush current is probably not something to take lightly. 470uf might be a reasonable increase - it might not. We'll have to wait until one of us tests it first and reports back.



    I can certainly confirm that the PS struggles with the cold heaters. I think there's a solution, but I'll let Pete comment on that when he gets a chance.

    As more of these get built I'm sure folks will tweak it. It's not like the BOM is a set of rules you have to follow, just one example of a working concept.
     
  7. rds
    Larger caps won't increase the inrush current, they will simply make the time that current is sustained for longer.

    I'd like to hear more details about the ps struggling. What exactly happens?
     
  8. rds
    Also, I'm wondering if some people would appreciate a parts kit?
    I'm in the process of placing orders and have some extra cash (a scholarship cheque appropriately).
    Since people are having trouble getting certain things and have to make 3 orders it seems to be a good candidate.
    I'm using pretty much pete's list with some minor changes (slightly better things here and there), with one major change being a hammond 1455. With the case and 4 tubes (2 extras) the cost for a kit would be about $60.
    Let me know, I'm ordering today.
     
  9. n_maher Contributor
    Quote:

    Originally Posted by rds /img/forum/go_quote.gif
    I'd like to hear more details about the ps struggling. What exactly happens?



    Basically on startup the PS starts, stops, starts, stops and then finally lights the heaters. As soon as the heaters come online the PS is fine but there are some hiccups at startup. According to Pete it's nothing worth worrying about and I've been power cycling my amp every day for the past couple months with no ill effects.
     
  10. bhjazz Contributor
    Quote:

    Originally Posted by n_maher /img/forum/go_quote.gif
    Moving to a 470uF coupling capacitor will take the -3db point down to a reasonable ~21Hz.



    470, eh? Interesting. I have 220's (210, 220...Don't rememeber. Need more coffee right now) on my parts list, but had not considered going higher. I'll definitely look into them, though. For the simplicity of the design, it would be pretty easy to swap parts in and out, so maybe I'll get both. [​IMG]
     
  11. trains are bad
    I pretty much followed the BOM when I ordered my parts; now I'm thinking I should have sprung for some more betterer caps. I didn't realize that the electrolytic caps (I got 220uF) were to be used for the output coupling caps. Electrolytics aren't usually used for audio, right?
     
  12. holland
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  13. holland
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  14. n_maher Contributor
    Quote:

    Originally Posted by trains are bad /img/forum/go_quote.gif
    Electrolytics aren't usually used for audio, right?



    In my experience they are used all the time. A fair number of tube amps have electrolytics on the output, mostly because high-value, high-voltage film caps tend to be huge. For example, the Menace only has 200uF on each output in the form of 2x 100uF motor-run oil caps, each one of those is 2-1/2" in diameter by 4-1/2" long. Not at all practical for most applications.
     
  15. tomb
    Quote:

    Originally Posted by rds /img/forum/go_quote.gif
    Larger caps won't increase the inrush current, they will simply make the time that current is sustained for longer.

    I'd like to hear more details about the ps struggling. What exactly happens?




    I'd like to hear more details of that statement up there.[​IMG] Maybe I'm missing something, but any number of equations show that peak inrush current is proportional to capacitance:

    I = C * dV/dT
     
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