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Millett "Starving Student" hybrid amp

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  1. bhjazz Contributor
    Quote:

    Originally Posted by trains are bad /img/forum/go_quote.gif
    I pretty much followed the BOM when I ordered my parts; now I'm thinking I should have sprung for some more betterer caps.



    No worries. The design is simple enough that you can start with the standard bom and swap parts in and out whenever you want to.

    Also, this brings up Nate's mention a few weeks ago in this very thread where he said, "That's the Y in DIY." YOU can do whatever you want with your own build. I say, stick with the caps you have, build it, enjoy it, then when you are ready to get back under the hood do so and have fun.
     
  2. rds
    tomb -
    The power source is dc in this case. Here's a good explanation of what happens when you flip the switch. Notice that Imax is V/R.
    Charging a Capacitor
    You may be thinking of the instantaneous inrush when the voltage is actually changing from 0 to 48. Yes here the inrush will be very large. Regardless of whether the capacitor is 150 uF or 1 F it will be much greater than 380mA and therefore the current limiting mechanism of the ps will kick in momentarily.
     
  3. trains are bad
    You know I ordered two different kinds of film caps for the .1uF application, just to have on hand. Whichever ones I don't use, I'll probably put in parallel with the electrolytics.

    Since I've ordered my parts, I came up with a cool circuit that doesn't let you turn the amp on or off, unless the headphones are unplugged. This way you actually couldn't turn the amp on or off if you forgot to unplug the headphones first. The parts are a dollar, but a special order from mouser would be another 6 dollars in shipping. Why o why can't they ship little parts out in envelopes for the price of a stamp?
     
  4. MisterX Contributor
    Quote:

    Why o why can't they ship little parts out in envelopes for the price of a stamp?



    I think it has something to do with them having to pay for the packing material, the envelope, the shipping label, the printer that prints the label, the person that picks and packs the order and all of the other expenses involved in operating the web site that you ordered the parts from. [​IMG]

    (given all of that I don't see how they make any money on small orders, especially since they don't have a small order surcharge like Digi-key does)
     
  5. bhjazz Contributor
    Quote:

    Originally Posted by MisterX /img/forum/go_quote.gif
    (given all of that I don't see how they make any money on small orders, especially since they don't have a small order surcharge like Digi-key does)



    I avoided a small-order surcharge due to an item being out of stock when I needed it. HOWEVER, I had to pay $5.25 to ship my tiny $1.25 Panny caps...and they put them in a standard-sized box! GAH!
     
  6. Gross
    I fingered it out! First of all I put some 100 Ohm Kiwame resistors on the output after C3 & C5( I am not a starving student anymore, so I can splurge a bit right?) to first and foremost make the pot much more usable, which it did for my HD-600s and RS-1s. I was also hoping it would bring up the Z of the headphones and lower the F3, but it did not make any difference in that respect. Second I replaced C3 and C5 with some 470uf Nichicon Muse ES, I would like to think it made a difference, but I only tried that config with my HD600s, and not the Grados, So I cannot be sure it fixed a little something, but I could tell even with my Senns, the lower bass was still missing. I thought to myself, hmm, it is still as if there is a HPF in place...what other caps do I have? I did not order anything for C2 and C4, as I have plenty of film caps in stock. Well it turns out I put in some .022uf caps in instead of .22uf. As soon as I replaced them, all that tubby tubey bass that I was expecting sprung to life. I would like to point out that it still sounds very crisp on the high end, however it has the lowend to balance it out now. Also, for those of you ordering parts do not be afraid to go a little crazy with the heatsinks. I have 1.5" sinks and they get pretty toasty, so I would say bigger is better.

    I think I may put the original 270uf caps back in the output, and see how it compare to the 470uf Muse, since those are supposed to go in my MOSFET-MAX. I am going to completely rewire this guy too, after all these mods, it is a bit messy inside.
     
  7. MisterX Contributor
    Quote:

    GAH!



    I have the same response to backorders.
     
  8. n_maher Contributor
    Quote:

    Originally Posted by MisterX /img/forum/go_quote.gif
    I have the same response to backorders.



    I have the same response to Newark!
     
  9. MisterX Contributor
    lol, I think we all have that responce to Newark.
     
  10. FallenAngel Contributor
    Quote:

    Originally Posted by MisterX /img/forum/go_quote.gif
    lol, I think we all do.



    Yep, but just wait until you've dealt with moving, then it's even more creative.

    I was in Canada and had a Newark.ca account (where they have kept an exact 50% chance of screwing up my order), then I moved over to San Francisco and opened a Newark.com account (because I couldn't change the country on the account, nor could I create an account with the same name because their database is shared stupidly). Now, I placed an order with Newark.com and they sent me an email with my Canadian billing info! [​IMG] Oh the joys of Newark. At least I got free shipping.
     
  11. tomb
    Quote:

    Originally Posted by rds /img/forum/go_quote.gif
    <snip>
    You may be thinking of the instantaneous inrush when the voltage is actually changing from 0 to 48. Yes here the inrush will be very large. Regardless of whether the capacitor is 150 uF or 1 F it will be much greater than 380mA and therefore the current limiting mechanism of the ps will kick in momentarily.




    That's exactly where I was talking about and that was the context. Also, momentarily may be an assumption. Sometimes there's an automatic reset, sometimes a manual one - sometimes, it's permanent. [​IMG]
     
  12. rds
    Before I place my order I'm interested to hear why Pete chose 150 uF and what he or anyone else thinks the disadvantages might be in using a 1000 uF cap.
     
  13. pmillett Contributor
    Hey everybody -

    Sorry I haven't been able to keep up with this thread and answer everybody's questions. I'm busy as heck with the "day job", plus various commercial headphone projects on the side, and have been travelling a lot, and will be travelling more...

    Feel free to send me an email or PM if you have a specific question. I try to answer reasonably quickly, though it can take a few days.

    A few comments:

    The switching power supply has a hard time starting up into the cold filaments, which have a pretty low resistance. So it "hiccups" - turns on and then current limits, and turns off - several times at about a 1Hz rate, until the heaters are warm enough that they don't draw too much current for the supply. You can avoid this by increasing the second filter cap (C6 on my schematic) from 150uF to something like 470uF. Bigger here will not hurt anything - 1000uF or more is fine, no worries. What this does is make the bias on the FETs rise more slowly, causing the filaments to heat gradually, and the power supply is happier.

    As for the output cap size - 150uF is OK for most headphones (even 32 ohms, IMO), 470uF is OK as well. Why did I use 150uF? 'cause I had them [​IMG]

    I think there's no benefit in going as high as 1000uF. Don't assume that having flat frequency response to 20Hz is necessarily a good thing; most headphones only produce distortion at that low of a frequency, and if the amp reproduces it you tend to increase IMD generated in the headphones. Just for kicks, try listening to test tones at these frequencies - what you hear is usually either nasty, or non-existant.

    I usually like to set a -3dB point of 20Hz - 50Hz for the "chosen" headphones. YMMV, IMHO, and lots of other TLA's and FLA's apply here. Another downside to going big (no, I'm not talking about "male enhancement") is that you will get a bigger thump (get your mind out of the gutter) when you turn the power on. You can mitigate this somewhat by lowering the value of the output load resistors (R6 and R12), but you will be stealing current that could otherwise be delivered to the headphones.

    I still haven't had time to try the Jameco linear supply, but it should not have the hiccup issue. Has anybody used it? Is there any hum? Many of the linear wall warts have pretty small filter caps. You can always add more capacitance (C1 in the schematic) if there is hum, even 10,000uF with a linear supply will probbaly work (but most switchers won't start up into a big capacitance like that).

    You can't really substitute a 6J6 tube for the 19J6, since it requires 600mA to heat it up. I don't think 19J6's are too hard to find - there are lots of tube dealers out there. The beauty of this particular tube is that it has high enough Gm that you can get reasonable gain and distortion from it on 48V B+, and it's heater voltage and current just happen to make a good load for a class-A MOSFET source follower to drive 32 ohms.

    If you want to try other tubes you need to find something with a filament voltage of (ideally) 10-25V and a current requirement of 100-200mA. 100mA into 32 ohm headphones is 3.2V peak, which is plenty loud. FWIW, for the engineering-inclined, you could design a similar amp using 12V car radio tubes (12AE6A, etc.) that need 12-14V at 150mA. You would bias the gates of the FETs at about 16-17V (Vgs of IRF510 is around 4V at this current). I picked the 19J6 partly because it is cheap and useless [​IMG]

    I wouldn't be obsessive about the coupling caps - sure, polypropylene is a tad better than PE to most people, but any film cap will do. I'd be more concerned about making sure you have decent electrolytics. Black gates are not necessary - good low ESR caps (Nichicon UHE's, UPL's, or similar low-Z parts from Panasonic of Chemicon) are fine.

    Did I read that right, that somebody gets to build this instead of taking an AP phyics final? That's waaaaay cool.

    Well, that's all that comes to mind for now... have fun!

    Pete
     
  14. bhjazz Contributor
    Quote:

    Originally Posted by pmillett /img/forum/go_quote.gif
    A few comments:



    Golden, golden information. Thanks, Pete. Much appreciated! This kind of info allows me some great insight into the design and gives some elbow room to experiment. Coolness! Thanks!
     
  15. tomb
    Quote:

    Originally Posted by bhjazz /img/forum/go_quote.gif
    Golden, golden information. Thanks, Pete. Much appreciated! This kind of info allows me some great insight into the design and gives some elbow room to experiment. Coolness! Thanks!



    x2 - as in chiseling every word into memory. [​IMG][​IMG] What sounds really neat is that slightly upping the size of C6 is enough to overcome the short-term PS current limit in starting the heaters. I was afraid to mention it after all the rest of the talk of upping the cap sizes (had visions of my PS croaking), but had selected 470uf (63V UPW's) for those, too. It didn't seem natural to have smaller caps on the input than on the output. [​IMG]
     
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