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The DtpaKiller headphone amp lives

Discussion in 'Electronics & Electrics' started by dakiller, Dec 22, 2004.

  1. dakiller

    dakiller (Oscillating & Impeding)

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    Being a bit of an experienced headphone builder, I done a fair few in the short time that I picked up this hobby

    There were always little things in designs that I didn’t like or thought that could be done better, so deciding that I should make my own from scratch

    just recently, Texas Instruments released a chip, the tpa6120, that was designed for headphones and unlike all the past dedicated chips that have been done, they did this one with extremely good performance (on paper anyway, 100mhz bandwidth, 1300 V/µs Slew Rate, current feedback, independent power supply pins, <0.00014 THD + N)

    Now this became the center of my amp and I went from there

    I got the basic schematic pretty easy, it needed an input buffer that also became the voltage gain as the tpa6120 was pretty strict with the value of its feedback resistor dictated by the current feedback design of it as well as in inverting mode it would play up with the volume pot being seen as a variable impedance to its input
    A pretty basic psu stage with adjustable voltage reg's with plenty of big, fast capacitors was also added in

    [​IMG]

    Then the fun part came, laying it out on a board

    one thing that I found is no one like surface mount parts, I guess they never tried working with them (too scared?) but I’ve grown to love them, so I was going to use them throughout this amp and it was a real advantage when layout the board, you can fit so much in such a small space

    After many days of playing around with layouts and with a bit of advice from the guy's at head-fi diy forums I finished up a design worthy of becoming a physical board

    [​IMG]

    [​IMG]

    I sent the board files off to get professionally made by Custom PSB and in 2 or so weeks they shipped me back the finished boards, 15 of them in the end for a bit over $140aus shipped, not bad I must say

    now it had been about 8 weeks of stuffing around until I got the components and the last lot of them finally arrived I built one up in extreme excitement to see how my masterpiece would sound (if it even worked at all :Paranoid: I was dealing with a 100mhz chip that would get real unstable if it had a bad layout and the datasheets stressed this point a few times though out it, nothing to just say 'she'll be right, mate')

    Now it didn’t take me long at all to get one soldered up and with crocodile clips hanging off it, I powered it up for the first time
    The led lit up a bright blue, so I knew I got that around the right way :) but did the rest work? :o

    first thing to check is dc offset on the output, 5 and 7mV each and I was happy with that
    The positive supply sat on 12.57, about where it should
    The negative supply..... -6.35 ? Uhoh

    Now being a freshly soldered board it still had flux residue all over it (flux is your friend when soldering, especially with SMD's) so after a scrub with metho and touchup on any suspicious joints I tried again...

    -6.35v

    Hmm, what the hell is going on here? Then it stuck me.
    the tantalum caps on the regulators are polarized and I just put them both with the + to the adj pin and other end to ground, well negative regulators are below ground, so everything has to be reversed (I knew this, the other big electrolytic were in the right way)

    Ripping this little cap out, I tried again...


    ....success!!!

    Plugging in some headphones and my iriver player on my desk, I had sound.
    Initial impressions of the sound - bass slam, this thing has plenty of power behind it and it had far more bass control than my meta42 I use with my Beyer dt531's. This amp is the best I ever heard (so far)

    [​IMG]
    [​IMG]

    so after playing a whole 2 songs, I set out to get a case to put it in, having done plenty of these before I drilled the panels out in under an hour (with lunch in between) then it was just a simple task of screwing/bolting everything in and attaching wires to the board

    [​IMG]
    [​IMG]

    Now there are a few little things to go, like chop down that shaft sticking out for one, but its good enough for today and it plays music so I been listening all evening

    so here comes the end of this journey, I think a matching DAC is to come next but I got a lot more reading and learning before I consider jumping into that
     
  2. krell

    krell Member

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    good work.
    board layout looks good.
    only thing i have to question is, after you obviously spent so much time and effort on the design and layout of the board, sorced what looks like a realy nice case, why the (dont take this the wrong way, just curious) slap dash assembly into the case ?
    i can only see one ac supply point, so why two transformers ?.
     
  3. bondy_112

    bondy_112 Member

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    tell me when you start building them. i wouldnt mind one ^^
     
  4. Jazper

    Jazper Member

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    the board needs 15v-0v-15v - he obviously didn't grab a 30v transformer, and instead got two 15's ..
     
  5. OP
    OP
    dakiller

    dakiller (Oscillating & Impeding)

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    I designed it to take in a 12.6-0-12.6v (15-0-15 would work fine too, just more heat on the voltage regs) transformer @ 500mA, and went and got 3* 12v center tapped transformers from jaycar. I’d use the others later on; you can never have enough spare parts. Then when I got home and opened one up it had 6.3-0-6.3 on the label inside so I stuffed up a bit there. Next time I should read the labels better :o
    Spending $30 on transformers was a bit of a waste, so I hooked 2 up to get 12.6-0-12.6, I know it is a bit dodgy but it works well

    also this particular one I’ve built for someone else, and he paid me nearly 2 months ago, so I don’t want to keep him waiting any longer

    and I’ve done more work on the case to finish it up, when I took those picks it wasn’t entirely finished and I’ve fixed everything in now and closed it up
     
    Last edited: Dec 23, 2004
  6. *iNVaDeR*

    *iNVaDeR* Member

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    how much are you going to be selling those for? if you dont mind me asking
     
  7. OP
    OP
    dakiller

    dakiller (Oscillating & Impeding)

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    $270, if people are interested
     
  8. mAJORD

    mAJORD Member

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    very nice! , i like ur use of smd, mm love to build a power amp one day full of smd 1206 and 2010 components wherever possible , I load SMD boards on a regular basis at work so I find them extremely easy to work with, and get neat results every time.
     
  9. OP
    OP
    dakiller

    dakiller (Oscillating & Impeding)

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    interested in a smd pwer amp, Per-Anders has a very nice design, he has done many other designs and was 1/2 the inspiration with his implementation the tpa6120 chip that is very similar in topology to mine and he also personaly helped out in his posts at head-fi with suggestions in my layout (thread linked in my sig below). though he opts for 0805 size which is just a little small for my likeing as well as many other little design ideas that differ from mine
     
  10. Etacovda

    Etacovda Member

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    Jesus, if you're going to be charging that much id make the insides a bit more presentable

    calling it 'dodgy' and trying to sell it for 270$ isnt the best practice :D
     
  11. bondy_112

    bondy_112 Member

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    i think that one was just a test one to see if it was working.

    when he builds the real ones, theyll be allot more presentable.
     
  12. OP
    OP
    dakiller

    dakiller (Oscillating & Impeding)

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    It really isn’t as bad as the pic make it look, there is a bit of hot melt glue on the 240v joints as well as the heat shrink to make it extra safe and keep my conscious happy

    About the transformers, while it isn’t the normal thing to do there is nothing wrong with it either, next time I’m going to read the labels a bit better and get proper transformers for it

    And if you concerned about the price, it doesn’t leave much left over from parts for my time. And for what you’re getting, it could easily retail for up to $600
    And I’m not 'trying' to sell it, it’s already sold
     
  13. Etacovda

    Etacovda Member

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    Sorry miswording, i meant 'them' rather than 'it' :)

    I bet it sounds excellent though.
     
    Last edited: Dec 24, 2004
  14. mAJORD

    mAJORD Member

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    ahh an smd gainclone, i like, that would be great for my future nearfield PC project.

    805s are fine they're the most common in my work, . but anyhting smaller than that and i think id be wanting a more delicate iron ;), thanks for the link.

    im thinking 2010s (maybe some 1206s) in a large power amp simply for the power disipation required for most designs around atm.
     
    Last edited: Dec 24, 2004
  15. bondy_112

    bondy_112 Member

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    now that youve built one. how much would it cost you for all the parts to build one ?
     
  16. OP
    OP
    dakiller

    dakiller (Oscillating & Impeding)

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    with current exchange rate, around $180 in raw parts excluding shipping costs from 3 seperate US vendors
     
  17. Jazper

    Jazper Member

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    you sure you're not forgetting the chips?
     
  18. OP
    OP
    dakiller

    dakiller (Oscillating & Impeding)

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    no, im not

    you got -
    Mouser for the bulk of the stuff
    Digikey for the tpa6120 chip and opa637's
    Tangent for the alps "blue velvet" pot and ad8610 chips if you want to swap them for the opa637's
     
  19. @sia@home

    @sia@home Member

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    If I didn't allready have PPA boards here I would offer to buy one of those PCBs for $20 :p
     
  20. Goth

    Goth Grumpy Member

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    Hey, did you use Eagle to design that PCB?

    How did you do the 3D rendering?
     

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