The Zero DAC headphone amp is a weird MF

  1. diditmyself
    I wonder how they came up with something like this. The input capacitor to the second stage seems unnecessary. The input unity gain buffering stage seems unnecessary. The 100 uF cap from neg input to ground to reduce DC offset seems unnecessary and is probably a remnant of the days when they used NE5532. I don't understand the meaning of the 22 pF cap, C2 in my schematic. Why use so large resistor values, resulting in unnecessary noise. Why bother using high quality output transistors when using such a low quality opamp as TL082. Why use the 1R resistor on the output. Why, why, whine, whine...

    All the fancy capacitors ELNA SILMIC II and CERAFINE are probably fake. Hitachi transistors, I wonder... The "metal film" resistors looks like cheap carbon film resistors with blue paint to look like the real stuff.

    I'm tempted to do a radical mod. I think it would be better to use a single opamp per channel and no capacitors in the audio path. The output stage is OK.

    The output transistors are B649/D669, but I don't have SPICE models for them.

    [​IMG]
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  2. Henmyr
    Do you know what the value of V+ and V- is? I will follow this thread closely[​IMG]
     
  3. hopeless
    It's about +/- 15V.


    BTW that schematic is just what I expected, except that I expected the unity gain stage to be after and not before. [​IMG]
     
  4. diditmyself
    Yes, it's +/- 15V.

    These are some very non-flattering square waves. And this is without load! It's only the amp that's tested with a function generator, not the DAC. It simulates just as bad, so it's not my measuring that's faulty. Well, do I need more motivation to mod it? Fancy PIO caps and other jewelry? I don't think so. It needs to be redesigned.

    10kHz 1V

    [​IMG]

    10kHz 3V

    [​IMG]

    100kHz 1V

    [​IMG]

    100kHz attemted 3V

    [​IMG]
    vbattach17751.jpg
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  5. hopeless
    But does it sound that bad? [​IMG]


    You could just bypass the first, unity gain stage, and see what happens (to the sound).
     
  6. diditmyself
    Quote:

    Originally Posted by hopeless /img/forum/go_quote.gif
    But does it sound that bad? [​IMG]


    You could just bypass the first, unity gain stage, and see what happens (to the sound).




    No, it doesn't sound as bad as it looks. It's just a bit dull, probably because of the very obsolete TL082. I think the cure will be:

    1. to use a single opamp on a dual BrownDog, just keeping the gain stage by hardwiring the input to the pot, I can see no need to keep the input buffering stage
    2. remove C2, it just **cks up the signal
    3. bypass C1, there's absolutely no reason for this if JFET input opamps, or modern bipolar input opams with low bias currents are used
    4. bypass C4, there's no need for this either for the same reason as in point 3, who wants electrolytic caps to mock up the audio signal?
    5. bypass R10, I want a low output impedance, this resistor will increase crosstalk and reduce the damping factor
    6. increase R7 to 1M to better match the 100k pot and JFET input amps
    7. remove the zobel (R11 and C3). If the amp is unstable without zobel, I'll change the values to 47R/47nF instead. 10R/10nF is mocking up the signal.
    8. depending on the results, I'll probably reduce the input R5 and feedback R13/R14 resistor values to reduce the noise

    I was thinking of bypassing the DAC opamp, so I'll have two OPA627's to go into the headphone amp.
     
  7. Currawong Contributor
    Have you seen the FrankenZero thread? You need dual opamps for the headamp, so 4x OPA627s are needed. The Lampizator's page on the Zero is also worth checking, though it probably covers everything you already have.

    The result of the FrankenZero, in case you were wondering, was that it ended up sounding close to as good, at least in the DAC section, as the Compass, which, in a sense, has replaced it (since the reliablility of the Zero was seriously suspect).
     
  8. diditmyself
    Quote:

    Originally Posted by Currawong /img/forum/go_quote.gif
    Have you seen the FrankenZero thread? You need dual opamps for the headamp, so 4x OPA627s are needed.



    No, dual opamps are not needed, only if you stick to the original scheme, which I find to be bad and plan to deviate from. I can't see the need for the unity gain input buffering stage. More stages = more degradation. I think this weird headphone amp section is optimized for yesteryears bipolar opamps (read NE5532). Why buffer a buffer? Why use coupling caps when JFET input amps are used? When using a 100k pot and 1M resistor to ground the load on the previous stage is almost nothing.

    Quote:

    The Lampizator's page on the Zero is also worth checking, though it probably covers everything you already have.



    Still Lampizator uses the original electrolytic capacitors. There's no reason to use electrolytics. Electrolytics are bad for audio, fancy brand or not. I'm going to use polypropylene. Anything larger than 2.2uF will do if the amps input impedance is "normal".

    Quote:

    The result of the FrankenZero, in case you were wondering, was that it ended up sounding close to as good, at least in the DAC section, as the Compass, which, in a sense, has replaced it (since the reliablility of the Zero was seriously suspect).



    Like I said in a previous thread, I'm not sure what to trust in that thread. Eg the 22pF caps in parallel with the output shouldn't kill any treble since the cutoff frequency is so very far from the audible range. It seems like most people did a lot of mods at the same time. I'm going to find out for myself. If the AD1852 can drive the load of a normal pre/headphone amp, why use an active stage after it? If some buffering is needed I'll be using a class A JFET buffer (or a bipolar class A diamond buffer) that is far more transparent than an ampliying stage. I don't think any voltage amplification is needed after the DAC.
     
  9. Voodoochile
    I'll preface with saying that I've never heard it, but you have to wonder about it. At the ~$140 price, and considering probably a third of that is the chassis, it doesn't leave much of a budget for components and labor. Those traces are pretty scary.

    Then again, it does still stand a good chance of performing considerably better than the DAC included in many big box store CD/DVD players, which is another scary topic alltogether.
     
  10. diditmyself
    Quote:

    Originally Posted by Voodoochile /img/forum/go_quote.gif
    I'll preface with saying that I've never heard it, but you have to wonder about it. At the ~$140 price, and considering probably a third of that is the chassis, it doesn't leave much of a budget for components and labor. Those traces are pretty scary.

    Then again, it does still stand a good chance of performing considerably better than the DAC included in many big box store CD/DVD players, which is another scary topic alltogether.




    The same goes for the nice brands as well. Those thousands of dollars nice amps and DACS, I would guess that the enclosure/components cost ratio is even higher, sometimes much higher. Not to speak of the price we have to pay for their ego, advertising and other marketing related costs.

    Regarding Zero and other cheap Chinesene DACs/amps, one can wonder if the expensive parts like the SILMIC II, CERAFINE, OPA627 and Hitachi transistors are genuine. There can't be much left for them to make a living. If I were to buy two OPA627s in my country, I had to pay about the same as I did for the ZERO with them included.
     
  11. crapback
    I had to try and revive this thread and see what you were able to do with the zero's amp section, diditmyself. How did your project turn out?
     
  12. audiophelia
    I just purchased a Zero DAC off of ebay, as I owned one previously years ago and loved it. I received it in the mail, and first thing I notice is that the Pre-Amp isn't working.  Headphones work, but unplug the headphones and nothing comes out of the RCA's.  So I thought okay, it must be a defective 1/4 inch jack and it thinks that there's something plugged into it when there isn't. So I remove the 1/4 inch jack, still the problem persists. 
     
    Anyone have any ideas of where to start looking?
     
    PS> to clairfy. pre-amp/phono light off, sound comes out RCA's but volume knob does not alter the volume, and it's very loud.  pre-amp/phono light on and nothing comes out of the RCA's.
     

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