High quality DAC-Headphone amp based on a Full Digital Amplifier Board

  1. ondesx
    Hi Guys,

    After testing several FDA units (Topping, FX802, YJHiFi, etc...) most of them based on STA326 chips, I found a board based on the 333BW circuit, and the idea was to derive a high quality audio DAC-Amplifier for headphones with it...

    Here is the board :

    Below are some pictures of the resulting DIY unit...

    The audio quality is really amazing for the price of the prototype. Perfect silence without modulation then no noise (the so-called "black background"), very high fidelity of the instruments and voices (I have a "personal" reference...), crystal clear sound and very good dynamics ! In addition, this is a fully balanced unit.

    The very important points here are that there isn't neither any DAC nor any Operational Amplifier in this device !... Thus, the path is very short between the digital signal and its audition through the headphones. These are the most important reasons of the very good audio quality obtained with this little board.

    My Best

    FDAPAdetail.jpg FDAPAinside.jpg FDAPAoutside.jpg
    Last edited: Sep 22, 2017
  2. maxxevv
    Pardon my very limited knowledge of electronics.

    I see that you've used the speaker output posts and converted them into a balanced output jack. However I also see that you have added 4 huge resistors in the 4 output lines leading to the balanced jack ?
    Those are for modulating the power output or something ?
  3. dw1narso
    Looks like voltage devider? 1/16
  4. ondesx
    As stated by dw1narso this is a voltage divider since the board is initially an amplifier for loudspeakers !...
    Then, as you probably know, such an amplifier is too powerfull to drive even the highest impedance headphones...
    Reducing the output power of the board in a balanced mode through a pair of resistances is a very good way to drive any headphones.
    I prepare a very simple mod of this device to change the values of the resistances to adapt the overall gain of the unit to drive as easily as possible any headphones, whatever its impedance.
    For example, with the presented bridge, the output level is too low for an AKG 240 Monitor (600 Ohms). With a better choice of the resistances the output level can be easily augmented.

    Resistances of such values wouldn't degrade neither the distortion nor the noise figure of the amp.

    I'll post the mods ASAP.
    maxxevv likes this.
  5. maxxevv
    Thanks for sharing as it seems like a viable route if one is looking for a balanced output jack implementation, be it a 5 pin or a 2.5mm TRRS. In fact, it might be possible to do a split switch allows one to toggle between the 2 systems ??

    I would put in some heatsinks for the resistors though, those things should get pretty hot at 20W per channel after an hour or so ?
  6. ondesx
    You're welcome !...

    Such FDA boards are only usable for "balanced" headphones. This is because the FDA doesn't share any reference/ground, in other words each output is totally independent from the other. Of course, it's possible to return to SE through an adapted circuit, but it's a pity IMHO... After comparing SE and balanced listening, I prefer the latter option. I modified all my cans to use them in balanced mode. It's very easy to make an adapter for SE mode.

    Of course, you can use any connector and even have several if needed, with a switch to toggle between them, provided you keep them "balanced", i. e. without contact with each other !

    After several weeks of use, the system, even loaded with high impedance headphones doesn't go any warm. As suggested in my last post, I'll modify the overall gain with a couple of resistances (3 or 4) in order to use the amp at its lowest level, this will prevent any temperature issue, whatever the impedance of the headphones.
    Last edited: Sep 19, 2017
  7. Jake142
    Looks nice, i wish i had the skill to do that.
  8. ondesx
    To be honest, I must confess that one of my friends helped me to build this prototype... Anyway, there isn't any difficulty in this built, excepted the volume control through two interrupters here : one to raise and the other to lower the output level...
    Last edited: Sep 19, 2017
  9. Jimster480
    So this is a Class-A type device then?
    But there is no DAC.
  10. dw1narso
    This is more like Class D (PWM power converter) or Class T (Tripath proprietary PWM processing)..

    It is still a DAC by nature.. (since we human hear analog wave)... I believe the way it works is more like powerful DSD (direct bitstream) conversion..
  11. Jimster480
    Are there more pictures of this with more descriptions?
    Because this is very vague... how is it a DAC without any DAC inside of it
  12. dw1narso
    My knowledge on EE and digital processing is actually very limited...

    The following article seems describe Class D architecture quite easily to understand...

    In Class D, analog signal is used to create into "digital" format of pulse and pulse width, PWM signal. Then this "digital" data are then amplified and then converted back to "analog" right at the final stage / output device powering the transducer.

    STA326 and STA333 in this case accept digital PCM data (in I2s interface) directly and then digitally convert the PCM into PWM signal (thus Digital to Digital conversion)...

    I hope I could make it clear now that in the sense of bigger function, this is still a "DAC"... Since the data is in digital, but then transformed into analog at the last stage.

    Wadia call their product PowerDAC.., which is make sense...

    But, there are also in the market, some final product, although have digital input and using class D (or class T) amp that actually consist of a DAC + "analog"/traditional Class D amp..
    For example products that use TPA3116D2, TA2024, TA2020 chips and a lot others. This chip accept analog line input. So in the case like this, you will need a "traditional" DAC to feed in the amp.
  13. Jimster480
    Yes I gathered that much, its just interesting although I don't have any time to learn about this specifically.
  14. ondesx
    Hi Everybody,

    Please find enclosed 2 picts of the mods we made to the gain in order to adapt the amp to any Headphones, whatever its sensibility. Now the gain can be adjusted at three different levels. Even my AKG 240 Monitor (600 ohms) or the Fostex planar T50RP are perfectly usable with the amp.

    The sound quality is really amazing compared to very expensive balanced amps from Sennheiser or Beyerdynamic. We also compared this unit to several amps (the Wire in headphone version we built, class A amps and several other amps, like the SMSL sAp9, please see picture). This little unit is definitely a big deal !...

    These FDAs are worth a try !...



    Last edited: Nov 1, 2017
  15. ondesx
    To Jimster : I think the explanations given by dw1narso are complete. Yes, there is NO DAC in the classical sense, and the transformation is a conversion from PCM to PWM ! In other words you're digital to the very end of the processing, there are no losses in the D to A chain !...

    Apogee was the first to use this processing. But the industry was not prepared to that quantum leap at this moment. ST Microelectronics launched several chips from low to moderate power the Sound Terminal STA3XY series...


    I think that this technology will be the next one before the creation of the "digital speakers" !...

    Hope this helps.
    Last edited: Nov 1, 2017

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