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 at 9:30 AM
  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 at 12:02 PM
  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 at 12:58 PM

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