Ever heard about Full Digital Amplifiers ??

(Latest update:  Jan-11-2021)


Full Digital Amps (FDAs) sometimes also called Power DACs , Direct Digital Amps or All Digital Amps!


No !?!? Never heard about them?


...the end of separate  DAC + AMP audio systems is near!!! ;)


That's what I thought when first publishing this article in 2012 after playing around with this breed.


I then wrote: "For those of you who got tired to run after every new DAC or Amplifier (I did), and those who've read the 1000th review of another miracle DAC (I did), or those of you who got more confused then enlightened with the endless number of DACs of choice out there (I did again), or those who look for a very small, cost efficient and still great sounding system ... 

Jee - just figured -  I'm talking about myself..  ;)

And now I am realizing after more then 8 years further down the road not much has changed.
After abandoning this FDA project I slipped back into old habits, chasing DACs and amps. 



Full Digital Amplifier (FDA)



What is a Full Digital Amplifier?

Full Digital Amps convert  PCM (Pulse Code Modulation) audio data to PWM (Pulse Width Modulation) signal.  You basically feed your digital CD data right into a special Class-D amplifier.

Sometimes Full Digital Amplifiers are also called Power DACs. Such a term is a bit misleading though. 
One would assume - as the name somewhat suggests - a Power DAC would run a DA conversion like a typical DAC and after that a high-power output stage - being used instead of the typical low power opamp based DAC output stage - would get the job finished.  Basically a fully integrated DAC-Amp.
(As a matter of fact these devices also exist.) 

The actual FDA is a slightly different beast though. Slightly?? 


The Digital PCM Signal is typically fed into a PWM Processor (modulator) From there the new PWM signal is fed into a PWM (Class-D) power stage and further processed through the typical Class-D output filters (high power coils).

The PWM processor stage is where the FDA related magic happens. Like with most todays DACs there's also upsampling involved. As you can find in above linked datasheet the digital input data will be upsampled to much higher samplerates. You'll find many rather low-cost solutions that do upsampling prior of the data hitting the PWM modulator.

The FDA and  Class-D amp output stages are pretty much identical. There are certain small issues related to the reconstruction filter. The most evident flaw is that the output filter interacts with the speaker (same as with pretty much all Class-D amps).
Impedance shifts throughout the audio band of the speaker or a poorly matched filter (speaker/amp) slightly influences the resulting signal quality.
Best results can be expected with impedance linear speakers and matched amp output filter- speaker impedance.

Though the audible differences running e.g. an output filter dimensioned for a 4R speaker on a 8R speaker can pretty much be neglected. That's my experience at least.

Some FDA models come with a low-power (20W) one-chip solution, where the processor and power-stage resides inside a single chip and most others come with above outlined 2 two-chip solution.

Why FDA ?


Let's try to gather some arguments that would support to go for such a technology.

  • No analog DAC output stage (and related losses)
  • No analog Amp input stage  (and related losses)
  • No impedance and interface issues
  • No cabling, no jacks, no RCAs, no coupling caps
  • No DAC amp integration issues
  • No ground loop or EMI/RFI issues
And then there's another very nice feature, which comes with "some" of those amps (if implemented by the manufacturer) :
  • PWM volume control
    TI e.g. is doing it  by changing the actual PWM output voltage (the supply voltage of the output stage). Volume control at the very last spot. It can't get better then that. Doing it this way is extremely simple and most effective in comparison to all other types of volume control I'm aware of.
  • PWM volume control eliminates the awful "gain" discussions that pop up when integrating
    DAC, Amps and speakers. 

And finally

  • the soundquality
    The first impression is usually. "I wouldn't have expected it"
    It sounds really clean and crisp. @such a cost. Unbelievable!


Where there is light...


I didn't stick to that amazing technology. Why?

  • Lack of affordable highest quality implementations
  • LowQ input sections
    If you look at the typical FDA input sections, you'll find that these are not
    in the same league as you'd find on a modern decent DAC. (clocks/regulators/XMOS/...)
  • LowQ output filter sections
    If you look at the output coils, these are not state-of-the-art compared
    to real highest-quality modern Class-D amps
  • LowQ power supplies
    Usually you'll find lowQ SMPS supplies that come with these affordable amps and these then
    also feed the digital input section. That's not good. I tested it myself.
  • LowQ digital processing
    The PWM processor does quite some DSP work, such as upsampling. This can be done better.
    I did tweak this weakness by feeding 352,8 or 384kHz material.
  • Inherent weakness in FDA digital modulator principle (no feedback loop from PWM to PCM)
  • Most of above pro-FDA list also applies to well integrated digital DAC-Amps.

I hit all these so-and-so's after playing a while with these kind of amps. I modified my amps as much as I could and it still wasn't enough back than. Perhaps it's different nowadays. I didn't have the opportunity to try the most recent devices out there. By looking at them a little closer I do see that several of the weak spots I identified back then do still exist.

And it's even getting worse. Modern fully integrated solutions come with a lot of software and firmware. You really need to make sure that a manufacturer supports such a device over a long period of time, which usually isn't the case. Before you buy such a device look at the forums about the manufacturers SW maintenance performance!

Wrap UP


The company called STM holds a patent on the PCM/PWM amp technology since 2000. You'll find informative technology information in the patent documentation about how such an amp works. Have a look at it. 

I just saw that the patent expires later this year (2021). Hopefully this causes new developments in the FDA field.


All above listed advantages that I associate to FDAs translate into crystal clear, very dynamic, very black and very neutral sound. You'll notice very low coloration artifacts, a wide and deep soundstage.

The funny thing. It doesn't matter if you read a review about a NAD M32 or a FX-Audio Amp. The outlined characteristics about these FDAs do look very similar.

Some people feel strange about their listening experience with FDAs first. Usually you'll hear comments like: the "body" and "warmth" is missing. Yep. "Body" "Warmth" -- result of filter effects, coloration and phase shifts due to whatever problems in the chain. I can tell you one thing. I'm more then happy to get rid off those nasties. The only good thing about the flaws which lead to  "body" and "warmth" is that they usually also mask other problems in the chain. 

However. 

Most recent moderately priced DACs and Class-D amps do also perform very impressive. Even with
all the above mentioned flaws that come with them.
Still. In Annex A I list a FX-Audio FDA. The solution will cost around ~$500. And the SNR performance looks pretty outstanding.

Bottom line. I'll keep an eye on further developments. I guess it's worth it.

Enjoy.


*********************************************************************************

Annex A:

I'd like to list a small selection of FDAs I looked up today (Jan/2021) just to give directions.
I figured - you really have to look twice to find out if they are talking about a "real" FDA. 

Mainly NON-DIY

  • HifiMeDiy UDP320 (120$)
    Connexelectronics SMPS300RS (65$)
  • FX-Audio D2160 (~$250)
    SNR -105dB
    THD: 0.04%
    (better with 2 x better PS @ 36V + ~$130) 
           (I'd give that one a try nowadays - Jan/2021)
  • Nubert nuConnect ampX (~$700)

  • NAD D7050  (~$1100)
  • NAD D3045  (~$699)
  • NAD M32     (~$4500)

  • Lyngdorf TDAI-2170 (~$2800)

**********************************************************************************

Annex B:



DDX320 V2 - Modifications (Status 01/2013)

The HifiMeDIY DDX320 V2 @ 106$/82€ is my Full Digital Amp of choice for the time being.
The stock device performs quite nice. Though I wasn't really happy with the performance of that  stock DDX.  It was good, but not what I'd call audiophile.



Over a period of 4 weeks I spent some time to apply this and that modification.
Basically I applied those mods I'd try on all my audio stuff.

But first, before you start your journey,  I highly  recommend that you let the amp break in.
It really improves a lot over the first two weeks.
You'd have a real hard time to distinguish between break-in effects or tweak effects.


My Modification -Summary:

* Decoupling needs to be improved. E.g. on clock/VDD STA320/Toslink.
   Decoupling caps can be soldered right to the pins or very close by.
   There is no local decoupling close to the clock and Toslink. The STA320 digital and
   analog supply pins need to be improved.

* All (just 6) rather low quality  Elna Stargets should be swapped
   The choice of small caps can be improved - IMO at least Silmic II can be used.
   I still had some small Blackgates NP around, so I used those.
   Extra cost are almost neglectable. For digital decoupling (e.g. clock and Toslink) ,  lowest
   ESR Oscons SEPC are highly recommended.

* I swapped the 330uf  Panasonic FC (used as 3.3V buffer) with Blackgate 1000uf NP.

* Output coils got swapped (e.g. shielded Wurth WE-PD). And best
   mounted underneath the board. Note: I still use my Mundorf aircoils (you might have space
   and radiation issues with these).
   If not possible to swap coils the existing coils needs to be stabilized with e.g. BluTack or similar.
   I use  Henkel Teroson sealant for cars (kind of modeling clay)
  The coil mounting is pretty poor - wires used as stands. These vibrate and ring like hell.

* Use of LipoFe4 3.3V battery supply instead of the Techcode DC1507 DC/DC converter + output
   filter. I use 3 A123 cells in parallel.
   Note: It's a bit tricky to desolder the DC7DC converter. That's why I cut off the legs of the chip.
             But obviously  that means you're entering the "way of NO return". ;)
   I also removed all DC/DC converter related parts ( e.g. inductor). The batteries are hooked up right
   to the 330uf Panasonic FC or in my case a large Blackgate.

   This battery tweak is IMO one of the - if not - the most important tweak(s)!

   For future board revs HifiMeDIY should consider an option for an internal/external 3.3V supply
   switch

* Lelon power caps need a bypass turbo. I used 0.52uF KP cap and a little silver-mica.
   You also should  stabilize the Lelons - with e.g. blue tack to avoid vibrations.
   You might take off the plastic cover off those caps.
   Note: I do have the impression that the Lelon's are not worse then e.g. Panasonic FC.
             That's why I did not replace these caps.

* In case of 24V DC operation, the diode you'll find right at the suppply input can be shorted!
   In the future HifiMeDIY should consider a 4th input for DC only.

* I use a Connexelectronics SMPS 3000RS set to 28V. 28V seems to be
   the best voltage.

* For those who run SPDIF. Get yourself a pulsetransformer ( e.g. from Pulse  - Pulse PE-65612NL -
   IMO the best out of 10 I tried (that inlc. stuff like Scientific Conversion transformers)) to isolate
   your source.
   Though --- avoid double isolation! Check if your source/transport comes with pulsetransformer
   already.
   Unfortunately HifiMeDIY didn't follow my earlier advise on this one. There are plenty of inputs.
   At least one should have been transformer-isolated

* On the SW side I turned the
          - bass section off and
          - activated automatic display off and
          - highpass and all EQs off



*  The DDX320 resamples everything to 24/96!!!!

    It excepts everything from 44.1 to 192, but resamples everything!!!

    I do highly recommend to do the resampling on the transport/PC side.



  Note: There is no AES/EBU input. And it's not possible to connect via I2S! The MCU
            expects a sample rate notification from the WM8805. That's not working if
            I2S would be used directly.


 Note:   On the DDX320 V1 I was using the crossover function ( highpass) at 80Hz.
            That lifted off some low frequency stuff from my fullrange speakers. I liked what I heard.
            With the tweaked V2 in place, I can clearly hear the DSP related losses.
            That's why I run the the V2 without highpass nowadays. 




Note: All caps (piggy-back style) and coils are mounted underneath. The board is mounted on stands.
          As you can see I put some modeling clay (Teroson sealant) on Caps, coils, Toslink and clock.



Summary:


As you can see above, most of my modifications relate to the power supply, such as cap swapping, decoupling, battery.
These mods should be manageable by everyone, who's done some basic modding.


The key modifications are IMO the 3.3V battery supply and the output coil replacement.
Still, I'd highly recommend you'd do them all. You might use your own parts of choice.


Bottom line. All above tweaks cost you (and HifiMeDIY in case of a little re-design) close to nothing.
With those mods being implemented, you'll end up with a pretty high-end full digital amp. As I said before. My  TP Buffalo (LiPoFe4 powered+ more tweaks) + amp chain is settling dust.

I'd like to say that even without all these tweaks the stock DDX320 V2 is a  nice sounding all-in-one solution. With the tweaks in place it's an outstanding value and value for the money.

Who needs more then that???

14 comments:

  1. Any recommendation for a good FDA? I am currently using the Sumoh TinyAmp and I know it makes some faults...

    Stefan

    ReplyDelete
  2. Thanks!

    I lost the idea of FDAs for some time as the TacT Amps have a sound quality i do not like so much.

    Thanks for the inspiration :)

    kind regards,

    Michael

    ReplyDelete
  3. Your fault...
    I ordered one ;)

    I am really eager to know on which level it plays - even with the "noise bug" which they hopefully can fix soon.

    cheers,

    Michael

    ReplyDelete
  4. Coreaudiotechnology.com seems to do the job and taking it further to active speakers.
    Anyway, the concept seems very promissing and judging from what I've heard in Stuttgart from your modified sqt through Silvercore to Bastanis, I believe you are in serious cause to work on it.
    Please keep us informed.
    Cheers
    Nondas

    ReplyDelete
  5. Been using Tact amps and room correction, since 2001, and with Squeezeboxes since 2005 or so. Shame that BOTH Tact and the Squeezebox seem dead.

    ReplyDelete
  6. Great job and tweaks!I must try. One step further for next release should be I2S input, the WM8805 has it ! on forum the answer is the controller cannot handle the selection but it's strange in my opinion .ciao.Matteo

    ReplyDelete
  7. Klaus,
    I have been using your TT3.0 with good effect. It is interesting that you are into FDA. I have been using Panasonic Digital Amplifier receivers and now a TACT S2150 integrated amp. These amps are known for their transparency which probably many audiophiles dont prefer because they prefer to have 'control' over the type of the sound they want. I also tried your upsampling options with my Touch and unlike you I seem to prefer that sound (yes, maybe a bit brighter but to me it seems to have more clarity). Thanks for all the great work you are doing.

    ReplyDelete
  8. Hey Klaus,

    you missed the Panasonic SA-XR Series in your overview. The all utilize the TAS5182 and TAS5076 PCM to PWM Modulators.

    Right now I'm thinking of sending the PWM-Stream to a Hifimediy T4, as it should have a second PWM Input. Source could be a TAS5518 or TAS3308. By this, it may be possible to have a new full digital high power system.

    Viele Grüße aus D,
    Ben

    ReplyDelete
  9. Hi Klaus,

    You mention V3 in your thread, how does this (stock) version sound compared to the previous versions?

    I can't find the V3 amp on the Hifemediy site....

    Cheers, Mark.

    ReplyDelete
  10. How is the sound quality of a HiFimeDiy DDX320V2 compared to a QLS QA100? Anyone else experienced both?

    ReplyDelete
  11. I recently tried this with the NAD D 7050 I am using Hawthorne Open Baffle they have been heavily modified So far very impressed music flowing and did not know how much background my previous set up was giving. I am just using a Blue ray player as source

    ReplyDelete
  12. Someone tried the SMSL Q5? Like to get assessment on the inside of this boy.

    ReplyDelete
  13. Do you know if there are any multi channel FDA? TI chips are 4ch e.g. but I have not found anyone building a module out of it.... Still crawling the Web...

    ReplyDelete
  14. Hi. Your experiences are very impressive. I had similar experience with providing external 3.3v with super-cap to digital modules like usb, bluetooth etc. Many digital parts uses 3.3v, thus providing good quality 3.3v is very effective.
    If you any full-digital-amplifier interested in, please let me know.
    Thanks.

    ReplyDelete