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NFB-28 (ES9028Pro )

Discrete  DAC+ Headphone amplifier / Preamplifier
With XLR / RCA analog inputs

DSD , DXD support from USB & I2S Input
32bit / 768K Sabre ES9028Pro
32bit / 384K Asynchronous Transfer XMOS U208 Chip


Price: USD708 (Free mini remote controller but it had not warranty.)

Extra, high quality metal remote :  USD45 

     


      
Summarize Use Manual Specs Custom Option Shipping Cost


Xmos U208 Introduce and drivers download
Amanero combo 384 Intruduce and driver download (For the unit with the Amanero Combo384 upgrade module)
Reviews:   
http://www.head-fi.org/t/676930/audio-gd-NFB-28
Audio-gd products had design and test based on the AP SYS-2722 (The top-drawer professional equipment in world)



Different to the last generation NFB-28 (2016):
The newly designed ES9028Pro can setting the built in filter parameter , it can auto detect the input signal DSD and remove the switch noise . In this design, the input signal had less signal journey feed to the ES9028Pro , improve on the sound quality .

Different to the NFB-29:
The NFB-29 is a single ended DAC with headamp and preamp (+/- 30V DC high voltage power supply ), The NFB-28 is balance DAC with headamp and preamp (+/-15V DC power supply ).  So They had same output power specs can drive most headphone as well.
If customer like to keep your headphone with single ended connector (6.3MM), choice the NFB-29 get better performance than the NFB-28.
If customer had the balance connector headphone or can modify the headphone with balance connector, choice the NFB-28.

Summarize
Fully balanced design built by discrete analog stages without any OPA and coupling caps in the signal channels, built in real balanced DAC. Output for headphone (4-pin XLR or 1/4") and  DAC (fixed output) with preamp (variable output)
Non- feedback ACSS analog output stage .
Large R-core transformers and Hi-fi grade parts like DALE resistors, WIMA caps .
Total of 6-group PSUs are used to purify the power supply , the analog amp has discrete PSU power supply
Four-channel 100 steps digitally controlled relay-based volume control avoids channel imbalance and improves sound quality .
Supper Exponential Volume Characteristic with Fast / slow selection, easy match the volume with very different sensitivity headphone / speaker .
4 digital inputs:   USB / Optical / Coaxial RCA / I2S
2 analog inputs:  XLR / RCA (The RCA input can conversion to XLR output native without extra convertor  )
DAC output:   XLR / RCA / ACSS 
Headphone output : 4 pin XLR / 1/4" (6.3MM) SE
User can change the digital parameter setting 
4 types gain modes , drive the high and low sensitivity headphones as easy .  

Modification guide on headphone jack for 4-pin balanced headphone output.
  
Click here to read:   modification guide

Parts for the modification is USD30. (Includes one female 4-pin plug and one male 4-pin plug, some tape and tin solder.)

 

Detail of the Current Conveyor (ACSS) Technology

Impedance matching:
             The NFB-28 had one group analogy RCA and one group analogy XLR inputs ,can connect external DACs , working as a decent headphone amp .There are a lot different DACs in worth, they had different design and output impedance . For avoid the sound quality effect by different DAC's output impedance, in the NFB-28, it had design 4 groups expensive class A FET buffers in the input . All the analogy  input signals through these FET buffers, the output impedance had become very low, even the source had high or low output impedance.
             The FET buffers working on pure class A state, and it is so short signal journey , it can keep the sound quality without coloration ,dry and harsh .
             

 Simple and short signal transfer is best:
    
  The NFB-28 applies the newest ESS high-end Sabre32 DA chip ES9028Pro which can support up to 32Bit/384KHz input .
       
ES9028Pro in voltage output model , there is -12DB THD+N worst , to be achieve  the best performance of ES9028Pro , must work in current output model .
       In some other ES9028Pro built in DAC design , ES9028Pro output signal have to through 3 step OPAs implement analogy signal process .
       The NFB-28 had different design , applies Non-feedback ACSS design. The ES9028Pro feed to the ACSS circuit by balanced model without any OPAs .
       The ACSS circuit naturally working in current model and have design difference current input , it can transfer the ES9028Pro current signal to output and implement the I/V conversion in current model in one ACSS amp . And this ACSS amp only have one current gain stage , the ES9028Pro output signal only through one stage can arrive the output stages .
      The shortest signal journey can reproduce the best detail and dynamic, and almost without sound coloration , It can show the  real performance of the ES9028Pro .
       So our ACSS design is the best combo with ES9028Pro achieve the best performance .
          
              Upside in the picture is the standard ES9028Pro headphone amp design. The bottom is the NFB-28 design .
                     

                       Super low THD  (Test by AP SYS-2722,0DBV output level)             Flat frequency response (Test by AP SYS-2722)
                                                                      

      

        The ACSS also is a non-feedback technology made with fully discrete amplifiers. Most people know the global feedback design can offer better specs in test measurements, and non-feedback can't do well in test measurements but can offer better sound for the human's ears. Here is a conflict of the classic circuits.
        But the ACSS opens a new field, it can offer a least coloration sound which is more neutral with very low distortion and high linearity. So it can retain the dynamics, detail and neutral sound but not sound bright or harsh.
        The output buffers are Non-feedback. For low impedance, we applied a diamond output stage which is quite less colored than most conventional circuits.
        The DAC is without couple caps to avoid coloration. There are two OPAs built in as the DC serve  to keep the DC offset .
        The 32bit / 384K USB interface outputs a I2S signal to ES9028Pro . 

Strong diamond difference output stages:
             The NFB-28 had design 4 groups diamond difference output stages as the balance output drive stages, every parts had exact matching before assemble so don't need any human adjustment . 

             The output stages had built in  4 pair 15W transistors  for offer strong drive ability , the balance output mode had 4X times power than the same state single-ended mode, so no matter drive the high and low sensitivity headphones  . The diamond different design can avoid switch distortion , it working as the class A but don't want large idle current . The balance drive mode can kill the noise and distortion in the end of the headphone, much improve the sound stage, background and the transparency ,ect.

             



Multi-ply   power supplies are the key of sound :

         NFB-28 applies the excellent DA chip ES9028Pro and excellent analog output stages, but these are not the only keys of the best sound. The power supply is most important. Even applying the best DA chip and the best amp, if matched to a average power supply, the total sound may still be average or sound musical but can't be neutral and detailed. That is why it is easy to find hi-end grade gears maybe without the best chips , but with plenteous dedicated DC supply circuits.
         The NFB-28 uses A total of 6 groups  PSU power supply for all digital and analog circuits.
         To achieve high S/N, the control circuit is powered with separate regulator.


 
               Strong power supply.
       
        

 The I/V conversion volume control without sound degradation:

           Usually the volume control can be implemented in the DAC + amplifier combo in two methods
           Method 1: The volume control is implemented into the digital stage. The major drawback is that 1 bit of the D/A conversion is lost in every 6DB volume reduction. So, even though the built-in DA chip is 32 bit, in fact, only 16 bit may be left at low volume level.
           Method 2: The volume control is implemented in the analog amplifier input stage through a volume pot to reduce the signal level. However, the volume pot affects the sound quality adversely. A low grade volume pot loses the details and creates channels imbalance resulting into soundstage distortion. Even a high grade volume pot inevitably loses the details. Both methods degrade the sound quality.
           The NFB-28  applies the I/V conversion volume control, the volume control is just a variable passive I/V conversion being placed at the ACSS amplifier output, where the output is current signal but not voltage signal. I/V conversion is to change the volume level from the current (I) signal to the voltage (V) signal. (Like R-2R D/A chips output passive I/V conversion) It can keep the signal frequency band flat while not losing any detail. It does not degrade the sound quality in every volume level. In front of the volume control, there are four groups of diamond non-feedback buffer output stages that offer very low output impedance.
            NFB-28  applies digitally controlled relay-based volume control in 100 steps. It has 90DB control range ( most other volume control only had around 70DB range, so the device want to design the gain switches ) allow users to control the volume level smoothly. NFB-28  has two relay-based volume control  channels through changing the DALE resistors to control the volume to avoid channel imbalance, achieving the best performance and sound quality of the gear. While change the volume, the combine relays break or close, it may had slightly switch sound on output and disappear while stop change the volume.

        

        Precise  digitally controlled relay-based volume control.            Fast / Slow two volume control characteristic

                                                         

 shield:

           The NFB-28 had built in Multi-ply shield boards, to separate the  transformer, power supply, digital parts and analogy parts for avoid the different parts had disturb . Even though the power input IEC socket, it had hide in a copper shield box .
          

Components:
           Uses all Hi-end grade components, including DALE resistors, WIMA caps, custom order NOVER (UK brand) caps, Neutrik headphone output socket, ect .


 

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