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NFB-28 (Version 2014 )(DSD / DXD )(Since 1st May 2014)
Discrete  DAC+ Headphone amplifier / Preamplifier
With XLR / RCA analog inputs

DSD , DXD support from USB
32bit / 384K Sabre ES9018
32bit / 384K Asynchronous Transfer  USB-32 Chip
Price: USD750

Extra, high quality metal remote :  USD50 



Summarize Use Manual Specs Custom Option Shipping Cost

USB-32 Introduce and drivers download
Audio-gd products had design and test based on the AP SYS-2722 (The top-drawer professional equipment in world)

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 .
LED display avoids RF disturbance for better sound quality
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 digitally controlled relay-based volume control to avoid channel imbalance and improve sound quality.
80 steps volume level control in both high gain mode and low gain mode. 160-step volume control ensures smooth control of the volume level.
4 digital inputs:   Coaxial RCA / Coaxial BNC /  Optical / USB
2 analog inputs:  XLR / RCA
DAC output:   XLR / RCA / ACSS  (User can set either variable output level as preamp or fixed output level as dedicated DAC )
Headphone output : 4 pin XLR / 1/4" (6.3MM) SE

Static storage mode control software for avoid disturb sound quality , the MCU output once control data and save in the perform parts ,then stop output data until users next operation.  

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.)

Technology detail

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 ES9018 which can support up to 32Bit/384KHz input .
ES9018 in voltage output model , there is -12DB THD+N worst , to be achieve  the best performance of ES9018 , must work in current output model .
       In some other ES9018 built in DAC design , ES9018 output signal have to through 3 step OPAs implement analogy signal process .
       The NFB-28 had different design , applies Non-feedback ACSS design. The ES9018 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 ES9018 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 ES9018 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 ES9018 .
       So our ACSS design is the best combo with ES9018 achieve the best performance .
              Upside in the picture is the standard ES9018 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 ES9018 . 

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 ES9018 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.
           So, 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.
            Mark Levinson also knows that current volume control has great benefits, so in the volume control of his top grade hi-end preamplifier No.32, he uses many components to change the voltage (V) signal to current signal (I), then through the R-2R network to control the volume, and finally changes back to voltage signal (V) again. NFB-28 simply works in current (I) signal domain, and technically, it is superior to the conventional technology due to less conversion.
            The volume control quality is much more important in a real balanced gear. It must guarantee the signals of the four channels are exactly the same to bring out the benefits of a real balanced gear. If the cold signal and hot signal of a balanced gear are not exactly the same, the balanced output will has a large distortion causing the sound quality and performance even much worse than a single end gear. This is a waste of a balanced design and the additional cost.
            NFB-28 applies digitally controlled relay-based volume control in 160 steps. It has 80 steps in both high gain and low gain mode to allow users to control the volume level smoothly. NFB-28 has two relay-based volume control boards (totally four 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 users change the volume, the different relays break or close, it may had slightly switch sound on output and disappear while stop change the volume.

Precise four-channel digitally controlled relay-based volume control.            High / Low gain modes volume control characteristic

Static storage mode control software:
       In most control softwares design, the MCU will continuously output data to the perform parts , the MCU had the very high speed, so the output data become high frequency impulses disturb the audio signal circuits make the sound worst .
This product had applied static storage mode control software , the MCU output once control data and save in the perform parts while just power on and users operate ,then stop output data until users next operation. This design make the sound quality improve on the clear and transparency .


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

           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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