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NFB-10.33
Discrete  DAC+ Headphone amplifier / Preamplifier

32bit / 384K Asynchronous Transfer  USB-32 Chip
Built in 2 Pieces Hi-end grade WM8741 Chips

Detail of the Current Conveyor (ACSS) Technology


Simple and short signal transfer is best:
            The NFB-10.33 applies the newest  high-end dual WM8741 which can support up to 32Bit/192KHz input .
            The standard current model  design usually need 3 amps (OPA) , the first amp is the difference amp ,  conversion the 4 channels output signal from WM8741, become a single ended signal  .
            The second amp  implement the low pass analog filter , then through  volume control  feed to the third amp, the headphone amp .
            Every amp usually have two gain stages built in so  the WM8741 signal must through total 6 gain stages arrive the output stages .
            The NFB-10.33 applies Non-feedback ACSS design. The WM8741 feed to the ACSS circuit without any OPAs .
            The ACSS circuit conversion, signal difference and headphone amp in current model in one ACSS amp . And this ACSS amp only have one current gain stage , the WM8741 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 WM8741 .
           So our ACSS design is the best combo with WM8741 achieve the best performance .
                                          Upside in the picture is the standard WM8741 headphone amp design. The bottom is the NFB-10.33 design .

                                       
           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 dual WM8741 .
                         
            Fully Discrete ACSS headphone amp.                               THD                                      Frequency band
                                

           NFB-10.33 applies the excellent dual WM8741 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 normal 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 or amp stages, but with plenteous dedicated DC supply circuits.
           NFB-10.33 uses a high performance transformer. More than 38,000uf audio grade NOVER capacitors are used to ensure ample and smooth power feed. The NOVER capacitors are specially custom made from NOVER (U.K.) according to Audio GD's requirements.
           The NFB-10.33 uses 7 groups of high-quality PSU with dedicated DC supply. Digital and analog voltages each have separately isolated internal power regulations.
           The analog power supply is more important for sound reproduce, in NFB-10.33, the +15V and -15V for the ACSS analog output stages are high speed Discrete PSU. They can offer pure and quick power supply for driving the headphone exactly. The high voltage power supply can drive most headphone from 15 to 600 ohms as well.
                                         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-10.33 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-10.33 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-10.33 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-10.33 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 .