Wisdom in mind,   enthusiasm at heart.

Home Products Warranty Shop Contact Us Shipment Chinese


Fully Balanced discrete Preamp / Headphone amp
5500mW @ 50 ohm output

Price:   USD399
Extra metal remote :  USD50        

(The unit had free a mini remote control but sometimes the remote had wrong control.)


Summarize Use Manual Specs Custom Option Shipping Cost

Product edit note:
30th June 2014:  Remove the buttons setting while power on. Improve the sound quality to  more neutral.
Audio-gd products had design and test based on the AP SYS-2722 (The top-drawer professional equipment in world)

The unit applied analog input short design :
Nowdays a lot high and middle range preamps, headphone amps and integrated amps had applied the input short design .
Without the input short design , if the amp connect to multi-ply sources and all source had playing , different sources will had the signal crossing cause the sound quality degrade .
For example, the input 1 connect to a DAC and the input 2 connect to a CDP, while both had playing, the CDP signal will crossing to the input 1 , mix with the DAC signal cause the sound degrade.
In world there are more than 99% DACs and the sources had allow the outputs short , if your DAC decalre can't short the output , please read the "Use Manual" page , pull off the jumpers for discontinue the analog input short design , but in the case , while you connect multi-ply sources, you are better only let one power on and others power off for avoid the signal crossing .

Fully balanced design built by discrete analog stages without any OPA or coupling caps in the signal channels.
Non- feedback ACSS analog output stage .
R-core transformers.
2 group class A dedicated PSU's are used to purify the power supply.
Four-channel digitally controlled relay-based volume control avoids channel imbalance and improves sound quality. NFB-6 has 80 steps volume level control in both high and low gain mode. 160 step volume control ensures smooth control of the volume level.
5 analog inputs: 
Preamp output :   XLR / RCA / ACSS
Headphone output :
4 pins XLR / 1/4" SE (6.3MM) (3000MW @ 100 ohm)
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 tapes and some WBT tin solder.)

Technology detail

Detail of the Current Conveyor (ACSS) Technology

Impedance matching:

             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-6, 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 .

About ACSS technology:

         ACSS (Audio-gd Current Signal System) technology is Audio-gd's signature technology for transmitting audio signals in the current domain. This concept, first seen in 1966, has been adapted by us for many years. Since 2005, we have been evolving this technology to include it into all audio signal transmissions in our systems from digital source to power amplifier. Since 2006, ACSS technology became Audio-gd's signature technology and gains a lot of popularity.


NFB-6 is a true balanced design applying the newest ACSS design which can improve the XLR connection to sound as good as the ACSS connection. They seem complex , but in fact they are the most simple design the world has ever had , their a lot components to process the power supply. The simplest and shortest signal journey brings out the most neutral and least colored sound.
         NFB-6 applies ultra-low distortion single stage ACSS amplifier and soldered directly on the motherboard. It is a fully class A preamp / headphone amp and applies class A power supplies.
        The ACSS is a non-feedback technology made with 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.
        All signal gain is in current, all transistors work in class A mode.
        The NFB-6 output buffers are Non-feedback, for low impedance and ultra-low distortion , we applied two pare  transistors  connect with cascode  , the output impedance is only 1 ohms , it can drive most headphones and amps as well  .

              Real fully balanced discrete design.


Strong diamond difference output stages:
             The NFB-6 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.


Power supply:    
         A high performance transformer are used to supply power separately for the audio and control circuit.
         NFB-6 uses 2 groups of high-quality class A parallel connection PSUs.
         The class A parallel connection PSU has very high input impedance to avoid pulse through the PSU from affecting the preamp and low output impedance with very fast speed and high linear, so it is a very clean power supply.
         NFB-6 applies another linear PSU for the display and control parts, avoid the disturbances through power line from affecting the amplifier.
             Strong power supply.

         NFB-6 can input not only XLR balanced signal or RCA unbalanced signal, but also the top-grade ACSS current signal. It can also output RCA signal, balanced XRL signal, ACSS current signal to the power amplifier. Using RCA input, NFB-6 can convert to balanced ACSS signal and output real balance signal from XLR and RCA.
         NFB-6 can output RCA, XLR and ACSS at the same time in preamp output model , in headphone amp model, NFB-6 can output XLR ,single end for headphone, and ACSS for power amp at same time.
        The input channels of NFB-6 adopt separated shielding method. When switching, the idle signal lines linked to the equipment and the ground wire are completely disconnected. The idle input ports of the equipment are shielded. In that case, the sound quality won't be affected even with more connected equipment.

Volume control:
        The volume control is also different from conventional technology. Conventional volume control technology is the volume control implement in the analog amp input stage through a volume pot to reduce the signal  . The volume pots quality effect the sound quality direct , the low quality volume pot cause lost the detail and channels imbalance cause the soundstage distortion  . Even the high performance volume pot still cause the detail lose. Whatever quality volume pots, must cause the S/N worst .
    But in the NFB-6 , we applied the I/V conversion volume control , the volume control just  a variable passive I/V conversion place at the ACSS amp output , where the output is the current signal, not the voltage signal. The volume control is I/V conversion, and the volume depends on the current (I) conversion to the voltage (V). (Like R-2R D/A chips output passive I/V conversion)  It can keep the signal frequency band flat and not lost any detail.
It can't degrade on sound quality in every volume level . After the volume , there are four groups  diamond non-feedback buffer output stages offer the very low output impedance  .
        Mark Levinson also knows that current volume control has great benefits, so in their Top End preamp NO.32 volume control, they use many components to change the signal to current signal (I), then implement the volume control , and then change back to voltage signal (V) again. But NFB-6 is simply a more complete work in terms of the current signal, and technically, it is superior to conventional technology. The less conversion bring much neutral sound .
        The volume control quality is much important in the real balance gear . It must guarantee the four channels signal control exact to achieve the performance of the real balance gear .  If the cool and hot of the balance signal can't guarantee exact, the balance output will has large distortion cause the sound quality and performance even much worst than a single end gear waste the  balance gear  design and the cost.
         In NFB-6 is applied a four channels digital control relays based volume control. .NFB-6 has total four channels through changing the DALE resistors to control the volume for avoid channels imbalanced ,achieve the best performance and sound quality of the gear.
        NFB-6 have 80 steps volume level control in both high gain  and low gain mode. The total 160-step volume control ensures smooth control of the volume level.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.


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 .  The 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 .



版权所有:          何庆华  睿志音响

Copyright(C) 2004   www.audio-gd.com    All Rights Reserved