Audio-gd :: NFB-28 (Version 2016 )H Discrete DAC+ Headphone amplifier, Preamplifier With XLR, RCA analog inputs, 32bit/384K, Sabre ES9018, Amanero 386, DSD , DXD support from US, 2xTCXO , with remote control
Price: 856.02 EUR ( 695.95 EUR Net Price)
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Audio-gd NFB-28

NFB-28 (Version 2016 )#

The best mid range DAC + amp we ever had
  • Discrete DAC+ Headphone amplifier / Preamplifier
  • With XLR / RCA analog inputs
  • DSD , DXD support from USB & I2S Input
  • 32bit / 384K Sabre ES9018
  • 32bit / 384K Asynchronous Transfer USB-32 Chip

Main features

  • 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
  • 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.

Audio-Gd NFB-28

Impedance matching#

The NFB-28(2015) 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(2015), 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(2015) 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(2015) 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 . 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 .

Audio-Gd NFB-28

Strong diamond difference output stages#

The NFB-28(2015) 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.

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(2015) 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(2015) 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(2015) 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.

Audio-Gd NFB-28

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 .

Audio-Gd NFB-28