Audio-GD D28.38 Headphones Amp/Dac/Pre ^Sale

WARRANTY:

EASY RETURN:

Since April 4th 2018, the model has upgraded volume ports:
The upgraded volume ports have built with Vishay or KOA resistors with dual resistors in parallel , tolerance as low as 0.05% .

Different between NFB-28.38 and NFB-28.28:
The difference is that NFB28.38 has built in ES9038 Pro, NFB28.28 has built in ES9028 pro, other parts are exact the same. As for sound quality, NFB28.38 has slightly better sound transparency.

Differences with NFB-29.38:
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 and can drive most headphone very well.
If customer has headphones with single ended connector (6.3MM), choice would NFB-29 you will get better performance than the NFB-28.
If customer has the balance connector headphones or can modify the headphone to balance connector, choice would be the NFB-28.

Summarize
Fully balanced design built by discrete analog stages .
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-groups of 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 avoid channel imbalance and improves sound quality .
Supper Exponential Volume Characteristic with Fast / slow selection, easy match the volume with very different sensitivity headphones / speakers
4 digital inputs: USB / Optical / Coaxial RCA / I2S
2 analog inputs: XLR / RCA (The RCA input can be converted to XLR output natively without extra converter )
DAC output: XLR / RCA / ACSS
Headphone output : 4 pin XLR / 1/4" (6.3MM) SE
User can change the digital parameter settings
4 types gain modes to drive the high and low sensitivity headphones very easy .

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

Detail of the Current Conveyor (ACSS) Technology

Impedance matching:
The NFB-28 has one group of analog RCAs and one group of analog XLR inputs that can connect external DACs , working as high quality headphone amp .There are a lot different DACs out there they have different design and output impedances. To avoid sound quality loss by differences in DAC's impedance output the NFB-28 has designed 4 groups of expensive class A FET buffers on inputs .All the analog input signals go through these FET buffers, That alowes output impedance to become very low, even if the source has high or low output impedance.
The FET buffers work in pure class A and it is short signal path can keep the sound quality without coloration ,(dry and harsh.)

Simple and short signal path is best for sound quality
The NFB-28 applies the newest ESS high-end Sabre32 D/Achip .ES9038Pro can support up to 32Bit/384KHz input .
ES9038Pro in voltage output mode, has -12DB THD+N worst performance . To achievethe best performance of ES9038Pro , D/A chip must work in current mode output.
The ACSS circuit naturally works in current mode , it can transfer the ES9038Pro current signal to the output and implement the I/V conversion in current mode in one ACSS amp . And this ACSS amp only have one current gain stage , the ES9038Pro output signal only go through one stage .
The shortest signal parh can reproduce the best detail and dynamic, almost without sound coloration , It can show Real potential of the ES9038Pro dac.
So our ACSS design is the best combo with ES9038Pro to achieve the best performance of that Dac.
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 quality .Here is a conflict of the classic circuits.
But the ACSS opens a new field, it can offer a least coloration to the sound which is more neutral with very low distortion and high linearity. So it can retain the dynamics, detail and neutral without being bright or harsh.
The output buffers are Non-feedback. For low impedance, we applied a diamond output stage which add much less coloration than most conventional circuits.
The DAC is without couple caps to avoid coloration. There are two OPAs built in as the DC serveo to keep the DC offset .
The 32bit / 384K USB interface outputs a I2S signal to ES9038Pro .

Strong diamond difference output stages:
The NFB-28 has 4 groups of diamond differential output stages and the balance output drive stages. Every part has been exactly matched before assemble.

The output stages have built in 4 pairs of 15W transistors to offer strong drive capability .The balance output mode has 4X times power than the same single-ended mode, so no matter if you drive the high or low sensitivity headphones . The diamond differential design can avoid switching distortion , it works in the class A but does not draw large idle current . The balance drive mode kills the noise and distortion ,improves the sound stage, background and the transparency.

Multi-ply power supplies are the key of sound :

NFB-28 applies the excellent D/Achip ES9038Pro 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 D/A chip and the best amp, if matched to a average power supply the total sound quality may still be average .Maybe sound musical but can't be neutral and detailed. That is why it is easy to find hi-end grade gear without the best chips , but with plenteous of dedicated DC supply circuits.
The NFB-28 uses A total of 6 groups of PSU power supplies 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 in current mode and not voltage mode . I/V conversion changes 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 at any volume level. In front of the volume control, there are four groups of diamond non-feedback buffers 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,) allow users to control the volume level smoothly. NFB-28 has two relay-based volume control channels through the DALE resistors to control the volume and avoid channel imbalances .Same time achieving the best performance and sound quality .While changeing the volume, the combine relays break or close, it will have a slight switch sound on the output and will disappear after you stop adjusting the volume.

digitally controlled relay-based volume control.

Fast / Slow two volume control characteristic

Shield:

The NFB-28 have built in Multi-ply shield boards, to separate the transformer, power supply, digital parts and analog parts to avoid the different parts interfering with each other.Even the power input IEC socket is wrapped in a copper shield.

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