Audio-gd products had
design and test based on the AP SYS-2722 (The top-drawer
professional equipment in world)
combo 384 Intruduce and driver download
Xmos U208 Introduce and drivers
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.
Fully balanced design built by discrete analog stages without any
OPA and coupling caps in the signal path , 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-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 /
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:
Detail of the Current Conveyor
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 .)
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 .
in voltage output mode, has -12DB THD+N worst performance . To
achieve the best performance of ES9038Pro , D/A chip must work
in current mode output.
In other applications of ES9038Pro output signal have to
go through 3 steps of OP-amps implement in analog signal path.
The NFB-28 has different design , it applies Non-feedback
ACSS design. The ES9038Pro is fed to the ACSS circuit in
balanced mode without any OPAs .
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.
diagram is the standard ES9038Pro 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 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
Strong diamond difference
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
Multi-ply power supplies are the key of sound :
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
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
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
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.
Precise digitally controlled relay-based
Fast / Slow two volume control characteristic
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 .
Uses all Hi-end grade components, including DALE resistors, WIMA
caps, custom order NOVER (UK brand) caps, Neutrik headphone output
socket, ect .