Xmos U208 Introduce and drivers
combo 384 Intruduce and driver download
(For the unit with the Amanero Combo384 upgrade module)
Audio-gd products had
design and test based on the AP SYS-2722 (The top-drawer
professional equipment in world)
Different to the last
generation NFB-28 (2016):
The newly designed ES9028Pro can setting the built
in filter parameter , it can auto detect the input signal DSD and
remove the switch noise . In this design, the input signal had less
signal journey feed to the ES9028Pro , improve on the sound quality .
Different to the NFB-29:
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 can drive most headphone as well.
If customer like to keep your headphone with single ended connector
choice the NFB-29 get better performance than the NFB-28.
If customer had the balance connector headphone or can modify the
headphone with balance connector, choice the NFB-28.
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
Non- feedback ACSS analog output stage .
R-core transformers and Hi-fi grade parts like DALE resistors, WIMA
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 /
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 )
XLR / RCA / ACSS
Headphone output :
4 pin XLR / 1/4" (6.3MM) SE
User can change the digital parameter setting
4 types gain modes , drive the high and low sensitivity headphones
as easy .
Modification guide on
headphone jack for 4-pin balanced headphone output.
Click here to read:
Parts for the modification is USD30. (Includes one female 4-pin plug
and one male 4-pin plug, some tape and tin solder.)
Detail of the Current Conveyor
The NFB-28 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, 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.
buffers working on pure class A state, and it is so short signal
journey , it can keep the sound quality without coloration ,dry and
Simple and short signal transfer is best:
The NFB-28 applies the newest ESS high-end Sabre32 DA chip
ES9028Pro which can support up to 32Bit/384KHz input .
in voltage output model , there is -12DB THD+N worst , to be
achieve the best performance of ES9028Pro , must work in current
output model .
In some other ES9028Pro built in DAC design ,
ES9028Pro output signal have to through 3 step OPAs implement analogy
signal process .
The NFB-28 had different design , applies Non-feedback ACSS
design. The ES9028Pro 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 ES9028Pro 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 ES9028Pro 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 ES9028Pro .
So our ACSS design is the best combo with
ES9028Pro achieve the best performance .
Upside in the picture is the standard
ES9028Pro 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
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 ES9028Pro .
Strong diamond difference output
The NFB-28 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
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
Multi-ply power supplies are the key of sound :
NFB-28 applies the excellent DA chip ES9028Pro 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 average 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 , but with plenteous dedicated
DC supply circuits.
uses A total of 6 groups PSU power supply 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.
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.
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 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
Precise digitally controlled relay-based
Fast / Slow two volume control characteristic
The NFB-28 had built in Multi-ply shield boards, to separate
the transformer, power supply, digital parts and analogy parts
for avoid the different parts had disturb . Even though the power
input IEC socket, it had hide in a copper shield
Hi-end grade components, including DALE resistors, WIMA caps,
custom order NOVER (UK brand) caps, Neutrik headphone output socket,