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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 .
Summarize
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: RCA X2, XLR X2, ACSS X1
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.)
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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.
Core:
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.
Input/Output:
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 .
shield:
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 .
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Summarize
