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Detail of the Current Conveyor
(ACSS) Technology
About NFB?
The NFB-27 applies the newest ESS high-end Sabre32 DA chip
ES9018 which can support up to 32Bit/384KHz input .
To
be achieve the best performance of ES9018 , must work in
current output model, in voltage output model, there is -12DB THD+N worst .
The standard current model design
usually need 3 amps (OPA) ,
the first amp implement I/V conversion, conversion the ES9018 output
current signal to voltage signal .
The second amp is the difference amp,
conversion the balance signal to single-ended signal and implement
the low pass analog filter , then through volume control feed to the third amp, the headphone amp
or preamp
.
Every amp in the standard design usually have two gain stages
built in, so the ES9018 signal must through
total 6 gain stages arrive the output stages .
The
NFB-27 applies Non-feedback ACSS design. The ES9018 feed to the ACSS
circuit without any OPAs .
The ACSS circuit naturally working in
current model and have design difference current input , it can
process the I/V conversion, signal difference and headphone amp in current
model in one ACSS amp . And this ACSS amp only have one current gain
stage ,
the ES9018 output signal only through one gain 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 , users will know why
the ES9018 had become popular in short time after release .
So the ACSS design is the best combo with
ES9018 achieve the best performance .
Upside in the picture is the standard ES9018 headphone amp design.
The bottom is the NFB-27 design .
Click image to enlarge.
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 .
Power supply:
NFB-27 applies the excellent
ES9018 with
excellent analog
output amps, 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 normal 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 or amp stages, but with plenteous
dedicated DC supply circuits.
The
NFB-27 uses
11 groups of high-quality class A parallel
connection PSU , which can
offer very clear DC power for the DAC and amps, producing
a very black background and neutral sound characteristic. The
class A PSU is very important for digital parts. Some designs
may have very low jitter in theory, but if the power is supplied by
a dirty or low-speed power supply, it will increase the jitter and
pollute the data, and degrade the sound quality obviously.
The class A parallel connection PSU has very high input
impedance to avoid pulse through the PSU from affecting the DAC and
low output impedance with very fast speed and high linearity, so it
is a very clean power supply. In my experience, its sound is better
than battery power supply, better human sound and neutral.
The NFB-27 uses three high performance transformers. More than
50,000uf audio grade NOVER capacitors are used to ensure ample and
smooth power feed. A total of 11 groups class A SPUs and 2 groups
linear PSUs are used to purify the power supply. To achieve high
S/N, the control circuit is powered with separate regulator.
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-27 , 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-27 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-27
is applied a four channels digital control relays based volume
control. .NFB-27 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-27 have 100
steps volume control, control range achieve 90DB (usually the
steps volume ports, potentiometer volume port, relays base
volume control all have around 70DB range ), very smooth
control range on first 50 steps , cooperate with 13DB high gain,
so can very
easy adjust the volume for both high / low sensitivity
headphone and speakers.
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.
NFB-27 Supper Exponential volume characteristic
The Volume steps comparison to normal volume knob
Shield:
NFB-27 has complete separation of left, right
amplifier channels, power supply, and transformer.
NFB-27 uses heavyweight all-aluminum alloy chassis to prevent the
eddy current of transformers from interfering on circuit board.
NFB-27 applies optical couple isolation for the MCU and the
control implementations to prevent the MCU affecting the audio parts.
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 . |
Summarize
