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R-7 reviews :
Pros and cons of R-2R DAC :
1.R-2R will not convert the clock signal into the output
2. R-2R is not sensitive to jitter while Delta-Sigma D/A is
much more sensitive to jitter.
3. The output signal is much more precise compared to
Delta-Sigma D/A .
1.THD today is extremely good with Sigma Delta chips; R2R
ladders are good too but not as good.
2. Glitches and accuracy of the ladder resistors are very
difficult to avoid and require complex technology to resolve it.
R-2R basic design in the market:
R-2R DAC is very popular nowadays and available from DIY kits and
all the way up to high-end products.
In the low range DIY market, the R-2R design is often
based on old technology designed a long time ago by MSB and only
includes basic R2R ladder design and do not include the wonderful
correction design of the original MSB technology. This design uses
data shift registers logic chips in series mode to convert the data
to an analog signal. The structural R2R technology issues cannot be
avoided, and performance is solely depending on the accuracy of the
the High-End market, the R2R design is much more complex and reaches
performance. A basic R2R ladder is simply not sufficient enough to
achieve good performance and sound quality! Some manufacturers are
using shift registers design. A less complex and lower performance
design based on traditional logic chips working in serial mode to
correct the ladder.
A far better design switches resistors in parallel mode. An
ultra-fast FPGA controls and corrects the R2R ladder. The parallel
design mode controls every bit respectively and therefore achieve
unprecedented performance. (In parallel mode only 1 clock cycle is
needed to output all data; serial design mode needs at minimum 8 up
to 24 clock cycles) The parallel design is much more complicated.
Once designed properly it can correct every bit of the ladder.
Photo below shows a design with such FPGA, can correct the
unavoidable imperfections of the R2R ladder caused by tolerance of
resistors, glitches to achieve best performance.
Accuracy of the ladder resistors
people believe the tolerance of the resistors in the ladder is most
important to reach best performance. Nowadays 24 bit resolution is
standard. What tolerance is needed to achieve 24 bit resolution?
When we look at 16 bit the tolerance of 1/66536, 0.1%
(1/1000) is far not enough, even a tolerance of 0.01% (1/10000), the
best tolerance available in the world today, still cannot handle 16
bit request correctly; we are not even calculating 24 bit here!
The tolerance of the resistor will never solve
Imperfections of a ladder. This would require resistors with a
tolerance of 0.00001% and can handle 24 bit resolution. This is only
in theory because the discreteness of the switch logic chips have
already too much internal impedance and will destroy the impossible
tolerance of a resistor.
The solution is to correct the ladder and not only
depend on the tolerance of resistors. Itกฆs a combination of both:
Ultra-low tolerance resistors controlled by a correction technology
using very high speed FPGA are applicable in in our design.
Importunacy of the FPGA in the R-7:
stands for Programmable Array Logic.
Nowadays the FPGA is applied in a lot high end grade
DACs; like the popular ROCKNA WAVEDREAM DAC.
The internal hardware design is fully controlled by
complex software. A huge advantage is the fact the software in the
FPGA can easily be upgraded offering new features or improve the
performance. Such design is much flexible and future proof!
R-7 has no built in mute switch to keep the signal
The FPGA is responsible for:
1. High performance SPDIF interface, replacing
traditional SPDIF interface chips like DIR9001, WM8805 or AK411X wich
are lower in performance in comparison to FPGA.
2. Full re-clocking process with FIFO design applicable
on all inputs. This way the output data keeps fully synchronized
with the clock signal to reject any jitter.
3. Built in 2X, 4X and 8X oversampling and digital
filters and on top of this 4 different true NOS (only analog 6dB
filtering) modes. To completely configure it to your liking!
.Fully discrete output stages
The signal last stage is the
analog output stages, which can greatly effect the whole DAC sound
After d/a conversion by the R2R D/A modules the analogue
signal is transported by fully discrete matched-transistor output
DC-coupled design with first class through-hole
components. No SMD transistors are applied. The high speed special
ACSS output stages are non-feedback and current driven design.
Almost all other designs need to convert the signal
multiple times from and to current or voltage, resulting in less
detail and less defined sound stage .
The output buffers are single ended FET. Two stages in
parallel to reach very low output impedance. All output stages are
in pure class A design without any (negative) feedback to achieve
purest and a real live sound reproduction.
The 4 OPA opampฃพs are functioning as DC servo, this way no
coupling-capacitors are needed and DC output is automatically
biased! Resulting in a perfectly neutral sound.
There are no relays or other switches in the signal path
after D/A modules to allow the best and purest sound quality.
Heavy power supplies design:
DAC has 3 high quality low noise, low flux leakage, R-cores
In total 130W power to supply all digital parts and the
left and right analog boards. The DC power is distributed over 23
separate power rails. 20 groups are pure class A low noise regulated
power; fed by 3 groups linear power supplies.
This results in ultra-high speed and ultra-low noise
performance. Clean and independent power for all different parts to
achieve highest quality.
The R-7 has completely separated power supplies .Digital, left and
right analog channel have their own dedicated transformer. All
boards and transformer compartment are separated by 5mm thick
aluminum plates for high isolation degree.
It improves the signal-to-noise ratio, isolate the
harmonic interference and reduce noise, which will make the sonic
background cleaner and blacker.
The Left and Right analog parts are placed symmetric
beside the digital board to keep the wires and distance identical to
ensure equal output performance on both channels .
The R2R DA modules are assembled between two aluminum
boards to avoid any RF interferences.