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STATE-OF-THE-ART
The Bryston BDA-1 is a state-of-the-art external Stereo DAC (digital
to analog converter) using fully discrete Class-A proprietary Bryston
analog circuits, two independent linear power supplies and dual
Crystal CS-4398 DAC chips. The BDA-1 features an impressive array
of inputs for USB, COAX, OPTICAL, AES-EBU and BNC equipped digital
devices. For audio outputs, the BDA-1 offers both balanced XLR as
well as unbalanced RCA stereo connectors on the rear panel. The
BDA-1 is RS-232 software upgradeable, making it the most flexible
high performance DAC on the market.
JITTER REDUCTION
Jitter is a mistiming of data being moved from point A to point
B in any synchronous digital system. Think of jitter as individual
ticks on a clock—however each tick is not occurring at exact
one-second intervals. Some are slightly less than a second and some
are slightly longer, and they average out so that no actual time
is being gained or lost over a large number of seconds. Jitter is
the difference between the shortest and the longest second, and
in digital audio systems this specification is usually measured
in nanoseconds. Both the frequency and the jitter characteristics
of the system’s digital clock will affect the accuracy of reproduction.
The frequency, if not accurate, can cause the pitch and speed of
the music to change, and in some systems cause drop-outs if there
is no data available.
THE BRYSTON SOLUTION
Bryston delivers superb sonic performance by re-sampling and re-clocking
the digital input in order to reduce jitter. The result, a significant
reduction in jitter (1/1000 of a nanosecond). But it isn’t
enough to just get the bits right; those bits have to be converted
back into music with the same timing reference as when the music
was first digitized. The input signal of the BDA-1 is re-clocked
and re-sampled to reduce any possibility of jitter affecting the
sound quality. Even the input receiver and the sample rate converter
serve to further reduce jitter.
IMPEDANCE MATCHING TRANSFORMERS
The best way to understand the Bryston BDA-1 External DAC is to
follow the flow of a signal from when it first arrives at the BDA-1
in digital form to when it leaves to drive an external analog input.
The digital signal first arrives at the BDA-1 via either the SPDIF
COAX, OPTICAL OR AES EBU inputs or the USB interface input. These
are the standard digital outputs from a CD Drive, Sound Card, Computer,
Music Server etc. There are 6 digital inputs, which are easily selected
using a front panel switch. This digital signal contains data at
over 1 million bits per second that requires a bandwidth of 5 to
10 million hertz (cycles per second). At these high frequencies,
it is very important to maintain the quality of the signal by having
the correct termination at the digital inputs. The BDA-1 provides
for this termination in the best possible manner using devices called
impedance matching transformers. Impedance matching transformers
provide the optimal interface to the incoming source under all sorts
of signal conditions. Lesser quality terminations will degrade the
signal, causing increased jitter.
UPSAMPLING
After the input stage, the signal goes to the SYNCHRONOUS upsampling
circuit (sample rate converter). This circuit converts the digital
signal from one sample rate and bit depth to another. In the BDA-1,
the sample rate is increased from the input sample frequency (32K,
48K, or 96K upsamples to 192K and 44.1K or 88.2K upsamples to 176.4K).
The 16 bits of depth (the CD standard) is increased to 24 bits.
The added 8 bits are filled with placeholder information. This upsampling
process provides a digital signal for later conversion to analog
by the Crystal 4398 DAC chip. The upsampling process doesn’t
add any new, but does put the data in a form which can better be
translated by the DAC as described below. The advantage of this
synchronous upsampling process is improved processing of the upsampled
signal by the DAC chip, which was designed for higher sample rates
and bit depths. There is also a noise shaping process implemented
where “noise” within the audible spectrum is shifted up
to frequencies above audible limits. An added advantage of this
upsampling process is that a totally new clock signal is applied,
which results in significant jitter reduction.
OPTIONAL UPSAMPLING FEATURE
A very unique feature of the Bryston BDA-1 External DAC is the
ability to disengage the upsampling feature. You can compare an
upsampled signal with a non-upsampled signal simply by engaging
a switch on the front panel. This feature is functional when using
sample rates of 44.1K, 88.2K, 48K, and 96K.
BRYSTON DAC
The DAC integrated circuit (chip) provides the conversion of the
digital signal to the analog domain. The two independent DAC chip’s
used in the BDA-1 are the Crystal CS-4398. Due to the requirements
of the conversion process, every DAC chip employs a digital filter
to the signal in the digital domain and an analog filter after the
conversion process has been applied. Without this upsampling technology,
these filters would likely effect frequencies at or near the audible
range, accompanied by unwanted level and phase changes. The CS-4398
is a hybrid multi-bit delta-sigma DAC. This is an advanced generation
chip, which uses several methods to optimize the conversion process.
This DAC uses a process similar to the previously detailed upsampling
process where it oversamples the incoming signal. The CS-4398 operates
in one of three oversampling modes based on the input sample rate.
Single-speed mode supports input sample rates up to 50 kHz and uses
a 128x oversampling ratio. Double-speed mode supports input sample
rates up to 100 kHz and uses an oversampling ratio of 64x. Quad-speed
mode supports input sample rates up to 200 kHz and uses an oversampling
ratio of 32x. This again allows for filtering that is safely out
of the audible range. The output of this process is a sensitive
analog signal. The timing of this process must be very closely controlled
by a low-jitter clock.
POWER SUPPLY
The stability of power in any audiophile equipment is imperative
to superb performance. The BDA-1 uses two independent power transformers
for the initial stages of filtering and regulation. In the BDA-1,
each stage in the digital chain (input receiver > sample rate
converter > digital audio converter) is independently regulated
to prevent any interactions and to provide a rock solid supply of
power for any up-sampling/over-sampling process. The Crystal DAC
chip also requires a very clean digital power supply if it is to
function at its optimum level. Noise on the digital supply could
cause added jitter and various forms of distortion. Incorrect circuit
board routing of the digital power supply or related ground may
also introduce digital noise into the analog circuits. The BDA-1’s
digital power supply is provided from a separate closely regulated
and filtered source. The DAC also requires a high quality analog
power source. The analog signal is at its lowest magnitude within
the DAC and as it exits the chip, so any induced noise or distortion
will be greatly amplified by circuitry upstream from the DAC. Bryston
engineers have employed a separate, heavily regulated and filtered
power supply with carefully routed grounds that is critical to the
superior sound of BDA-1. Careful trace routing eliminates the risk
of noise via capacitive coupling and provides the extra dB’s
of noise and distortion reduction which separate truly great audio
equipment.
DISCRETE CLASS-A ANALOG STAGE
The most critical part of the circuit design in the Bryston BDA-1
is the DAC’s analog outputs—connected directly to a pair
of proprietary Bryston Class A discrete operational amplifiers rather
than the typical IC chips employed in most other products. These
exotic amplifiers make a huge difference in transparency, resolution
and dynamic performance. The use of discrete devices allows the
design of a circuit that exactly matches the needs of the DAC, whereas
the use of general-purpose integrated circuits always involves compromises.
Discrete devices allow engineers to generate more output power since
the heat from the output driver transistors is physically separated
from other sensitive components. Discrete devices also allow specific
matching of important characteristics such as input and output impedances
based on the specific in-circuit requirements. Discrete operational
amplifiers can also be designed to more closely match their power
source leading to additional reduced distortion and noise. Bryston
consistently utilizes discrete devices in our product designs because
our circuits are engineered to require closely matched devices for
optimum performance. Bryston does ultra-fine sorting and grading
of discrete devices, which leads to superior sound quality.
HAND ASSEMBLED WITH CARE
Bryston hand assembles and individually tests each and every product
we manufacture. We exclusively use only the finest components, 1%
metal-film resistors, 0.1% polystyrene capacitors, and hand selected
and matched transistors in order to reduce noise and distortion
to the absolute minimum. Bryston applies techniques and employs
custom materials in our everyday construction of electronic equipment
that are typically utilized by military and aerospace industries.
Our traditional adherence to the use of proprietary parts, sophisticated
construction, and refined testing techniques guarantees that the
Bryston BDA-1 DAC will perform superbly for many years.
PERFORMANCE WITHOUT COMPROMISE
Bryston has always maintained that the final arbiter of this exercise
is to provide products that are as transparent and accurate to the
original recorded performance. The BDA-1 DAC is the ultimate representation
of this design goal.
SPECIFICATIONS
Frequency response - 20 Hz - 20 KHz -.1dB
Signal to noise – Audio Precision AP2700 analyzer FFT digital
measurement
140 dB unweighted
THD plus noise - .002%
IMD - .002%
Jitter – below the measurement capability of the AP2700 analyzer
Output Level - 2.3V unbalanced - 4.6V balanced
Shipping weight - 18 Lbs / 8.2 Kg
Dimensions - 17 or 19 w / 11.25 d / 1.75 h inches
43.2 or 48.3 w / 28.6 d / 4.4 h cm
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