NAIM NDX - User Manual - page 8
Copyright Naim Audio 2010
Page 8 of 8
Analogue stage filter capacitors that work in the audible
band are prone to microphony, so susceptible components
are lifted from being in direct contact with the board to
minimise this effect.
PCB design for digital circuits is as critical to performance
as it is for a high-end power amplifier – possibly more
so – and different techniques are required for digital
and analogue board design. There are high-speed
DSPs, high-resolution digital circuits and low noise
analogue circuits present, all of which require different
design approaches. In the NDX the DSPs are placed
well within the boundaries of a six-layer PCB. Multiple
power pins are decoupled using multiple decoupling
capacitors mounted on the bottom of the board, and the
DSP also relies on inter-plane capacitance to provide
low inductance decoupling. External peripheral devices
such as the SDRAM are placed as close as possible to
the DSP ports to minimise the loop area of high speed
circulating currents. All high speed traces are measured
and resistively damped accordingly to ensure that digital
signal edges are not subject to ringing since ringing
radiates high frequency electromagnetic energy that
could affect the analogue circuits.
The master clock drivers and timing gate are decoupled
differently to slow-speed PCB design. For best
performance the decoupling capacitors are placed
topside to avoid using the inductive PCB vias. Decoupling
traces dominate the topside, being as wide and short as
possible since every millimetre of PCB trace length has
a significant effect when clock signals are at 24MHz with
nanosecond rise times. All high frequency traces are
sandwiched on the central layers of the PCB where they
are shielded by the ground and power planes that create
the high frequency return current paths. Outer layers are
used for low frequency signal and clock decoupling.
In the analogue circuits, different techniques are
required again. Here large planed areas of conductor are
detrimental to sound quality and are replaced with the star
ground techniques that can be found in all Naim analogue
products.
Gapless playback
The NDX supports gapless audio playback with both
compressed and uncompressed audio formats. In this
mode there is no two-second gap between tracks, which
can be so annoying when listening to a live recording or a
classical music recording where the microphones remain
‘open’ between movements.
Digital output
A single S/PDIF output is provided on the NDX via a BNC
connector, chosen for best sound quality and impedance
matching. The S/PDIF output is both buffered and
isolated, the isolation being achieved using a high quality
pulse transformer. The buffer, which is used to drive the
S/PDIF currents through the pulse transformer to the
digital receiver, isolates the DSP from the high transient
currents required for S/PDIF output. As the S/PDIF signal
is RAM buffered in the same way as the digital data going
to the internal DAC chip, the same level of jitter removal is
applied whether the output is analogue or digital. Digital
output is switchable and is should be disabled when the
analogue output is being used.