Copyright Naim Audio 2010
Page 7 of 8
VCXOs are a good solution too but since a VCXO’s output
frequency is controlled by a voltage, if noise is present on
the control line then the VCXO’s output will exhibit phase
noise. Normally the control line to the VCXO is feed via a
PLL which means that it is constantly changing to match
the incoming clock rate.
The NDX approaches the problem of matching its clock
frequency to the source in a quite different way. Its master
oscillator offers ten switchable fixed frequencies which are
selected so as to keep the average clock frequency the
same as the source’s. The master clock in the NDX is a
VCXO controlled by two 9-bit DACs connected to its control
line to give 10-bit resolution (1024 discrete steps). This
method was chosen to give greater resolution, ie smaller
step changes, in the master clock frequency. The 9-bit
DACs are controlled by the DSP in software so the rate at
which the master clock is altered can also be controlled.
This is important because if the master clock is changed too
quickly this can cause audible artefacts, ie introduce jitter.
The SHARC DSP monitors the rate at which the RAM buffer
is either filling or emptying and changes the clock frequency
only if the buffer is going to either overflow or underflow. This
way the incoming clock jitter is completely isolated from the
NDX master clock. When the system has settled, it will only
modify the master clock frequency every 10 to 15 minutes.
DAC
The DAC chip in the NDX is a Burr-Brown PCM1791A used
in external oversampling mode. This is the same Delta-
Sigma DAC used in other high quality Naim products but
in the NDX we switch off the internal oversampling and
digital filter. The oversampling and digital filtering is instead
done externally by the Naim-written code in the DSP. At
44.1kHz and 48kHz sample rate the oversampling factor
is 16x, which runs the DAC section of the PCM1791A at its
maximum sample rate of 768kHz. The internal Delta-Sigma
modulator in the DAC oversamples the incoming data by a
further factor of four, so the effective oversampling is 64x.
Analogue output filter
The DAC’s output signal has to be filtered to remove the
remaining unwanted frequency components using an
analogue low-pass filter. In the NDX this is multi-stage seven-
pole design using Burr-Brown OPA42 op-amps; as these are
single-chip devices a total of six are required. This op-amp
was chosen from the plethora of available audio op-amps
after many hours of listening tests as having the finest sound
quality. A combination of Sallen-Key and multiple feedback
low-pass filters are used to implement the seven-pole filter.
The highest quality through-hole components are used
in the signal path rather than surface-mount equivalents.
They are tested and matched into tight tolerance groups to
ensure correct frequency response.
Low noise power supply
Reducing power supply (PSU) noise has long been part
of Naim’s design philosophy. To increase perceived and
measured dynamic range, PSU noise in the NDX has
been reduced to an extremely low level. The NDX can be
powered either from its internal PSU or, as an upgrade,
from either the XPS or NS 555PS external PSUs.
The internal PSU begins with a custom-designed toroidal
transformer. Toroidal transformers have very low magnetic
leakage, which ensures that electromagnetically induced
mains noise is low. The NDX transformer has four isolated
secondary windings, feeding four sets of rectifiers and
reservoir capacitors:
1) Digital (front panel; ARM9 microcontroller; Wi-Fi
module; streamer module; optional DAB/FM module)
2) DSP (SHARC DSP; RAM buffer; local DSP clock)
3) Audio clock (master clocks; master clock control
circuits; re-clocking gate; digital section of the DAC)
4) Audio filters (differential to line level; first-stage
analogue filter; second-stage analogue filter;
output relays)
Reservoir capacitors are larger than those typically used,
in order to reduce the unregulated voltage noise and
provide increased short-term current capability. The four
separate PSUs form part of the electrical isolation of the
digital circuits from the DAC chips and analogue circuits.
Low noise LM317/337 regulators smooth the unregulated
voltage from the reservoir capacitors. Voltage supplies to
many of the digital circuits are double- and in some cases
triple-regulated to reduce noise still further.
External PSU upgrade
When the PSU upgrade option is used with the NDX,
power supply separation is maintained as the external
power supply also has independent power supplies
and ground connections. In addition it provides a larger
toroidal transformer and bigger reservoir capacitors to
reduce noise still further. Outputs of the external PSU are
regulated using selected low noise LM317/337 regulators
to achieve a lower noise floor ahead of the internal voltage
regulation.
Other sonic influences
As with all Naim designs, the influence of vibration –
induced microphonic noise – has been minimised in
the NDX. The starting point is a rigid chassis made from
high-grade aluminium. Once assembled in its aluminium
sleeve the NDX has a 7.5mm-thick base, giving the
electronics an extremely rigid foundation. To minimise
the transmission of vibration to the FR4 fibreglass printed
circuit boards, the PCBs are screwed to the chassis only
at carefully selected points.