If you
own a Cambridge Audio 640C and want to upgrade then you are going to need to
spend in the region of £800 - £1000 on a new CD player to hear any marked
improvement. Audiocom have developed two levels of upgrades which drastically
improve upon the 640C and surpassing stock players to at least £1K (level-2).
Here is how. The 640C is considered budget or ‘entry level’ in the Hi-Fi market
but in fact uses one of the very best DAC’s available, the Wolfson WM8740. The
WM8740 is the choice by Arcam in their top of the range FMJ CD36 player.
Cambridge Audio have scaled down the performance of the 640C to meet the
‘budget’ market, and used cheap, lower grade parts for the WM8740 ‘s
associated audio circuits to place it below their costlier 840C. With the stock 640C the WM8740’s performance is
understated, performance held back and you do not hear anywhere near the
potential of these 24-bit player.
The
Audiocom level-1 upgrades install an active analogue stage using superior
op-amps to those in the stock 840C, add an ultra-low jitter reference master
clock, improve component level quality for the power supplies, decoupling and
signal coupling. Level-2 advances performance by upgrading the audio board DC regulated
power supplies and upgrading the power supplies and regulation of the Precision
Servo 2.0.
24-Bit DAC Magic
Like other Cambridge Audio products gone before it, the 640C’s excellent
circuit topology means serious improvements can be unlocked. With a justifiable
and practical expenditure the 640C V2 loses its foibles and becomes a
thoroughly capable and refined CD player.
The 640C is divided in to two main PCB’s, the servo PCB (digital) and
main PCB (digital & analogue), it is the main PCB which is the focus of the
Level-1 upgrades. Containing main power supplies, regulation, master clock, DAC
& analogue output stage this PCB is certainly where most of the action
happens.
First off we replace the ‘cooking grade’ capacitors used for power
supply reservoir (smoothing AC) for Panasonic FC type. The FC series although not
selected as “audio grade” outperform several types of electrolytic pitched at
the audio market. The main PCB contains several fixed voltage regulators for
the +/- 15V analogue supply, +5V DAC supplies. High grade bypassing is
essential to reduce noise and impedance on the power supply lines; here more
general purpose capacitors are upgraded to Panasonic FC & Rubycon ZA type.
The
master clock in the 640C hides underneath a screening can and is labelled ‘low
jitter’. Beneath the can we find a TCXO which is powered by a 7805 regulator.
The output of the TCXO is buffered by a 14-pin CMOS inverter and the outputs
from this sent directly to the Wolfson WM8740 DAC’s.
This arrangement has several drawbacks, the limited power supply rejection of
the 7805, output noise of the 7805, CMOS gate oscillator in the TCXO has poor
signal to noise ratio; the selected crystals inside the TCXO have low Q and a
used in parallel mode, a method which is highly susceptible to digital noise
injection. To attain the highest level of sound quality from these circuits the
OEM clock is removed and replaced with the Superclock 4-S ultra-low jitter
master clock.
The post DAC filter in the 640C is a little unusual, it uses what is
called a ‘double balanced’ filter rather than the normal low-pass filter. A
total of 6x NE5532
operational amplifiers follow the Wolfson WM8740 for filtering and buffering.
The 5532 is an ordinary consumer grade op-amp, a great workhorse in its day but
it can be significantly better by modern op-amps. Here we use a mix a top spec
op-amps costing 20x the price of the 5532 to wring the most out of the WM8740
DAC’s. The superb LME49720 are enrolled for the filter stage, then AD825 on
Brown Dog adapters for output signal buffering. The AD825’s we found to be
superior to the LM4562 or LME49720 for signal buffering.
The
concept of low noise power is fully realised with the level-2 upgrade. The 4
voltage regulators which populate the audio board for digital DAC supply,
analogue DAC supply are all upgraded. The OEM fitted 78 & series
regulators have poor noise performance and are not very linear. It is vital for any digital-to-analogue
converter to be powered from a low noise supply, noise + signal on the DAC input
= signal + errors on the output. We upgrade the digital supply
for the left & right channel DAC’s with a pair of specially selected low
noise regulators. For the DAC analogue supplies we use a pair of the
Super-regulators 3’s, this is a discretely designed regulator with ultra-low
noise and wide bandwidth capabilities.
Cambridge
Audio 640C Level-1 Upgrade
1x Superclock 4-S ultra-low jitter master clock.
2x Panasonic FC analogue power supply capacitors.
2x Panasonic FC analogue regulator bypass capacitors.
4x Rubycon ZA capacitors for WM8740 DAC decoupling.
6x Beryllium Copper op-amps sockets.
4x LME49720 op-amps for differential filter stage.
2x AD825 modules for output signal buffer stage.
Sontech SFRT-AL resonance control.
WBT 4% Silver solder used throughout.
Cambridge
Audio 640C Level-2 Upgrade
1x Superclock 4-S ultra-low jitter master clock.
2x Panasonic FC analogue power supply capacitors.
2x Panasonic FC analogue regulator bypass capacitors.
2x Specially selected regulators for WM8740 DAC analogue supplies.
2x Specially selected regulators for WM8740 DAC digital supplies.
4x Rubycon ZA capacitors for WM8740 DAC decoupling.
6x Beryllium Copper op-amps sockets.
4x LME49720 op-amps for differential filter stage.
2x AD825 modules for output signal buffer stage.
Sontech SFRT-AL resonance control.
WBT 4% Silver solder used throughout.
