On How We Make Digital Sound like Music
Audio Note pioneered a fundamentally different approach to D/A conversion than that taken by the rest of the industry, rejecting oversampling and digital filtering in favour of an architecture that takes the native data stream, converts it to analogue, then passes it through a simple filter.
Our DA Converters use no over sampling, no jitter reduction, no noise shaping and no re-clocking.
Using the Audio Note DAC strategy calls for a Resistor Ladder DAC architecture as opposed to the more common Delta-Sigma computer processing style DAC. Delta-Sigma DACs and R-2R resistor ladder DACs are quite different. Rather than using feedback loops and high frequency oversampling clocks to produce an 'approximation' of the digital data as Delta-Sigma types, an R-2R DAC will turn on each required bit in the digital 'word' and the exact amount of current will flow from the chip to reconstruct the analog waveform. The method is perfect in its simplicity and is reflected in the sound quality.
Having removed all the digital filtering that is part of the over sampling, all filtering in our DAC's is done in the analogue domain, where it appears to be easier to retain good wide band phase-frequency and dynamically coherent behaviour than in the digital domain.
The signal from the CD-Transport enters the famous Crystal CS8414 receiver chip via a toroidal input transformer, which greatly enhances performance of the digital interface.
Less is more
Note how simple our digital board looks when compared to the elaborate computer like PCB's we see in today's DAC’s. Apart from the DAC0.1x all our DAC’s have similar digital boards whereby our higher level DAC’s are treated with increasing quality parts for decoupling and filtering.
Extensive research into the fundamental properties of the data stream itself have shown, beyond doubt, that regardless of the theoretical and measurable advantages of the signal manipulation employed in all currently available digital products, such as higher over sampling, noise shaping, re-clocking or jitter reduction, will greatly interfere with the critical time domain requirements of the signal.
Current theory is based on an assumption that music is similar to book keeping data, which off course it is not. So, at Audio Note we take the digital signal out of the digital-domain as quickly as possible.
All our DAC’s, except for the DAC0.1x and the DAC build in the Cobra, use the revered Analogue Devices AD1865 R-2R ladder DAC chip. Insiders know that this must be the 'best' sounding DAC chip ever designed, however production of this chip was stopped a long time ago.
Delta Sigma vs R-2R
So why did manufacturers move away from R-2R chips that they developed in the 90’s? Well basically it is an expensive process to create integrated circuits with laser trimmed precision resistors to correctly implement the R-2R circuit. Its much easier to throw a few million gates at the problem!
The Analog Devices AD1865 chip used in the Audio Note DAC’s is the highest bit resolution DAC chip that was made with 18 bits and provides 108db of potential dynamic range. So basically when you put a 24/96 signal through an Audio Note DAC you have a potential dynamic range capability of 108db which is approaching twice the dynamic range ever recorded.
If you are truly concerned with the very best of 2 channel audio the R-2R DAC’s will clearly outshine the computer processing Delta-Sigma (DSD) DAC's for obvious reasons. If you have a high end audio system such as a Single Ended Tube 300B or 2A3 amp or possibly CLASS A push Pull tube amplification, transformer coupled pre-amplifiers, high efficiency speakers, it’s the only way to go.
Listen to a saxophone with an R-2R DAC and you are going to pick up all the subtle nuances associated with this instrument – the attack, the timbre, the decay, these subtle nuances that immediately allow the brain to identify the instrument, are absolutely critical to an audiophile.
Audio Note recognised the qualities of the AD1865 long ago, and realised that the upcoming Delta-Sigma DAC's were a step backwards. As a result, Audio Note UK bought the remaining stock (which was quite sizeable), which enables us to continue supply.
From the DAC chip, the signal is feed into specially designed I/V transformer, to maximise the energy transfer during the Current-to-Voltage phase of the conversion, resulting in increased dynamics.
The Analogue Domain
From the I/V transformers, the signal is fed to a tubed series stage of which the topology is depended on the level/version. Our Balanced DAC’s have output transformers, using 33:1 step down ratio, improving bandwidth, and dynamic signal transfer whilst also reducing hum and RF interference.
In the case of the DAC3.1 Balanced for example the copper wired output transformers use the new IHiB core material in a double C-core configuration.
Note the copper shrouds on the output transformers, a small detail perhaps, but an important one and not just cosmetically, commercially available transformer frames and shrouds are all made from steel, which is magnetic and when an air gapped transformer is assembled with a magnetic frame or shroud, its performance is adversely affected, because the air gap is shorted by the magnetic currents flowing in the frame or shroud and a loss of low level detail is evident.
We therefore decided to move over to copper frames and shrouds, as they do not affect performance.
Every amplification stage is no better than its power supply, so extreme care has been taken here in any of DAC’s. Some versions even use tube rectification for both the rectification tubes and output tubes.
In the DAC3.1x Balanced for example, we see the thoughtful copper shielding of the power transformer and the use of Audio Note KAISEI capacitors and even an Audio Note Copper foil capacitors in the power supply which we discussed in the former episodes of this series.
More background about how our topology came into existence and how we proceed to make digital source sound like music you can read in these articles by Jeff Day of Positive feedback.
Peter Qvortrup on How to make Digital Sound Like Music part 1
Peter Qvortrup on How to make Digital Sound like Music part 2