R2R | DIGITAL TO AUDIO CONVERTER

The Kassandra Signature Limited Edition is our highest level in the Kassandra Series and completely redefines the status of digital replay

“We believe the Kassandra DAC is currently one of the best audio sources available today and can only be matched by top analogue sources in timbre and musicality, but keeping well ahead in aspects of dynamic presentation and resolution. While in the process of setting up and building the company, at the earliest stages, the need of a true SOTA source was mandatory. You cannot claim to build speakers that aim to be the absolute best, top amplifier designs and so on, if you do not have a true reference source. So the Kassandra was born…”

DESIGNER

The Kassandra Signature Limited Edition - a design with no limits or boundaries, a statement on what is possible in digital to analogue replay.

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Designing the Kassandra

Our approach in designing the Kassandra Signature Limited Edition, was to build on our already SOTA performing Kassandra II Ref, and push the boundaries further, much further. Adopting a 2 chassis design approach along the lines of our amplifiers in the Limited Edition range allowed extra freedom and space. We employed many techniques that proved so successful in our Impera Limited Edition pre-ampliifer, particularly in the power supply unit.

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The Kassandra Signature Limited Edition is a ladder DAC. Typically a ladder DAC uses a resistor network switched by a N number of switches, N being the bit depth. It is a passive sort of speak procedure with no manipulation of the incoming data stream.

It also has no digital filter, and completely avoids the negative sonic aspects created by fitting the common Delta-Sigma or Multibit architecture.

In the case of the Kassandra Signature Limited Edition we choose to use an R2R IC – the classic Analogue Devices AD1865N-K, a much praised chip which we believe is the best sounding IC ever made. No less than 24 R2R converters per channel. The AD1865N-K has a very straight forward data handling logic, and does not process the data stream in any way. This gave us the freedom to fully exploit the IC in our converter system.

The input data is latched and directly refreshes the resistor network, with absolutely no additional complex logic or data handling/processing. This way we have full control over the resistor network to use it as part of our converter system. Our Super Clock circuit directly re-clocks and drives the “refresh” signal, so absolutely no additional jitter is induced in the conversion.

There are no less than 24 R2R converters per channel in the Kassandra Signature Limited Edition working as parallel switched resistor ladder converters, cancelling the deviation of resistor values down to Zero. Paralleling R2R converters improves linearity and noise figures are improved on the same principles, along with the dynamic range and channel separation. Sonic wise, paralleling converters elevate performance to another level in all aspects significant in high end audio.

Jitter

In the Kassandra Signature Limited Edition we use many techniques regarding lowering and cancelling jitter, in both passive and active ways. Bouncing signals, overshoots, low rise times etc greatly increase jitter. We solve this by using special driving circuits and carefully tuned digital line terminations for all digital signals.

we use 35 LC filters implemented using RF chokes and high speed capacitors to decouple digital circuits and ICs from the power rails. These are extremely effective in cancelling out any form of PSU sourced jitter and solve this fairly complex problem. Jitter originated in USB and S/PDIF / Toslink sources are eliminated right at the “refresh” signal of the converters.

Having an IC that has no complex data handling logic and no multi stage data logic, the precisely timed signal resolves in a extremely accurate jitter free conversion. In our opinion, this is not possible with any other type of converter system.

The Super Clock is also used upstream for re-clocking the XMOS asynchronous USB controller, as well as the SPDIF receiver. With the internal clock, there is no need for a word clock input.

Jitter present at the word clock of the transport before S/PDIF transmission to the dac. Almost identical Jitter measurement was also present at the word clock using a USB transport.

Jitter distribution measurement of the word clock synchonising the 16 converters of the Kassandra DAC with the internal reclocker enabled, and using the same USB transport.

Jitter distribution after double reclocking of the word clock.

I/V Stage

The R2R ICs are current output devices. Their impedance is very high and close to an ideal current source. To drive the next stage in the Kassandra, the current must be converted to voltage. Avoiding using sonic degrading Opamps or a simple resistor, we use a specially designed transformer that converts the current differential to a single ended voltage output. The specially designed transformer is an ideal solution, and is second to none in direct comparison to the many alternative I/V methods available.

Gain Stage

Using technology from our world class tube amplifier and preamplifiers, the analog stage in the Kassandra Ref II is a small Single Ended tube amplifier. It is consists of a transformer loaded tube, driven directly by the I/V transformer, and biased with ultra low noise power supplies. The tube used is the best sounding tube from the super tube family of tubes – the E280F. It is triode connected and it’s specific parameters like transconductance, plate impedance and especially linearity and noise are levels above any other small signal tube.

The triode is loaded with a special quality large core step down transformer. The tube power supply is an oversized choke regulated supply, making sure that the tube is up to the task of following the converter’s flashing dynamics. Furthermore, the step-down transformer reduces the output impedance to 40 ohms, with a maximum swing @ full scale of 30V pp sine wave (10V rms).

 

ARIES CERAT SYSTEM: KASSANDRA SIGNATURE LIMITED EDITION DACIMPERA LIMITED EDITION PREAMPLIFIER, CONCERO 65 LIMITED EDITION POWER AMPLIIFERS

THE DEVELOPMENT OF THE KASSANDRA SERIES

DISCOVER
Technical Specifications

DIGITAL SECTION

IC Chip Array:

24 R2R converters per channel, complimentary current output using the top grade Analogue Devices AD1865N-K  with 12 converters per bank, 24 per channel, 48 in total

Current to Voltage Conversion:

Transformer I/V conversion with custom wideband transformers, balanced current to  single ended voltage conversion.

Additional Clock:

Internal Super-Clock by-passable on the fly,  triple regulated supply

Regulators:

Eight discrete ultra-low-noise regulators for the 4 converter banks

POWER SUPPLY SECTION

PSU:

Massively over engineered power supplies, power input filters

PSU filering:

Extensive local decoupling,using tuned LC filters, 76 in number

Mains Transfomers:

5 torroidal transformers,over 3 Farad total system capacitance, wideband local decoupling

Pseudo-battery system:

Pseudo-battery system for isolating all analog stages from the power grid

Tube Regulation:

Xenon gas rectifiers for the tube PSUs

INPUT & OUTPUT SECTION

Gain Stage:

Transformer loaded super tube output stage using the E280F tube.    5:1/10:1 step down transformer & double choke filtered supply

Gain Stage Capacitors:

Super-capacitors used in tube analog stage. 100 times less ESR/ESL than any film or electrolytic capacitor

Output Range:

RCA: 30Vpp output @ 0db 10Vrms (5Vrms on -6db setting)
Balanced output: standard

User Selectable Output:

10v @50ohms or 5v @12ohms

Output:

2 x RCA, 2 x Balanced. True Balanced output standard.

Floating RCA output switchable (ground loop resolver)

Output Impedance:

50 ohms (balanced and SE output), 12 ohms at -6db setting

USB input:

Data rates up to 24/384KHz

Jitter attenuation:

Down to femtosec level

Dimensions:

540mm W x 530mm D x 165mm H

Weight:

120kg total