Model S DAC :: Technical Information
Inputs
The Model S has two true 75 Ohm BNC S/PDIF inputs. The inputs are transformer coupled, with advanced termination scheme for proposed characteristic impedance over required bandwidth. Switching between the two inputs is accomplished by adequate 75 Ohm RF relay.
The ground of S/PDIF line can be capacitively coupled or left completely floating with respect to the DAC, by the switch at the back plate. When left floating, differential input network forms effectively symmetrical input, thus achieving a highest possible common mode rejection of unbalanced line.
D/A Chip Classic
Just as its predecessor, the Model S employs Philips TDA1541A D/A converter chip, introduced two decades ago, but still with ultimate performance among multibit D/A chips.
In addition to that, the Model S employs improved clocking scheme for TDA1541A internal Dynamic Element Matching circuit.
Non Oversampling
The Model S doesn't use an oversampling, and it especially doesn't use an "upsampling" (sample rate conversion) of any kind. The things with such a signal processing indeed have improved over the years, however it is still not possible to equal naturalness, ability to convey musical content and ability to make a long term listening enjoyable, of well implemented non oversampling device.
No Feedback
The output circuits of the Model S run in healthy class A and don't employ a global feedback. A current to voltage conversion is performed by the hybrid circuit which uses both integrated circuit and discrete parts.
So, no opamps here, but also no discrete transistors just to claim "discrete output stage", essentially using the same old opamp topology. Frankly, as long as one settles on the opamp architecture, a monolithic opamp is not bad choice at all, simply because there are excellent monolithic opamps nowadays.
A high signal slopes, as those coming out of the D/A chips, however push the opamps to work beyond the limits of proper feedback operation. That is why important sonic improvements can be achieved once the feedback is abandoned in favor of adequate no feedback circuit; natural scale and dynamics, and exceptional transparency and soundstage are the first descriptions that come to mind. Abandoning feedback, of course, additional effort was needed to achieve static performance specifications required by the modern criteria. With 0.002% THD, the Model S sets new standard in this regard. Please note that this figure includes multibit DAC, zero feedback output stage, and transformer at signal path.
Of course, the active parts used must be able to follow high signal slopes. The Model S uses a high bandwidth active parts, and current to voltage converter is a common base circuit, which in itself brings a highest possible bandwidth. The output buffer, with bandwidth well above 100 MHz and slew rate higher than 1000V/us also doesn't appear a bottle neck.
The output buffer is capable of driving any line load one can think about, and its internal output impedance is 3 Ohm.
Power Supply & Layout
The Model S employs probably the most serious power supply on the audio market.
First, the Model S comprises isolating mains transformer feeding the main power transformer by two bifilarly wound half-voltage secondaries ("balanced AC power"). Both isolating and main mains transformer are hugely oversized toroidal units (80VA each), and both comprise internal electrostatic shield between the primary and secondary windings, as well as the outer electromagnetic screen. The audio circuits of the Model S are fed by nine secondary windings so supplies of individual sections are kept completely separated. (There is also tenth winding that feeds the input switching relay.)
Schottky diodes are used for their lack of recovery overshoot. Voltages are then filtered by three stage common mode RC filters. Consequently, each stage of the unit has its own dedicated regulator and these are completely discrete, low noise, wideband regulators, and none employs a feedback. As a result, the power supply lines are quiet with practically eliminated crosstalk between particular sections.
Such a supply scheme made possible to design layout without compromises, where not only supply sections are kept separated, but different signal paths are kept separated too, as they are kept separated from supply paths. In addition to that, all the signals and their return paths are made short, making loops physically as small as possible. No star ground approach is applied, since it can't address requirements set by high speed circuits. The PCB is double sided, with ground fill on top side used mostly for shielding purposes; so, the ground fill is not used for routing.
High Quality Parts
The Model S uses high quality parts like Schottky diodes, Black Gate series N and NX Hi-Q, and Panasonic FC and FM capacitors for supply, Panasonic ECHU (polyphenylene sulphide) capacitors for TDA1541A active divider decoupling, and polystyrene capacitors for HF filtering, Allen Bradley resistors, Toshiba low noise audio grade transistors, Jensen signal transformers, Neutrik connectors etc. Everything is housed within anodized solid aluminum chassis, with all the markings engraved. The net weight of the unit is about 9kg.
Transformer or Directly Coupled RCA Output?
The outputs of the Model S are normally transformer coupled. Audial uses high quality signal transformers made by Jensen, and these are the Jensen's flagship units JT-11-BMCF, a vanishingly low distortion, high bandwidth units.
Still, on request, an RCA output can be directly or capacitively coupled instead.
As said, the output impedance of the Model S output buffer is 3 Ohm and that is also the output impedance of directly coupled output of the Model S. Transformer as a coupling device adds about 80 Ohms to this. Thus, in some environments where lowest possible output impedance is needed, a direct coupling can be preferred.
Customers who don't need balanced output can request two single end outputs, one directly and the other transformer coupled.
Directly coupled output exposes negligible offset (up to several milli Volts), however please note that such an output is not DC protected.