The Formula DAC is the first in the aqua’s production to show a proprietary D/A conversion system, that we name Optologic. The signals sent to the resistor ladders are generated by a proprietary digital circuit, implemented inside an FPGA (Field Programmable Gate Array). Formula has a sign-magnitude converter structure, and this requires not two but four 24 bit resistors banks, each one made of very low tolerance discrete resistors.
Aqua believes that the more musical D/A conversion system is based on the circuit usually identified as “resistor ladder.” This technology has been substituted in the mass-market by the so-called sigma-delta converters. The sigma-delta converter is a near-completely digital circuit and for this reason it can be can be integrated on silicon and produced at low-cost. A quality resistor ladder DAC instead uses a circuit based on a set of high precision discrete resistors: at least 2 resistor banks, one for each stereo channel.
In order to separate the digital section from the sensitive analog circuits, galvanic and magnetic isolations are implemented between the FPGA and the four branches of the R2R ladder converter. This creates complete isolation of all digital circuitry – including the clock – from the analogue section, for a clean, noiseless ground plane. It also plays the main role in the differential management of the R2R ladders, achieving perfect symmetry between the counter-phase halves. The result is the highest tonal contrast and naturalness.
Pure 1x native sample rate DAC
The Formula, like the other aqua models, is a digital filter-less DAC. Our conviction is that in spite of the instrumental measurements showed by upsampling/oversampling DACs, they rarely are able to produce the same emotional and realistic results.
A third important aspect in the aqua production is the modularity of the various electronic circuits. aqua equipment are not short life products: as modular, they are open to future improvements and to follow the evolution of the technology. Formula is made of a set of circuits every one developed on a separate printed circuit board easily interchangeable. As an example, each of the four resistors banks of the converter is separate and mounted on connectors.
State of the art digital and analog electronics
As explained before, the Formula DAC uses no digital filter or any other compensation, relying instead on maniacal attention to each sub-circuit. The separate analog and digital power supplies are executed purely by extensive use of discrete BJT, Mosfet, J-Fet and ultra-fast diodes. Passive parts were selected not just for the best sonic result but also for their top quality, longevity and reliability. With the exception of six superior electrolytic capacitors, all other caps are Tantalum, solid-polymer OS-CON and double-metalized MKP. The signal path itself is direct-coupled and completely capacitor-free. The two output stages (one single-ended, one transformer-coupled balanced) use discrete BJT and J-Fet with metal-film resistors. All the electronics parts show an ultra-low RF noise. An advanced ARM microcontroller manages the user interface on the front panel, the IR remote command and the communication port (serial, RS232 standard).
The xHD update
This update has required significant optimization of code for the main FPGA of R2R ladder DAC which has improved sound quality. The new hybrid USB XMOS xCore 200 (dual core) + FPGA circuitry enables the high-resolution file playback up to 768kHz PCM and native DSD512
- Proprietary hybrid 2-stage XMOS xCore XE216 + FPGA
- Fully-floating (isolated) USB decoding and clock generation by FPGA with proprietary code
- High quality, long life, low noise parts and modular design that distinguish aqua – acoustic quality equipment
- Sample rate in “Bit Perfect” up to 768 kHz PCM and DSD512
- Latest version of customized XMOS X Core Driver on Windows OS (W7, W10)
- Native X Core Audio on Mac OS (Bit Perfect)
- USB Audio 2.0 operation on Linux (Bit Perfect)