Introducing the new Formula xHD

The Optologic R2R conversion system

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.
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.
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 up to DSD256 (Quad DSD)

xHD improvements:
• 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 quad rate DSD (DSD256)
• 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)
Formula xHD
• Proprietary Optologic D/A conversion system. Pure R2R ladder - FPGA (Field Programmable Gate Arrays) based without digital filter

• Galvanic and magnetic isolations between the FPGA and the four branches of the R2R ladder converter

• Jitter free digital interface AQlink PRO (I2S protocol), uncompromising digital connection to La Diva cd transport

• Discrete R2R ladder DACs with low noise precision resistors

• Zero S/PDIF Jitter design, digital receiver stage PLL (phase locked loop) technology

• High-performance AQ Discrete Regulator (MOSFET, J-FET, BJT) for analog and digital DAC’s power supply

• 2 separate low noise C-Core power transformers, one for the analog and one for the digital section

• MODULAR DESIGN with upgradeable multi board platform

• Transformer-based true balanced audio output stage

• Fully upgradeable high-speed USB hybrid 2-stage XMOS xCore XE216 + FPGA, Fully-floating (isolated) USB decoding and clock generation by FPGA with proprietary code

• Proprietary USB Firmware / driver :
Apple MAC OS - Linux OS : USB asynchronous native support, no need to install drivers software

• Digital phase selector on front panel

• High-quality parts selected for sound quality:
- Tantalum, solid-polymer OS-CON and double-metalized MJP capacitors
- low noise Metal Foil ultra-precision resistors
- double metallized film pulse capacitor
- ultra-fast diodes
- halogen free cables

• Aluminium anti-resonant cabinet with Nextel

• RC5 IR remote controller (optional)

• Designed and handmade in Italy


Digital to analog conversion type
Proprietary Optologic D/A conversion system
Pure R2R ladder - FPGA (Field Programmable Gate Arrays) based without digital filter
Supported Native Sample Rates
AQlink / I2S serial bus  - USB PC Audio : 44.1kHz to 768kHz PCM up to 24 bits
DSD64, DSD256
Supports DSD via DoP on all inputs
DAC architecture
Multibit sign magnitude R2R ladder (upgradable)
Asynchronous USB (High Speed)
USB Audio Class 2 with Type B connector
Digital Receiver
PLL (phase locked loop) technology 128 or 256 FS internally selectable
AQlink (I2S bus)
LVCMOS level
Oversampling factor
Digital inputs
- RJ45 AQlink (I2S serial bus) - PCM 24 bit / 768kHz - DSD64, DSD256 via DoP
- BNC coax (S/PDIF) 75 ohm  -  PCM 24 bit / 192kHz - DSD64 via DoP
- RCA coax (S/PDIF) 75 ohm  -  PCM 24 bit / 192kHz - DSD64 via DoP
- AES/EBU balanced 110 ohm  -  PCM 24 bit / 192kHz - DSD64 via DoP
- USB port  -  PCM 24 bit / 768kHz - DSD64, DSD256 via DoP
Modular input:
- AES/EBU balanced 110 Ohm -  24 bit / 192kHz - DSD64
- RCA coax (S/PDIF) 75 Ohm -  24 bit / 192kHz - DSD64
- AT&T (ST Fiber) -  24 bit / 192kHz - DSD64
- Optical TOSLINK -  24 bit / 96kHz
Analogue Outputs
UNBAL 2 RCA Output 2.4 V RMS
BALANCED ( passive transformer's symmetrical ) 2 XLR Output : 3.8V RMS
Output Impedance
10 ohm RCA - 600 ohm XLR
Load Impedance
10 kohm (min.) RCA - 600 ohm XLR
Frequency Response
20Hz to 22kHz +0.5dB/-0.5dB
Phase Response
Linear phase
<0,016% 1KHz -10dB
Main processor
STM-ARM microcontroller
9 Button on front panel, IR Remote, Isolated RS-232 D-SUB 9-pin connector
Front Panel
Power, phase invert, mute, input switch, RC5 remote sensor
Power Consumption
100-115V / 220-240V; 50 or 60Hz - 58VA
( W x D x H ) 450 x 370 x 100 mm
9 kg
Front finish
Satin Alu Silver or Satin Black
Case finish
Grey Nextel powder coated

USB Audio Class 2 Driver for WIndows (NOT for xHD)
Owner's Manual

"With the original La Scala 2 being one of my two reference converters and the top deck in contributor John Darko's DAC index, exploring audible space beyond it seemed, truth be told, a rather tall order. How much room was there left for improvement? What ungodly reference gear would it take to hear it? As it turned out, the Formula delivered a demonstrable lead with €6'500-class ancillaries like a Pass Labs XA30.8 or pair of Zu Audio Druid V speakers. If you're an audio aboriginal who goes deep walkabout into gargantuan soundstages to cherish size, scale and utmost specificity about recorded space, this Italian formula has your number. But it's not merely about the usual 3D markers and how precisely and widely spaced they are. It's about an added dimension. That accesses the emotional mood or climate of the playback scenery. Whilst costlier decks came through earlier, none of them did it at that potency. Granted, to fully matter relies on superior recordings. Lesser or purely synthetic efforts reveal that they lack that particular magic. Finally there's this peculiar easy-digital aspect as though your brain's usually active aural processor went on holiday because it's not needed. As such, I must highlight Aqua Hifi's Formula with one of our sparsely administered awards. True, it's serious money. But it's seriously well built. And it delivers very serious performance. Yet all this multi-stage seriousness melts away when one is faced by such deep holistic listening satisfaction. That's what the 'high' in hi-end should be all about. Cachet, pride of ownership, resell value and bling to impress the ignorant with all factor. But they pale to nothing against delivering the actual experience. The Formula does. So it pays to study its recipe in detail. It really is something else."

Srajan Ebaen

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"Yea what he (and I) said! The Aqua Formula joins the rarefied air of music making DACs that shatter digital's glass."

Michael Lavorgna
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X Core Driver on Windows OS (W7, W10) for new Frormula xHD

"Although at times I have been tempted by the extra refinement and unbeatable purity of NOS, filterless DACs such as the Mosaic, Metrum and some other designs, ultimately I always came back to the Wadia and later the Bricasti, mainly because they simply had more conviction, more drive, more dynamic power." "The great thing about the Formula xHD DAC is that it offers unprecedented purity and all the advantages of the aforementioned R2R designs and adds to this all the dynamic power and drive that I could ask for, while totally avoiding imposing any character onto the sound."
"But in the end all the technology in the world means nothing if it does not distil into great sound. Fortunately, after having heard the Formula xHD DAC, I can testify that this DAC is truly magnificent!"

Christiaan Punter
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Mono and Stereo Formula xHD update REVIEW PART ONE
Christiaan Punter of HIFI ADVICE: “WOW”, this is a seriously great DAC!
"The Xhd upgrade brings quite a substantial step up in transparency and density, where both micro and macro details are forming a new experience. Have no worries, the "new" translates to positive. There are simply more pieces of information and cues coming through the digital veins of the updated Aqua Formula Xhd DAC."

"Acoustic Quality Formula Xhd DAC has now climbing up the ladder and reaching out to a plane, where the realism is more closely mimicking the real thing. Such, exhibited performance is is usually reserved for the bigger and more expensive "players", so this alone makes a Formula Xhd DAC a serious contender at a sensible price."

Matej Isak
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