Grace Design m802 A/D ADAT Converter

The Grace Design m802A/D provides eight-channel, high performance A/D converter option for the m802 remote controllable preamp system.  Designed to be seamlessly integrated into a Pro Tools|HD compatible m802 preamp, the new 24 bit/192kHz capable A/D converter option represents a new level of sonic performance and reliability.

Physically, the A/D consists of a hardware module that's installed internally in an m802 chassis and whose connectors are brought out to a rear panel slot.  Two versions of the A/D are available.  The first includes a DB-25 that provides two parallel eight-channel sets of AES3 format outputs in single-wire mode or eight channels in dual-wire.  There are also four BNC connectors that accommodate AES3-id/S/PDIF formats.  These BNC's provide a total of eight channels in single-wire mode or four in dual-wire.  For clocking duties, a Word Clock input and Word Clock/Loop Sync output are provided on BNC connectors.  The 2nd version of the A/D trades off the AES3-id/S/PDIF BNC outputs for two optical connectors that allow two parallel eight-channel sets in ADAT format or eight channels of SMUX at 96kHz.

An A/D converter can be divided into three basic functional blocks: the analog input stage, the A/D converter, and clock generation (or regeneration).  The analog input stage provides fully differential circuitry that contains no electrolytic capacitors and utilizes precision 0.5% tolerance thin film resistors, which guarantees no signal coloration from capacitor dielectric absorption and resistor thermal modulation.  Careful attention to grounding and power supply layout guarantees no sonic aberrations caused from unwanted interference from digital circuitry.

The A/D converter itself is the Cirrus CS5381.  This converter employs a fifth order delta-sigma modulator and a low pass filter that exhibits very wide dynamic range and low latency.  With a gentler slope digital filter than other ADC chips, the CS5381 is the most musical of the high end converter ICs and is capable of 120dB dynamic range with distortion and noise artifacts at less than -110dB.  Separate power supplies for the analog and digital portions of the A/D converter ensure that the precision delta sigma modulator will not be affected by other surrounding circuitry.  As well, the differential inputs provide additional input noise rejection.

The m802 A/D converter employs a two stage PLL system to provide practically jitter-free recovered clocks with extremely high incoming jitter attenuation.  The first stage is a wide lock range, ultra low noise, analog PLL.  This stage can lock to incoming word clocks and Avid Loop Sync signals with a +/-8% lock range.  Rather than using standard CMOS based PLL chips for the first stage PLL, Grace uses an extremely quiet, discrete VCO.  When using a quality external clock source, this PLL alone will provide a nearly crystal quality clock.

When the incoming sample rate is within +/-200 ppm of the selected sample rate, the secondary (our proprietary s-Lock) crystal based PLL will lock to the first stage PLL, providing the quietest possible recovered sample clock to the A/D converters.  The s-Lock PLL is capable of extreme rejection of incoming jitter, which begins at less than 0.1Hz and is better than 60dB at 1kHz.