Outboard DACs

Now dubbed the 'SuperTubeClock', the special sauce that governs the timing of data through PrimaLuna's XMOS-based USB input, SRC4192 asynchronous upsampler and PCM1792 DAC (both from TI, née Burr-Brown) is based around a tube rather than transistor circuit. This master clock is still referenced to a standard 24.576MHz crystal – the small metal can just visible at the bottom of the inset picture – while a miniature 6S6B triode is used in the subsequent widebandwidth oscillator. This same circuit, and the choice of monolithic 192kHz/24-bit DAC, now 14 years young, was first revealed in PrimaLuna's ProLogue Eight CD player [HFN Nov '08]. I discuss the performance differences with its EVO 100 Tube DAC in my Lab Report but the 'tube clock' itself has also witnessed some refinement.

PrimaLuna's claims for the efficacy of the tube oscillator really boil down to the exceptionally wide frequency response of this signal generator and, thus, its ability to deliver a recognisable 'square' timing waveform with good edge definition. The absolute frequency accuracy is determined by the choice of crystal, and its mere +20ppm error is well within Class I tolerance. Jitter, in practice, is influenced by noise and other interference emanating from every corner of the digital circuit and is reduced from the 330psec measured with the ProLogue Eight to 160psec (48kHz) and 60psec (96kHz) in the EVO 100, with ±100Hz and ±200Hz PSU modulations the key components. PM