Vintage

Some ‘fresh’ (for the era) tech was debuted in Micromega’s Stages – Philips’ CD-ROM-compatible linear-tracking CDM12.4 transport and the TDA1305 DAC. The latter was an early hybrid type, combining a cascaded 16x FIR filter with 6x oversampling (a total 96x upsampling) followed by truncation to 5-bits with 2nd-order noise-shaping. The core D/A conversion in each chip uses two 4-bit DACs (4+4 bits = 5 bits) to accommodate the 32 possible sample sizes (2⁵ = 32). The digital filtering is a linear phase type that provides slightly greater stopband attenuation of 62dB vs. the 52dB offered by Philips’ earlier SAA7220 o/s filter [Vintage Review, HFN] Dec ’25]. In-band ripple is also reduced, while the Stage 2’s response rolls away gently from –0.15dB/10kHz to –0.6dB/20kHz. A standard 2.1V output is offered although the super-low <1ohm source impedance is – then as now – uncommon. The 105.4dB A-wtd S/N ratio is generous while low-level linearity (a +0.6dB error at –100dBFs) is aided by the Stage 2’s use of two stereo DACs, each in differential mode.

Distortion is far lower than recorded for the test sample back in 1994 at 0.0005-0.0007% (re. 20Hz-20kHz at 0dBFs), increasing to 0.004-0.016% down to –30dBFs [see Graph 1, below]. The undulations in THD vs. level at 20kHz [cyan trace] are a reflection of non-linearities in the DAC process at HF. That said, despite the complexity of the DAC, and perhaps assisted by Micromega’s ‘Acutrans’ internal clocking and data sync’ing, jitter is respectably low at 240psec [see Graph 2]. Finally, the TDA1305 DAC featured a double-speed mode to service 2x dubbing tape decks – here a 1kHz signal appears at 2kHz and the 20Hz-20kHz response ‘stretches’ to –2.8dB/40kHz! PM