Phono Preamplifiers

While passive RIAA networks are often employed for their perceived subjective advantage, conventional RC stages soak up a lot of gain (~20dB at 1kHz) and present a variable load for the input/buffer preamp. Bridged T-network LCR filters can be designed for a constant input impedance without sacrificing as much gain, but optimising the inductor value (against its series resistance) is a trade-off, especially in the low-pass arm of the network. Aurorasound's latest circuit offers about 2dB extra MM gain vs. the VIDA MkI (+40.5dB vs. +38.4dB) with a +1dB 'boost' for MCs (+65.1dB vs. +64.2dB) at the expense of slightly reduced input overload margins – 128mV vs. 163mV (MM) and 7.0mV vs. 8.6mV (MC). The MM margin is still more than sufficient, offering +28dB headroom for a 5mV/1kHz (5cm/sec) rated pick-up, this coupled with a worthy 82dB A-wtd S/N ratio. The MC input has a good 76dB S/N but is arguably less well suited to very low output types where a 70dB+ gain is required, or very high output MCs where the +22.9dB input margin (re. 500µV) may be insufficient.

Distortion is reduced to 0.0015% through the midrange in the MkII although low frequency THD is slightly higher at 0.017% [see Graph 2, below] – all these figures being two or three orders of magnitude lower than that from a 'real world' pick-up. The equalised response shows a mild +0.2dB lift through the bass (~60Hz) and treble (~10kHz) and boasts slightly greater extension over the MkI, reaching +0.11dB/20kHz and –5.3dB/100kHz. The low bass extension of its DC-coupled output stage is extreme, just –0.6dB/1Hz, while the AC-coupled subsonic filter shows an improved –10dB/5Hz attenuation [see Graph 1, below]. Finally, the MkI's high 1.1kohm output impedance has been slightly reduced to 850ohm in the MkII. PM