In these days when amplifiers (and pre-amplifiers) often have line-only inputs, there are several ways of dealing with a phono cartridge, and indeed several types of phono cartridge. Ignoring specialist devices like strain gauge and optical types, the vast majority of cartridges use magnetism to convert the mechanical groove energy into an electrical signal.
One characteristic of these magnetic devices is a lack of LF and excess of HF output, so they require something called RIAA equalisation in order to deliver a ‘flat’ response. There is also a substantial – usually around 20dB – difference in gain between ‘high’ (typically moving magnet) and ‘low’ (normally moving-coil) output levels.
Nowadays most amplifier manufacturers have moving-magnet style phono stages in their portfolios, to be used alongside a line-level integrated amp or pre-amp; some also have a built-in moving-coil option that boosts the output by around 20dB. That said, Rega regards high- and low-output cartridges as having separate and distinct amplification requirements, which explains the absence of what could be a ‘legacy’ MM input in the Aura.
Although the RIAA equalisation is invariably handled electronically; the boost part may be electronic, but the alternative of a transformer can vary dramatically in size and cost. The predecessor of Rega’s Aura, the £4,000 Ios, uses a couple of small trsnsformers to boost moving-coil cartridges; at the other end of the scale, Audio Note’s large and heavy S9 booster transformer costs nearly £20,000 – several times the price of an Ios!
However, the Rega Aura phono stage is entirely electronic, and is only suitable for low-output (moving-coil) cartridges. It has three symmetrical stages, and unlike its Ios predecessor, uses FETs (field effect transistors) in place of step-up transformers. One bonus of using FETs is that it avoids any bias current flowing within, or indeed any coupling components between the FETs and the cartridge. Careful matching between channels is undertaken, along with the gain needed to handle the passive (upper) part of the RIAA equalisation. The second stage is a Class A differential design and deals with the RIAA boost at low frequencies, while a similar third stage reverses the phase for the balanced outputs.