The IOS is a dual mono discrete symmetrical preamplifier using a high performance transformer input stage. It has been primarily designed to match the Apheta, but with its selectable loading and gain it will easily match any other cartridge. It combines the organic low noise gain properties of the transformer and low noise, high bandwidth discrete transistor circuitry. Housed in the Rega extruded aluminium case and fitted with full metal front panel, it may be Rega's most complete product to date.

The function of a phono preamplifier is to match the output from a pick up cartridge to the input of a line level amplifier. It is required to amplify the very low signals produced by the cartridge and also to equalize in accordance with RIAA record cutting standard.

The IOS is a four stage all symmetrical amplifier. It is dedicated to amplifying the very low level signals that are generated by moving coil cartridges. The first stage uses a 1:10 step-up transformer, which can be optimized for any cartridge through selection of the correct loading. The second stage is a symmetrical compound pair class A amplifier, using paralleled low noise Hitachi transistors in the input stage. This drives the passive high frequency part of the RIAA equalization stage. The third stage is a symmetrical class A differential amplifier with a common base driver stage. This is also used as the active amplifier in the low frequency part of the RIAA equalization stage. The fourth stage is the active filter. This configuration uses the same circuit found in the second stage. The filter element uses a very high bandwidth operational amplifier gyrator as the simulated inductance part of the tuned filter.

The power supply is a using fast diodes and a low noise voltage reference. Each stage has its own localized power supply and LED referenced current generators. Very high quality polyproplene capacitors have been used in the signal path. 1% tolerance polypropylene capacitors are used in the RIAA EQ stages and audio grade capacitors are used in the rest of the circuit.