1. Apparatus for synthesising an output signal comprising a first audio channel signal and a second audio channel signal, the apparatus comprising; a decorrelator stage for generating a decorrelated signal comprising a decorrelated single channel signal or a decorrelated first channel signal and a decorrelated second channel signal from a downmix signal, the down-mix signal comprising a first audio object downmix signal and a second audio object downmix signal, the down-mix signal representing a downmix of a plurality of audio object signals in accordance with downmix information; and a combiner for performing a weighted combination of the downmix signal and the decorrelated signal using weighting factors, wherein the combiner is operative to calculate the weighting factors for the weighted combination from the downmix information, from target rendering information indicating virtual positions of the audio objects in a virtual replay set-up, and parametric audio object information describing the audio objects.