The invention relates to a multi-channel (e.g. quadrature) MRI transmit system in which RF power amplifiers having different power capabilities are used in different transmit channels. This results in reduced system costs, due to the avoidance of an unused excess of RF power capability when the power demand for obtaining a homogeneous B1-field (RF shimming) is asymmetric and the asymmetry is qualitatively the same for different imaging applications. The multi-channel transmit unit may also comprise a commutator which enables to selectively connect each RF power amplifier to each drive port of transmit coil arrangement (e.g. a birdcage coil).