We describe a new high-sensitivity experiment for observing cosmic microwave background (CMB) anisotropies. The instrument is a two-element interferometer operating at 33 GHz with a ~3 GHz bandwidth. It is installed on the high and dry Teide Observatory site on Tenerife where successful beam-switching observations have been made at this frequency. Two realizations of the interferometer have been tested with element separations of 11.9lambda and 16.7lambda. The resulting angular resolution of ~2 deg was chosen to explore the amplitude of CMB structure on the large angular scale side of the Doppler (acoustic) peak. It is found that observations are unaffected by water vapour for more than 70 per cent of the time when the sensitivity is limited by the receiver noise alone. Observations over several months are expected to give an rms noise level of ~10-20 2 muK covering ~100 resolution elements. Preliminary results show stable operation of the interferometer with the detection of discrete radio sources as well as the Galactic plane at Dec. =+41 deg and -29 deg.