We investigate the temporal contrast of the Light Wave Synthesizer 20 (LWS-20): a powerful, few-cycle source based on the optical parametric synthesizer principle. Saturation effects in the RF amplifier driving the acousto-optic programmable dispersive filter (AOPDF) were found to degrade the coherent contrast for non-monotonic group delay corrections. We subsequently present a new dispersion scheme and design a novel transmission grism-based stretcher optimized for LWS-20. The resulting temporal contrast of the amplified, compressed output pulses is improved by 2-4 orders of magnitude compared to the former design.