The paper reports on development of a stochastic slow extraction system prototype beam-tested during the 2004-6 MD runs of the 70 GeV proton synchrotron of IHEP-Protvino. Flat-topped 2-3 sec long and smooth (r.m.s. AC-to-DC ratio < 0.3) spills were obtained. The scheme implemented has a few inventive features. Protons are forced into the 3-rd order horizontal resonance with a supplementary 200 MHz RF system driven by sum of a non-random RF carrier and an additive base-band low-pass phase noise. Waiting beam stack is kept coasting in a close outer vicinity of 200 MHz buckets, in the longitudinal phase half-plane opposite to that housing momentum image of extraction resonance. RF buckets are kept repopulated by inward diffusion from the outer stack. Shape of noise power spectrum is invariable (no conventional frequency sweeping is applied), magnitude of noise being controlled via a proportional DC-coupled feedback loop acquiring data from a beam loss monitor. Terminal travel to resonance is carried out via a fast trapped cyclic motion inside buckets. It drastically improves immunity of extraction to ripples in power supplies of beam optics. No mains harmonics are observed in spills.