  We discuss properties of Alfv\'enic fluctuations with large amplitude in
plasmas characterised by low magnetic field compression. We note that in such
systems power laws can not develop with arbitrarily steep slopes at large
scales, i.e. when $|\delta \bf{B}|$ becomes of the order of the background
field $|\bf{B}|$. In such systems there is a scale $l_0$ at which the spectrum
has to break due to the condition of weak compressibility. A very good example
of this dynamics is offered by solar wind fluctuations in Alfv\'enic fast
streams, characterised by the property of constant field magnitude. We show
here that the distribution of $\delta B=|\delta \bf{B}|$ in the fast wind
displays a strong cut-off at $\delta B/|{\bf B}|\lesssim2$, as expected for
fluctuations bounded on a sphere of radius $B=|{\bf B}|$. This is also
associated with a saturation of the rms of the fluctuations at large scales and
introduces a specific length $l_0$ above which the amplitude of the
fluctuations becomes independent on the scale $l$. Consistent with that, the
power spectrum at $l>l_0$ is characterised by a -1 spectral slope, as expected
for fluctuations that are scale-independent. Moreover, we show that the
spectral break between the 1/f and inertial range in solar wind spectra indeed
corresponds to the scale $l_0$ at which $\left<\delta B/B\right>\sim1$. Such a
simple model provides a possible alternative explanation of magnetic spectra
observed in interplanetary space, also pointing out the inconsistency for a
plasma to simultaneously maintain $|\bf{B}|\sim$const. at arbitrarily large
scales and satisfy a Kolmogorov scaling.
