Devices comprising switching amplifiers such as class D amplifiers and comprising loads such as loud speakers are provided with controllers for controlling switching circuits for in respective four states introducing respective four voltage signals across the loads, which four voltage signals are different from each other. The hem has a pair of openings through which a drawstring is inserted such that the two ends thereof protrude through the openings. This decreases the proportion of longer-chain hydrocarbons in the natural gas, which may simplify subsequent processing, and may enable the size of the plant to be decreased. The gate of the FET is connected with a variable voltage source.