The invention refers to a coffee machine having a brewing chamber and a beverage outlet. A valve assembly is inserted between the brewing chamber and the beverage outlet. The valve assembly comprises a valve seat and a valve body member movable with regard to the valve seat. The valve body member is biased by a spring and can be moved, against the force of the spring, by the pressure of the coffee beverage flowing through the valve assembly. Moreover, these are provided for adjusting the bias force of the spring. In such a design, the operator or user of the coffee machine can directly influence the amount of coffee to be brewed per time unit. Moreover, within the limits of the user selected value, the brewing water flows through the coffee powder received in the brewing chamber with an essentially constant flow rate, regardless the amount and granule size of the coffee powder received in the brewing chamber.