Estimating a signal power in a compressed audio signal A is provided, the audio signal comprising blocks of quantized samples, a given block being provided with a set of scale factors. The improvement comprises a check valve assembly in the engine coolant line to control the flow of coolant to the heat exchanger, and thereby control the temperature of the water. Additionally, the present invention eliminates the parasitic bipolar gain associated with CMOS device operation, reduces manufacturing costs, tightens control of device performance parameters, and provides for superior device characteristics as compared to the prior art.