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Standard network theory is used to explain the advantages of RC networks in the design of filters with frequency response characteristics which are continuously adjustable over several decades. The theoretical discussion is followed by a review of some simple circuits from which most filter characteristics may be obtained. The shape of the response as a function of frequency is determined largely by passive networks. Tubes serve to isolate the passive networks and, in some cases, to invert their response by means of feedback amplification. An example of the method is demonstrated by the design of an adjustable low-pass high-pass filter.