Active, Adjustable Audio Band‐Pass Filter

An active, adjustable band‐pass filter having seventh‐order Butterworth attenuation characteristics (42 db/octave slopes) is described. It has 34 separate high‐pass cutoff frequencies between 16.2 cps mad 16 200 cps and 34 low‐pass cutoffs between 20 cps and 20 000 cps. The filter weighs six pounds exclusive of power supply mad uses six miniature double triodes and no inductors. Its insertion gain is 4.5 db, its dynamic range exceeds 110 db, and it will supply more than 50 v rms output with low distortion. Second‐order harmonic distortion is much reduced by running tube heaters at about 3.7 v, leading to greatly increased tube life. Intermodulation distortion at 10 v rms output is only 0.065%.