Zobel network
（重定向自Constant resistance network）
![BBC engineers equalising audio landlines circa 1959. The boxes with two large black dials towards the top of the equipment racks are adjustable Zobel equalisers. They are used both for temporary outside broadcast lines and for checking the engineer's calculations prior to building permanent units](/Images/godic/202501/06/SB_test_room_19591834.jpg")
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- For the wave filter invented by Zobel and sometimes named after him see m-derived filters.
Zobel networks are a type of filter section based on the image-impedance design principle. They are named after Otto Zobel of Bell Labs, who published a much-referenced paper on image filters in 1923. The distinguishing feature of Zobel networks is that the input impedance is fixed in the design independently of the transfer function. This characteristic is achieved at the expense of a much higher component count compared to other types of filter sections. The impedance would normally be specified to be constant and purely resistive. For this reason, they are also known as constant resistance networks. However, any impedance achievable with discrete components is possible.