Passive filters fill the bill at audio frequencies

By Richard Kurzrok, Queens Village, NY -- EDN, 2/1/2001

Low-frequency filters, particularly at audio frequencies, usually take the form of active filters. These filters eliminate expensive inductors with windings of many turns. Both analog and digital active filters are most compatible with large-scale integration at the subsystem and system levels. However, passive filters remain a viable option when you quickly need low-cost prototypes and test pieces (Reference 1). These filters use no external dc excitation and require no complex pc boards. You can easily wind some filter inductors using manual techniques. Moreover, inductors can handle greater power levels than small-signal active devices. You can construct a simple lowpass filter with a 3-dB cutoff frequency of 10 kHz; a source/load impedance of 50Ohm; five poles; and 0.02-dB-ripple, Chebyshev response. Figure 1 shows the filter's schematic; Table 1 provides the parts list.

Table 2 shows the measured frequency response with 50Ohm source and load impedances. The extremely low passband-insertion loss indicates that the inductors' unloaded Q is greater than 100. You could use smaller inductors, such as toroids with 0.5- or 0.625-in. diameters with acceptable insertion losses (Reference 2). Note that expensive Litz wire is unnecessary. Lowpass filters need much lower inductor unloaded Q values than do most bandpass filters. At very low frequencies, both inductors and capacitors can become large. By using moderate filter-impedance levels, such as 50 or 75Ohm at kilohertz frequencies, inductor values can be lower than 10 mH. With high-permeability inductor cores, fewer turns are required, and hand-winding is usually feasible. However, capacitors become large for lower filter impedances. For the traditional 600Ohm impedance used at audio frequencies, inductors are larger by an order of magnitude. If you reduce the cutoff frequency from 10 to 1 kHz, the inductor values also increase by an order of magnitude.

References
Wetherhold, Ed, "Audio Filters for EN 55020 Testing," Interference Engineers' Master, 1998.

DeMaw, MF, Ferromagnetic Core Design and Application Handbook, Chapter 3, Prentice-Hall, 1981.

ACKNOWLEDGMENT

I acknowledge Ed Wetherhold (Annapolis, MD) for three decades' worth of significant work on low-frequency passive filters and related circuits.