The HDR-2007 audio filter BPF consists of a 7-pol elliptic HPF and a 9-pol elliptic LPF. This audio roofing filter after the PPN is very important. The filter should have a steep frequency-cutoff characteristic with a sharp transition band (300- 2700 Hz), a stopband attenuation greater than 60 dB and a small passband ripple.
It is well known that large audio inductors are incompatible with the homebrewers fun. However, their true to nature equivalents are also well known. Both filter are made without coils but with modern parts and old knowhow published by Gorski-Popiel, Bruton and Antoniou.
The design process begins with the selection of an appropriate common passive filter topology (600 Ohm terminated) that meet above goals at reasonable homebrewing complexity and contains the maximum number of inductors! This circuit is then transformed to an active circuit without any inductors.
One of the best circuit topologies to handle this type of an elliptic HPF design is the Gyrator ladder. A gyrator is a GIC (Generalized Immittance Converter) that uses two operational amplifiers, four resistors and one capacitor to mimic the properties of an inductor. An active GIC filter can simulate the transfer characteristic of a passive LC ladder network without using an inductor. But note that the GIC must always be grounded for proper operation.
All impedances in the LPF are transformed with the Bruton Transformation but leaves the filters amplitude versus frequency response unaltered by applying impedance scaling without influence the transfer function. L becomes R, C becomes FDNRs and the terminating resistors R becomes C. These FDNR (Frequency Dependent Negative Resistor) may be realized using a special GIC with two capacitors.
This filter types have a lower sensitivity to component value variations than the other RC active filters that tuning is not usually needed and it can provide some dB increase in signal to noise ratio over a Sallen and Key filter for instance.
For details use an internet search engine with the keywords gyrator, GIC, FDNR and Bruton transformation.
 First version |
 HPF schema |
 HPF with GIC |
 FDNR LPF |
 GIC HPF |
 GIC HPF diag |
 FDNR LPF |
 LPF FDNR diag |
