AN-8: Virtual Analog (VA) Moog Half-Ladder Filter
by Will Pirkle
This App Note derives the equations and shows the implementation of a 2nd Order Moog “half” ladder filter. I created this filter to give to my students as a project for deriving VA equations and implementing VA filters. I have not seen anything like it, though I only did a preliminary web search. This filter produces a 2nd order LPF using a similar Moog loop but with two LPFs and one APF instead of the cascade of 4 LPFs in the ordinary filter. As with the full ladder filter, passband gain is reduced as Q increases, but in the half-ladder filter the reduction is not as high. The sample RackAFX plug-in implements both the full (4th order) and half (2nd order) filters for comparison.
This filter is nearly identical to another “half-ladder” filter that a well-known Japanese synthesizer company uses in its virtual analog model of one of its classic late 1970’s synths. In other words, half-ladder filters are far from useless! 😉