Each filter has a choice between a two-pole (12dB/octave) filter or a four-pole (24dB/octave) filter with resonance. There is also a choice between high pass or low pass filters but not both at the same time. The original Moog Filter Coupler connected the two filter sections, a high pass and a low pass to create either a band pass or band reject filter.

I loved using the built-in envelope follower to track or trace the dynamics of incoming audio and change the cutoff frequency of the filter. The Amount control, when turned past the "+" sign, will dynamically raise the cutoff frequency and create a new sound treatment for snare or percussion instruments. When the Amount control is turned to the "-" side, the cutoff frequency is lowered and is equally useful. Using a fat sounding analog envelope filter is instantly gratifying and just fun!

The Ladder has both fully adjustable Attack and Release controls for the envelope follower and I have to think of Dr. Moog when I use the Drive control because it distorts the filter itself and is more adjustable for subtle analog coloration.

All sorts of filth from nasty transistorized fizz to realistic frequency/amplitude dependent distortion are possible and the multi-colored input level LED is useful to set the amount and (peak) moments of its 'intrusion.' The Envelope LED indicates the Ladder's dynamic operation and is also good for recalling your favorite front panel setups for all the best filter treatments.

You may stereo link two Ladder filters with an internal linking cable and link resonance and envelope follower functionality. An internal DIP-switch (there is no room on the front panel) sets the exact operation of the filter's cut-off frequency. This feature enables external side chaining where an external source could be processed in one Ladder filter and linked to a second Ladder that is processing another audio track. But I do wish there was a way to set all this up without pulling them out of the rack. I'd love to see a double wide 500 version of two Ladders and this intriguing linking feature and the Moog Filter Coupler included.

The Ladder Filter from Moog is currently at a street price of $769 each and is an analog dream to use. I've enjoyed a pair of them for about a month now and insert them both into my mixes whenever I can. Much more than any EQ or filter, The Ladder filter is not just another filter effect because it is so useful in many subtle ways as well.