Exploring the Classic Moog 24dB Ladder Filter

What made it so special? Find out with our step-by-step and audio examples
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If any single component defines ‘the Moog sound’, it’s the filter. Moog’s circuit made use of transistors – cutting edge technology when his first instruments were introduced. They were configured in four pairs with a capacitor in between – an arrangement that looked a lot like a ladder, as in the diagram below, hence its common description as a “transistor ladder” filter.


The filter’s design allows for switchable 12dB/oct slopes as well as high-pass operation, though the one at the heart and soul of the legendary Minimoog offers only the basic 24dB/oct low-pass mode, albeit with rich resonance and self-oscillation.

The first Minimoog players were quick to discover that they could plumb external signals into the filter around back and, in fact, routing the Mini’s own ‘Low’ output back into the filter allowed for some tasty overdriven effects.


Take a listen to our example audio files – at http://bit.ly/TheVirtualMoogSessions – to hear what an actual vintage Minimoog filter can do to an acoustic drum loop. Let’s approximate that legendary Moog filter mojo using software clones such as Arturia’s MiniV. Load up your own favourite drum loop to get started.


Our loop was played at 107 BPM, and since we’re using Live, our tempo is automatically matched. Listen to the original to get a feel for the untreated sound. If necessary, we can cycle the playback so it’ll play indefinitely while we work. It’s time to drag our MiniV plugin in as an insert effect. The MiniV installs a version especially for use as an effect.


Let’s go to the Mini’s Mixer section and switch out all three oscillators. Next, we click twice on the External Input switch and bring its Volume knob up to about 8. Go to the Filter section, crank the Cutoff up full, and reduce the Emphasis and Contour Amount knobs. Click and hold a key on the keyboard. The loop can now be heard playing through the MiniV.


For an easier playback option, we’ll click the Open button in the top-right and de-activate the Arpeggiator’s Play button. We must also set the switch just to its right to the middle position. Now, any key we play will be held. Let’s have a play with the filter’s Cutoff and Emphasis knobs. Our patch allows us to modulate the filter Cutoff with the mod wheel.