Login or Sign Up
Welcome, !
Join the HC Newsletter
Subscribe Now!

Stompbox Envelope Filters in the...Studio?

Time for another installment in the “Stompboxes Reloaded” series


by Craig Anderton




The envelope filter Is a variation on the wa-wa pedal. However instead of moving a physical pedal, the envelope filter measures the guitar signal’s level, and uses this to control a bandpass filter’s frequency—higher levels are like pushing the pedal down, while lower levels are like raising the pedal.


Envelope filters were popular for guitar, and sometimes bass and keyboards, primarily with funk and R&B music. Bassist Larry Graham, of Graham Central Station and later bassist for Prince, used the Funk Machine envelope filter (Fig. 1), which I designed. Steve Cropper also used it.



Fig. 1: The Funk Machine is an extremely rare envelope follower from the mid-1970s. And here’s some interesting trivia: Martha Davis, who later moved to Los Angeles and founded the Motels (a popular 80s band with multiple gold records), did final quality control on the first generation of Funk Machines.


The most popular envelope filter was the Musitronics Mutron-III, introduced in 1972. In the 90s Electro-Harmonix asked inventor Mike Beigel to re-create that sound, which resulted in the Q-Tron pedal series (introduced in 1996). The very first envelope filter effects were created with Moog modular synthesizers, and today, Moog Music’s Moogerfooger (Fig. 2) packages that sound in a stompbox.




Fig. 2: The Moogerfooger is an envelope filter based on the Moog filter. This is a lowpass filter type, which is different from the usual bandpass response used in wah pedals.


The main envelope follower control is for sensitivity, so the envelope can track the proper range of your playing. A compressor between the guitar and envelope filter can help reduce the dynamic range if the filter variations are excessive. Optional controls typically change the filter resonance, filter range, type (e.g., bandpass or lowpass), envelope direction (i.e., higher signals make the filter frequency go lower), and attack and/or decay controls. Increasing these gives smoother filter changes, but don’t follow dynamics as accurately. (The Funk Machine used a photo-resistor as the filter control element, which had an inherent attack and decay time, so these controls weren’t needed.)




Vocals. One trick with vocals is to set the frequency as high as possible, preferably in the 2-3 kHz range, and sweep over a narrow range. Mix this in parallel with the vocal, but at a level where the effect is just barely noticeable—or better yet, at a level where you can’t really tell it’s there, but you can hear the difference if it’s bypassed. This can add a dynamic effect that gives the voice more animation, interest, and definition (people never believe me until they try it!).


Bass. Although bass was one of the original uses, like any wa effect filtering thins the bass. For a fuller sound, split the bass into a dry path and a parallel path that includes the envelope filter. With a DAW, patch the bass as a channel input, then use an aux bus to send some of this signal to a spare audio interface output that you patch into the envelope filter input. Connect the envelope filter output to a spare audio interface input, and bring it back into the DAW to provide the processed sound.


Drums. Envelope filters can give a trashy, funky sound with drums. As with bass, this often sounds best when you put the envelope filter in parallel with the dry drum sound. However, note that while this is useful for processing a mixed drum track, envelope filtering can also sound good on individual drum tracks, like snare, toms, and kick, giving an almost “Simmons” type of drum sound.


Electric piano and keyboards. Because the filter frequency depends on an instrument’s dynamics, anything with a percussive envelope like electric piano will work well with envelope filters. A sustaining sound, like organ, will simply “switch” the filter between high and low settings. This can sometimes produce interesting results, but in general, you’ll want dynamics.




Many modern guitar multieffects include envelope filter effects, and the sound will be similar to older units. Also, amp sims usually include envelope filters (Fig. 3).


Fig. 3: Virtual envelope followers, clockwise from top: NI Guitar Rig AutoFilter, Line 6 POD Farm Clean Sweep, Waves G|T|R Wah Wah, and IK Multimedia Ampeg SVX SCP-ENV for bass.


Another option is to use synthesizers with external audio input jacks. These usually don’t track the input signal dynamics, but some of them can trigger an internal envelope generator that controls the filter. The triggering occurs when the input signal exceeds a certain level. An added benefit is that these filters will usually be multi-mode types.   -HC-


[Note: For a related topic, see the article Stompbox Compressors in the...Studio?]




 Craig Anderton is a Senior Contributing Editor  of Harmony Central. He has played on, mixed, or produced over 20 major label releases (as well as mastered over a hundred tracks for various musicians), and written over a thousand articles for magazines like Guitar Player, Keyboard, Sound on Sound (UK), and Sound + Recording (Germany). He has also lectured on technology and the arts in 38 states, 10 countries, and three languages.


No comments
Join the discussion...
Post Comment
More Cool Stuff
  VocoPro Launches IEM-Digital Band, a True Stereo/Dual Mono Professional D...
Softube Tape Can a plugin really give your DAW the sound of analog tape?  &...
sign in
contact us
*Indicates required fields
Name *
Email Address *
Issue Type *
please wait