FL 201 Instant Flanger - Eventide Audio

The Instant Flanger, released in 1975, was designed to simulate true tape flanging. Flanging first became popular in the 1960s when recording engineers discovered that they could mix the output of two tape machines, one running slightly slower than the other, and get a cool new effect. As its time-delay circuit produced many more “nulls” and offered a much deeper flanging effect than anything previously available, the Instant Flanger became widely used on many legendary recordings. If you’ve ever listened to Led Zeppelin’s “Physical Graffiti” album, you’ve heard its legendary sound.

Electronic Flanging: The Bucket Brigade Chip

Eventide’s first effects box from 1971, the Instant Phaser, was designed to simulate tape flanging but used a series of ‘all-pass’ filters rather than delay (this was before audio delay was practical). Flanging requires a relatively short delay, and that delay must be smoothly varied from ~0 to ~10msec to achieve the effect. Digital delays had recently been introduced, but they were prohibitively expensive for such a short delay. It was not until the invention of a chip called a Charge Coupled Device (CCD), commonly referred to as a Bucket Brigade, that the Instant Flanger was made possible. This chip, analog though it was, could be used to delay audio. We contacted its manufacturer, Mitsubishi, in Japan. As Jon D. Paul, the designer of the black-meter Omnipressor®, recalls:

“Analog delay lines with charge-coupled devices were a new concept in 1974 and Richard Factor realized they could be used to simulate the analog tape delay. Richard assigned me to study the IC and design a bucket brigade analog delay with variable delay time.

“Ashes to Ashes”

The Instant Flanger has been used in many ways, from subtle to dramatic. Here is Tony Visconti discussing how he created the haunting sound of David Bowie’s “Ashes to Ashes”:

Better than Tape in So Many Ways

The Instant Flanger was designed to do everything that tape flanging could and to do it as well or better. Once studios installed an Instant Flanger in their rack, setting up a tape machine to create the flanging effect was no longer necessary.


  • Release – Determines the rate at which the Envelope Follower returns to a lower state after the loudest signal has passed. Clockwise on this control increases the release time and makes the Envelope Follower track more slowly.
  • Depth: Depth controls the mix of the swept-delay signal and the ‘dry’ signal. With the knob set to ‘noon’, only the varying delay is output, resulting in a doppler effect. Set to full counterclockwise, the output is an equal mix of the original signal and the varying delay added in phase. By adding the two signals in phase, low frequencies are reinforced. Set to full clockwise, the varying delay is added out-of-phase, resulting in a deep null at low frequencies down to nearly DC.
  • The Big Knob: The Big Knob allowed the user to manually control the delay with the LED’s brightness, indicating the amount of delay—dim for short delay and bright for max delay. Users could turn the knob to sweep the delay back and forth to create the doppler or flanging effect.
  • The Oscillator: The on-board LFO (Low Frequency Oscillator) automatically swept the delay. It varied the sweep rate from .05 Hz to 20 Hz. Of necessity, the oscillator was an analog circuit and consequently the sweep shape can only approximate that of a sine wave.
  • The Envelope Follower: The Instant Flanger incorporated one of the most important and popular innovations of the Instant Phaser, the ability to have the level of the signal vary the delay. The Threshold and Release knobs gave users the ability to accommodate different input signals.
  • Combined Controls: The Instant Flanger’s rich complement of ways to control the flanging effect manually, by the on-board oscillator (LFO), the signal’s envelope, or by remote control voltage (in any combination!) was quite the innovation. Obvious now, to be sure, and yet we know of no other effects device that offered this capability.
  • Bounce: The bounce control was designed to emulate the ‘bounce’ effect of a tape recorder’s servo motor trying to get back up to speed, which occurs in tape flanging after pressure is released from the flange.


The User Manual’s APPLICATIONS section begins:

The Instant Flanger is designed primarily for the generation of the flanging effect. However, through theory and experience, we have determined some other applications for which the flanger is suitable.

At the time of writing the Manual, little did we know that the Instant Flanger would be largely used for its non-primary function. In particular, using the Flanger to create pseudo-stereo became a go-to technique. It worked well because we designed the Flanger with two outputs and those outputs were decorrelated in two ways. First, each output was fed by a different Bucket Brigade Device so the delays were always a bit different. Second, we added analog phase-shift circuits (all-pass filters) à la the Instant Phaser.

The Phaser Plug-in Card

A couple of years after introducing the Instant Flanger, we gave owners the ability to turn their flanger into a phaser by replacing the small plug-in circuit card that held the bucket brigade chips with one that had all-pass filters, creating one of the first plug-in effects. Jeff Sasmor designed the phaser card with four additional all-pass filters for more dramatic phasing.

Check out Gear Club Podcast:

Gear Club Interview with Bill Wittman
Gear Club Interview with Tony Visconti

FL 201 Instant Flanger Images

Want More?

Interested in more Instant Flanger history? Check out our Flashback blog.

50th Flashback #5: FL 201 Instant Flanger
From Phasing to Flanging
Instant Flanger Plug-in


Instant Flanger Manual
BPC101 Expansion Manual