Flashback #5: FL 201 Instant Flanger - Eventide Audio

Flashback #5: FL 201 Instant Flanger

The Flanging Effect

Flanging 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. The Small Faces iconic pop hit, “Itchycoo Park,” is a classic example of the early use of flanging.

The Flanging effect is created by mixing the original signal with a slightly time-varied version of itself. This creates many amplitude nulls and peaks in the sound, and a spectrum that looks like the teeth of a comb. By slightly varying the speed of one of the tape recorders, the delay time changes and the so-called comb filter sweeps across the spectrum with the familiar ‘jet plane’ effect. How were the tape speed and delay controlled? Digitally—with the engineer’s thumb! Some developed quite the skill of ‘playing’ the effect by artfully pressing on the flange of the spinning tape reel and applying varying degrees of pressure to adjust the tape speed.

Is Flanging Phasing & Phasing Flanging?

In days of yore, engineers used the terms flanging and phasing interchangeably. Our first product, the Instant Phaser, used analog filters to create phase shifts which resulted in a new effect, a slightly anemic simulation of true delay-based flanging. It seemed logical to call the box a Phaser without considering the confusion that we would cause. We got away with it for decades until producer and engineer Bill Wittman suggested that we should have coined a new word for our effect because back in the day phasing and flanging were the same thing. Sorry Bill. From day one, Eventide distinguished analog phasing from delay-based flanging. We covered phasing in Flashback #1. Here’s a brief history of flanging, starting with the ‘flange’ itself.

Exactly What is a “Flange”?

The magnetic tape is held in place by a pair of flat metal plates called flanges which prevent the tape from disastrously unspooling.

Tape Flanging—DIY

Tape flanging gave recording engineers an opportunity to perform. They would have to apply varying pressure on the flange to change the playback rate, and the recorder’s servo motor would do its best to maintain a constant speed. It took a deft touch and was a true performance because the engineer would be listening to the ‘reel’ time effect in real-time. Here’s a video of an engineer demonstrating the technique:

Electronic Phasing Vs. Tape-based Flanging

Eventide’s first effects box, the Instant Phaser, was designed before audio delay was practical. Instead, it simulated the tape-based effect by using a series of analog ‘all-pass’ filters. As the name implies, they pass all frequencies unaltered but introduce a frequency-dependent phase shift.

By combining the phase-shifted signal with the original signal, frequencies that are out of phase are canceled. By varying the resistor value “R” in the circuit, the phase shift sweeps through a range of frequencies. The Instant Phaser connected eight all-pass filters in series to create a comb-filter-like and musically very interesting effect across the audio spectrum. However, with flanging, which results in many more nulls and peaks, the difference is quite pronounced. The nulls are harmonically related, and since there are always nulls at high frequencies, the ‘jet plane’ effect is always present. The difference between the two approaches is demonstrated below. Notice the difference of density and spacing of the spectral ‘combs’ created by varying the delay time (2 msec vs. 0.5 msec) for flanging vs. varying the resistor value (50K Ohms vs. 10K Ohms) for phasing.

Clearly, flanging has a more ‘intense’ effect for at least three reasons:

  1. Higher density of nulls and peaks
  2. Harmonic spacing of the nulls and peaks
  3. Nulls and peaks cover the entire spectrum; This is particularly true at high frequencies

That said, phasing has its own special charm. But more on that later…

Electronic Flanging: The Bucket Brigade Chip

By 1975, electronics had advanced, making audio delay 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.

The Instant Flanger: Model FL 201

When breaking news hit of a Charge Coupled Device (CCD), commonly referred to as a Bucket Brigade, it filled us with excitement and anticipation: 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 (see Flashback #3), recalls:

“The Flanger concept was to simulate tape flanging, where the audio is delayed by a tape recorder and the delayed and original signal are mixed to create a comb filter effect. 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.

The first production bucket brigade charge-coupled delay was the Mitsubishi Panasonic MN3001 Dual 512 Stage BBD Delay IC. Two of the ICs in series could give a 1024 unit delay, enough for a Flanger. The just-released chips were noisy (1% THD, 70 dB SNR) and no datasheet was available yet. A delegation of Mitsubishi Engineers from Japan visited us in New York City at our original 54th St. location with samples and a 3”-thick loose-leaf of blueprinted data and application info in Japanese.

The design required audio buffers and filters, as well as a high-current clock driver for the capacitance of the clock line. I designed a small board with two MN3001 ICs, the audio filters and buffers and a CMOS 7400 variable clock generator.”

Here is Jon’s design (note the 7 Trimpots which needed to be adjusted for best performance):

Here’s the Printed Circuit Board artwork:

Jon went on to design the rackmount box with a socket to accept his BBD card:

And, voila, the Eventide Instant Flanger was born! Here’s the original data sheet:

Better than Tape in So Many Ways

Quicker and Easier—Dare We Say “Instant!”?

The Instant Flanger was designed to do everything that tape flanging could, and to do it as well or better. More significantly, it was designed so that it could be played like an instrument. Remember, this was back in the days when engineers flipped switches, rode faders, and turned knobs made of matter instead of phosphors. 

Depth—From Doppler to Flanging and Everything in Between

Depth allowed a wide range of effects to be dialed in. It 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 Depth full counterclockwise and 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 Depth full clockwise and the varying delay is added out-of-phase, resulting in a deep null at low frequencies down to nearly DC. Of course, this wide range of effects was also possible using a couple of tape machines and routing the signals to the console, but what a nuisance to play with the mix while thumbing a flange!

The Big Knob—Manually Sweep the Delay

Dominating the center of the Flanger’s front panel is the Big Knob and an LED. (LEDs were still quite the novelty. The earlier Instant Phaser used incandescent lamps!) 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 in an analogous way to pressing a thumb on the tape flange. The Big Knob could also be set to a fixed delay amount to use the Flanger for ADT (Automatic Double Tracking).

The Oscillator—Automatically Sweep the Delay

The on-board LFO (Low Frequency Oscillator) varied the sweep rate from .05 Hz to 20 Hz. Of necessity, the oscillator was an analog circuit and consequently the sweep shape only approximated that of a sine wave. In fact, the shape distorts quite dramatically at its maximum rate.

The Envelope Follower—The Signal Level Sweeps the Delay

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.


The Instant Flanger even had a bounce control! “Bounce? Why? What for?” you may well ask. A tape recorder does its best to run at a constant speed and really doesn’t appreciate some random human pressing down on the flange and slowing things down. When pressure is released, the recorder’s servo motor kicks into high dudgeon to get back to speed. And, in fact, it tends to overshoot and bounce around as it seeks its proper rate. The Instant Flanger’s ‘bounce’ circuitry was designed to mimic the action of an abused, annoyed, angry servo motor to add yet another element of emulation; a certain frisson of underdamping.


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. Engineers quickly discovered that adding a bit of ‘flanging’ could sweeten a track and spread the stereo image in a most appealing manner.


In particular, using the Flanger to create pseudo-stereo soon 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 resulting stereo image created from a mono source was quite pleasing and many engineers employed it on everything, everywhere, every time. Funnily enough, the User Manual only devoted two sentences to this non-flanging application:

The Instant Flanger in Song—Classic, Subtle, Dramatic

Classic Flanging—Neither Tape nor Deft Fingers Required

Once studios installed an Instant Flanger in their rack, the bother of setting up a tape machine to create the flanging effect was no longer necessary. And, engineers no longer needed to perform the delicate art of flanging. The result was that the classic flanging effect became accessible to all and, some might argue, all too easy to add to a track or even a mix.

Subtle—Time After Time

The Instant Flanger’s ability to take a mono source and create a stereo image was used broadly, but we can think of no better application of this subtle technique than Bill Wittman’s artful treatment on Cyndi Lauper’s Time After Time. Here’s a video of Bill describing how he made that magic happen:

Dramatic—Ashes to Ashes

Talk show host John Oliver was recently asked, “If you could only listen to one song for the rest of your life, what would it be?” Oliver replied, “Ashes to Ashes. There’s so much going on in there, if you get tired of that song it’s your fault.”

One of the many things ‘going on’ is the quirky, unsettling piano riff that repeats throughout the song. Tony Visconti recalled how he created those amazing notes and – Wow – shared a few seconds of the isolated track from his master.

But What About Phasing? We Loved Phasing. We Miss it…

While time-delay flanging is a much more dramatic effect than all-pass-filter-based phasing, there’s something sweet about the sound of the phaser. And so, a couple of years after introducing our Instant Flanger, we decided to give 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. Jeff Sasmor designed the phaser card and, by adding four additional all-pass filters, achieved more dramatic phasing than our original Instant Phaser rack-mount box. Plug-in card??? Hmmm, have we just unearthed the first plug-in effect?

Stay tuned for Flashback #6, the H949 Harmonizer.

Try the Instant Flanger Plug-in

The Instant Flanger Mk II plug-in authentically emulates the original studio rackmount to bring the great sound of vintage tape flanging to your plug-in toolbox. Request Demo

That’s all for the Instant Flanger! Up next, we explore the first digital guitar FX device, the HM80 “Baby” Harmonizer!

Discover more:

Emulating the Instant Flanger
Tony Agnello & Woody Herman on Emulating the Instant Phaser
Gear Club Interview with Bill Wittman
Gear Club Interview with Tony Visconti

Check out our previous flashbacks!