Is a phase shifter the same as a phaser?

A "phase shifter" or "phaser" effect is a very short delay effect created by electronic filter stages.  Each stage requires an active device setup as an "all-pass filter."  The filter network is usually a RC type where the "R" is variable, and electronically controlled.  A single stage will only provide a subtle effect.  For dramatic effects, several stages are placed in series, and the variable resistances must be electronically ganged together.  The variable resistance component can be a FET, an optocoupler (LDR + LED), an OTA (CA3080, CA3094, LM13600/700), silicon carbide varistors (obsolete, used in '50s/'60s Magnatone amps; modern zinc oxide varistors are more like switches and will not work as linear variable resistances), or even diodes (see Steiner-Parker Synthasystem phaser).

For the classic "whoosh" effect, the dry signal must be mixed with the wet.  This causes phase interference, which creates notches in the frequency response of the overall system.  Electronically sweeping the notches creates the classic "whoosh."

If only the wet signal is monitored, the effect becomes vibrato.  This is the "true vibrato" effect in the Magnatone tube amplifiers of the 1950s and 1960s.  Some phase shifters have a "dry kill" switch to toggle between phase and vibrato.  The early Electro Harmonix Bad Stone and the Love Pedal Doppelganger both have this feature.

If the final output is fed back into the input of the first phase stage, resonance is added, and the effect becomes more intense, eventually breaking out into self oscillation if there is too much feedback.  The earliest phasers, the Maestro PS-1 and the MXR Phase 90, lacked this feedback feature.  Electro Harmonix claims they introduced it with the early Bad Stone and popularized it with the Small Stone ("Color" switch).

Manually adjustable phase shift processors are also found as utilitarian devices for aligning speakers or aligning the phase of multiple signals picking up the same source.  For the latter, multiple mics on an acoustic guitar or multiple mics on drums may pick up the sound at slightly different times, and adjusting the phase on one can result in finding a sweet spot where the two mics become in phase.  The Radial Phazer is a commercial example.

Especially in the early days of effects, the terms "phaser" and "flanger" were often interchangeable.  Both are short delay effects with generally similar sonic results.  This difference comes from the nature of the resulting phase interference from mixing the dry and delayed signals.  In a phase shifter, the filter networks are ganged together to create notches at specific frequencies, and those notches are electronically swept.  In a flanger, there are no changing filter networks, just pure delay.  Pitting the original signal against the delayed signal creates the same phase interference effect, but the notches are determined by the signal itself, not a filter network.  When the signal, pitted against it's delayed self, is creating its own phase interference, the notches are numerous and "track" to the signal, as opposed to a electronic filter created notch, whose frequency response is indifferent to the incoming signal, unless some very elaborate pitch to CV scheme is used.

When servicing old equipment, a short delay might be referred to as phase or flange (Marshall Time Modulator comes to mind).  Technically, the "flanger" effect is a result of phase interference, so calling either a "phaser" can be rationalized as "technically" correct, if confusing.  The term "flanger" is not a technical term, but a nickname given to the use of tape recorders as the delay element.  To change the speed of tape playback, you only need the slightest pressure on the flange of the tape reel.  It is widely believed that John Lennon came up with term when their engineers started to use the effect for Beatle's recordings in the mid 1960s.  The story goes that John was not particularly technical, and liked to use his own imaginative terms when talking with the engineers.  See Beatle engineer Geoff Emerick's "Here, There, and Everywhere" for his account of the story.

General Troubleshooting Tips

#1 phaser repair tip: If its a JFET based phaser, there is almost certainly a JFET bias trim, and quite often this just needs a little turn to either put the JFETs back in their operating range or to clear out the dust and dirt that is blocking the bias voltage from reaching the JFETs.

Because each stage is a filter, it is most helpful to use a swept function generator if you want to "look" at what's happening in the phase shift path.  Otherwise, if you just choose a single frequency as a test tone, you probably won't see much except obvious breaks in the signal path.

If your scope has a XY mode or inputs, they are useful for observing phase effects.  The diagrams on the MXR Phase 100 switch are showing you roughly what changes in the signal when observing the output in XY mode.  This mode plots the input as X and the output as Y, as opposed to the usual input as X and time as Y.  When input and output are plotted against each other, you get a diagonal line when the signal is the same.  When you add phase shift, the line becomes an oval.  When you sweep the shift, the oval spins around.  When you add resonance, the oval expands and contracts.  Observing the output this way, you can get nice visual indications when adjusting trim pots.  Some service manuals (Ibanez PT9 is a good example) will even give you pictures of what the XY display should look like when calibrated correctly.

A Method For Calibrating (or Troubleshooting) Phase Shifters

There are many ways to calibrate a phase shifter, but recently I started using a method I like very much because I can see the phase shift effect very clearly on my oscilloscope.

My method requires an oscilloscope, a swept function generator, and you need to be able to trigger the scope with the function generator's sweep oscillator.  When properly set up, you should be able to lock in and fill up the scope screen with a wide range swept signal.  20Hz-20kHz is great for covering the entire audio range; a narrower range like 100Hz to 10kHz would be fine as well.

Observing the phase shifter output this way, you should be able to see those "notches" you've read about that are created by the phase shift effect.  On the original Phase Shifter effect, the Maestro PS-1, you can use a trim pot to completely turn off the LFO signal, and just manually shift the phase with the FET bias trim.  Calibration is simple - you simply sweep the FET bias, observe the motion of the notches, and set the trim to where you think the "center" of the sweep is.  Then you add just enough LFO signal to get maximum sweep.  If you have too large of a LFO signal, the FETs will be abruptly cutting out and starting up again - this is audible as a "lopsided"/asymmetrical sounding phase shift instead of the "round and round"/symmetrical type of effect you normally want.

Not every phaser has a trim to turn off the LFO.  You don't have to do so to use this method.  If the LFO is running, it is simply a matter of watching and/or listening until the sweep appears symmetrical.

Not all dates are able to be confirmed.  +/-1 year error (or more) possible.

First Leslie rotating speaker.  These were designed for the Hammond organ, and became part of the classic Hammond organ sound.  The first phaser designers tried to make a compact Leslie simulator.

Various recording engineers experiment with phase interference effects created by mixing delayed signals together.  Many hit records are released featuring phase effects.  Such effects are confined to the recording studio since there are no commercial "phase shift" products available yet.

Magnatone amplifiers use phase shift to realize their vibrato effect (silicon carbide varistors)

Honey Psychedelic Machine (later repackaged as the Univox Uni-Vibe, optocouplers)

Countryman Phase Shifter Model 967 (1971?, optocouplers?)

EMS Synthi Hi-Fli (Designed by David Cockerell, OTAs)

Eventide Instant Phaser PS-101 (Notable for having an envelope follower and being used on the drums of Led Zeppelin's "Kashmir")

Electro Harmonix Small Stone (Designed by David Cockerell, OTAs)

Mu-Tron Bi-Phase (Optocouplers)

MXR Phase 100? (No release info, Optocouplers)

Mu-Tron Phasor II (Optocouplers)

Why does the timeline end here?  In 1974, Panasonic released the MN3001 bucket brigade delay chip.  In a few years, a wide variety of delay chips were available that could do very short delay effects usually called "flange" and "chorus," and these became the new darlings of the effects world.  The 1980s brought (relatively) cheap digital implementations to the stompbox.  The 1990s saw a major demand for the old analog designs, and old phaser designs were reissued.    

What is the difference between flanging and phasing?

What is meant by phase shifter?

What are the types of phase shifters?

What is a phaser?