Understanding the Classic MXR Phase 90 Guitar Pedal Schematic and Circuit Design

Understanding the Phase 90 Guitar Pedal Schematic

The Phase 90 is a classic guitar pedal known for its warm, swooshy phaser effect. First introduced in the 1970s by MXR, the Phase 90 has been used by countless guitarists over the years, from Eddie Van Halen to Trey Anastasio. But what exactly makes this pedal tick? Let's take a look at the Phase 90 schematic and break down how this iconic stompbox works its magic.

The Basic Circuit

At its core, the Phase 90 is a four-stage phaser that uses an all-analog design based around operational transconductance amplifiers (OTAs). The input signal from your guitar hits the first OTA, which drives the phase shift network. This network consists of resistors, capacitors, and OTA amplifier stages connected in a feedback loop. The phase shift network splits the signal and shifts one path out of phase by up to 90 degrees compared to the direct path. These two signals are then mixed back together to produce phasing effects.

The phase shift network on the Phase 90 uses four sets of resistors and capacitors to divide the signal into different phase-shifted paths. By changing the resistor values, the center frequency of the phaser sweep can be adjusted. The capacitor values affect the range or depth of the phaser effect. There are also fixed resistor and capacitor values that control the feedback within each OTA stage.

Controlling Speed and Intensity

Two key controls allow guitarists to tailor the Phase 90 effect. The Speed knob controls the rate of the phaser sweep by adjusting an internal control voltage. Turning the Speed knob changes the voltage on a variable resistor, which feeds into the OTA stages. Higher control voltage speeds up the phaser rate.

The Intensity knob adjusts the feedback within the phase shift network. More feedback accentuates the peaks and notches in the phaser sweep, resulting in a more pronounced "whooshy" sound. Lowering the feedback smooths out the phaser effect.

The Phase 90's Distinct Sound

So what makes the Phase 90 sound so unique? A few key factors contribute to its distinctive swooshy, almost vocal-like tone: The four-stage OTA design - Having four phase-shifted paths allows for a more dramatic swept phaser sound compared to two-stage designs. The extra stages increase the number of peaks and notches. Feedback - The Intensity control allows guitarists to dial in a lot of feedback, accentuating the phaser effect. The Phase 90 is known for its almost over the top whooshing sounds at higher Intensity settings. The sweep range - Having a wider sweep range spanning up to 90 degrees (as the name implies) allows the Phase 90 to cover a broader spectrum of phasing effects. Warm analog tone - The all-analog circuitry gives the Phase 90 a fat, warm sound compared to digital phasers. The OTA amplifiers and phase shift networks impart natural fuzz and overdrive when pushed hard.

Combined, these elements create the Phase 90's signature vocal-esque tone. From subtle pulsing backdrops to intense jet engine whooshes, this pedal delivers iconic phaser effects perfect for funk, rock, blues, and more.

Getting to Know the Controls

Let's get familiar with each of the knobs and switches on a standard Phase 90 pedal: Speed - Controls phaser sweep rate. Turn clockwise for faster speeds. Intensity - Controls feedback and depth of the effect. Higher settings create more dramatic phasing effects. On/Off Switch - Engages/bypasses the effect. LED indicates when effect is active. Script/LED Switch - Toggles between "Script" and LED modes. Script mode emulates the original models with an orange "Status" LED. LED mode has a green LED. Power - Standard center negative 9V DC adapter input.

That covers the basic controls. The Phase 90 also features an expression pedal input for external control of the Speed or Intensity. Pretty straightforward! Now let's dive into actually using the Phase 90.

Using the Phase 90 for Great Tone

Here are some tips for getting the most out of your Phase 90 guitar pedal: Mind the intensity - The Intensity control has a huge impact on the sound. Try lower settings as a starting point, increasing slowly to taste. Max intensity can sound harsh in some amps. Adjust speed to match tempo - Synchronizing the phaser rate to the tempo often sounds best. Faster speeds work for effect accents. Place before distortion - The Phase 90 tends to sound best before overdrive/distortion. This preserves clarity and prevents an overly muddy tone. Use sparingly - A little Phase 90 goes a long way. Resist overusing it, and opt for slower speeds in most applications. Cut bass for clarity - Reducing bass guitar tone knobs slightly can help the Phase 90 cut through the mix better.

With a bit of tweaking, the Phase 90 can do everything from add thickening effects to a solo tone to provide a radical rotary speaker impression. Let your ears be the judge as you dial in speed and intensity settings.

Classic Phase 90 Pedal Tones and Settings

To nail iconic Phase 90 tones, try these speed and intensity combinations: Eddie Van Halen rhythms - Speed 10am, Intensity 9am. Thick, rich shaping. Swooshy soloing - Speed 2pm, Intensity 1pm. Vocal, dramatic tones. Trey Anastasio rhythms - Speed 9am, Intensity 10am. Nuanced movement and color.

As you can see, lower speeds and moderate intensities are the sweet spot for many classic Phase 90 applications. But of course, each player's tone is unique. Let your ears guide your knob choices!

From Legacy to Modern Era

The Phase 90 has remained in continuous production since the 1970s, a true testament to its legendary status. Here's an overview of the pedal's evolution and key models over the decades: Original "Script Logo" (1974-1981) - The hallowed original with hand-painted script logo. Used by EVH, Robin Trower, and others. "Block Logo" (1981-1984) - Updated version with block letters. Built-in LED instead of separate lamp. Reissue models - Numerous reissues aimed at capturing the mojo of the originals. Differing design approaches. EVH Signature (2004 - ) - Updated circuit tuned to Eddie Van Halen's specs, plus stripes! Modern Phase 95 (2010 - ) - Adds a Script/LED switch and expression pedal input. Increased headroom.

The Phase 90 circuit continues to evolve, but always remains true to the essence that made this pedal famous. From vintage NOS units to the latest updates, it delivers that swirly, dreamy phaser magic that guitarists have come to know and love.

Take Your Playing to the Next Phase

For over 40 years, the Phase 90 has been sculpting guitar tones with its signature swoosh and vocal character. This pedal demonstrates how a simple, timeless circuit design can inspire countless musical ideas. Whether providing a subtle undercurrent or a radical jet-engine tone, the Phase 90 delivers classic phasing effects that will boost your creativity. Experiment with the speed and intensity controls to find colors that spark new ideas. From funked-out rhythms to cosmic lead tones, get ready to take your playing to the next phase with the Phase 90!

FAQs

What type of circuit is used in the Phase 90?

The Phase 90 uses an all-analog circuit with four operational transconductance amplifier (OTA) stages to create its phaser effect. The OTAs drive the phase shift network that creates notches and peaks in the frequency response.

How does the Speed knob work?

The Speed knob adjusts an internal control voltage that is fed into the OTA amplifier stages. Higher control voltage speeds up the rate of the phaser sweep.

What does the Intensity knob do?

The Intensity knob controls the amount of feedback within the phase shift network. Higher feedback settings increase the depth of the phaser effect and accentuate the peaks and notches.

Why does the Phase 90 sound different than other phasers?

Key factors like the four OTA stages, high feedback, 90-degree sweep range, and analog circuitry give the Phase 90 its distinctive vocal-like, swooshy phaser tones.

What are some classic Phase 90 speed/intensity settings?

Eddie Van Halen's rhythm tone uses Speed around 10am and Intensity at 9am. For dramatic solos, try Speed at 2pm and Intensity at 1pm. Lower speeds and moderate intensities are common.