This is the original MXR™ Phase 90™ Phase Shifter (script logo) Replica known as P90. This is a classic phase shifter. We chose to use this layout since the 741 ICs seem to be part of the “magic” sound of the old Script Logo version. The layout is very similar to the original MXR layout.
Has anybody done the R28 or C11 and C12 mods to their Phase 90? Did it make a differnce in sound? Would removing these resistors change the output lev.

Mxr Phase 90 Script Schematic
Vintage Mxr Phase 90 Schematic
Mxr Phase 90 Circuit Diagram
What Does A Mxr Phase 90 Do

The Phase 90 is a phaser guitar pedal designed by Keith Barr in 1974. It was the first product sold by MXR with great success. MXR’s contribution to phase-shift effects was the high-quality, small size and reliability of the stompbox, in contrast with previous models that tended to be dodgy, bulky and noisy.

Production ceased when MXR went bankrupt in 1984. Jim Dunlop bought MXR in mid-late 80's, resuming production and adding extra features like LED and external power supply jack.

MXR Phase 90 US$ 11.00. Possible mods: 'The resistor you pull to get rid of the feedback (and change the sweep) is the red 22k at the top of the schematic.

The old releases by MXR included the so-called Script and BlockLogo models. Dunlop nowadays offers few variants: Handwired 1974 Vintage model, Custom Shop Script, MXR M-101 Phase 90 and EVH Eddie Van Halen signature model. This study is focused on the first Script Logo model which is considered to perform the best.

Table of Contents.

1. The Phase Effect.
2. MXR Phase 90 Circuit.
3. Power Supply.
4. Input Stage.
5. Phase Shifting Stage.
5.1 MXR Phase 90 Shifting Unit.
5.2 FET as Variable Resistor / 2N5952 Matching for Phase 90.
5.3 Phase 90 Notches Frequency.
5.4 The R28 Feedback Resistor.
6. Low-Frequency Oscillator.
7. Output Mixer Stage.
8. Interview with Don Morris - MXR Engineer
9. References.

A Phaser Effect pedal splits the guitar signal into two copies: the first copy suffers a phase shift of 180° (is inverted) at a certain frequency f_phaser, the second copy is the same as the original. Both signals are recombined in a single signal, as a result, a notch (signal cancellation) at f_phaser is created.

The notch can be swept up and down through the frequency band, creating a rippling, doppler-like effect.
The effect is more dramatic by placing more than one notch. MXR Phase45 has 1 notch, Phase90 and Univibe have 2 notches and Phase100 uses 3 variable notches.

The MXR Phase 90 schematic can be broken down into 5 simpler blocks: Power Supply Stage, Input Buffer, Phase Shifting Stage, LFO and Output Mixer Stage.

The circuit is designed around a Phase Shifting Stage with an Input Buffer to avoid tone sucking. The last Output Mixer Stage will combine the wet and dry signal in order to create the signal cancellation. The LFO creates the sub-sonic carrier signal that modulates the notches, shifting them among the frequency band.

The MXR Phase 90 modulation is controlled by one knob which set the speed of the effect (frequency of the LFO).

MXR Phase 90 Part List / Bill of Materials.

Although this study covers the Script Logo circuit, the components are labeled following the Block Logo model. The differences between both models are small and in this way, the Block logo can be modded into the Script logo easily.

C₁,C₂,C₃,C₄,C₆,C₉: 47nF
C₅: 0.01u
C₇: 0.01
C₈,C10: 15uF
D₁: 1N914
D₂: 5.1V Zener
Q₁: 2N4125
Q₂,Q₃,Q₄,Q₅: 2N5952
R_trimmer: 250K
R₁,R₃,R₅,R₁₀,R₁₁,R₁₂,R₁₃,R₁₅,R₁₇,R₁₈: 10K
R₂,R₇,R₈,R₁₆,R₁₉,R₂₄: 150K
R₄: 56K
R₆,R₂₃,R₂₅,R₂₆,R₂₈: 24K
R₁₄,R₂₁,R₃₆: 470K
R₂₀: 1M
R₂₂: 3M9
U1b,U1a,U2d,U2c,U2b,U2a: TL061

MXR Phase 90 Guts / PCB Layout.
The PCB is a single layer board stuffed with op-amps, fets, and passive components. The original MXR and Dunlop layout suffered modifications; the first models used 6 single op-amps while the latest utilize 3 dual op-amps. The layout and jacks/switches placement changed during the different productions. Last models include SMD components and PCB mounted jacks.

Phase90 Models: This is the summary of the different versions of the Phase 90 circuit.

MXR era:

Script Logo:using 6 single 741 op-amps by National or TI (like the current MXR CSP-026 '74 Handwired version).
Block Logo: started with the script logo version PCB inside. During the years some circuit modifications appeared: first revisions include 6 single op-amps later a feedback resistor R28 was added. Finally, the six single op-amps were replaced for three dual op-amps. The LED and the adapter jack were added during the process.

Dunlop era:
First releases started with the last revision of the Block Logo PCB. Again circuit modifications happened: around 1989-1990 the reissue line added an input buffer stage to avoid tone sucking, the PCB layout was completely redone. Improvements also affected the DPDT switch and adapter jack.

MXR CSP-026 Handwired 1974 Vintage Phase 90 ($119): Reissue of the original 1974 vintage phase pedal, basically a copy of it. It is hand-wired and uses 6 single op-amps like the original 1974 Phase 90 Script Logo. Not true bypass, no LED, no AC connector (runs on a 9V battery).
MXRCSP-101CL Custom Shop Script Phase 90 ($99): It is a regular '75 block model in a and script-like case, actually the same EVH circuit, with the 24K resistor missing. Not true bypass comes with a LED and AC adapter.
MXR M-101 Phase 90 ($79): It has the block logo circuit, with feedback resistor and an added buffer stage on the circuit input, to avoid tone sucking. People clip out R28 (the feedback resistor on) to make them sound closer to the script logo versions. Not true bypass comes with a LED and AC adapter.
MXR EVH Eddie Van Halen Phase 90 ($129): This is signature pedal designed by Dunlop for Eddie Van Halen. It has a script button to toggle into a script circuit or a regular crunchy circuit (switching the R₂₈ feedback resistor). It has a completely redesigned circuit layout, using SMD components. Not true bypass comes with a LED and AC adapter.

Concerning the op-amp, the original MXR Script Logo had a Texas instruments UA741CP single op-amp, next versions with dual op-amps contain the TL062. Any compatible standard op-amp like TL072, TL082, TL022, 4558, etc is suitable for the job.

The Power Supply Stage provides the energy and bias voltage to the circuit.

The 9V supply will feed the op-amps. The Zener diode is used to generate a voltage reference of 5.1 volts. The resistor divider R_trimmer adjusts the bias voltage is to be

Mxr Phase 90 Schematic