Watching ball games tonight and waiting on the wife with cold so I got to reading up on my Phase 90 after some playing with a JHS Phaser I keep at home. I've droned on several years ago on how nothing I tried sounded like my beloved '81/82 P 90. This is kind of a history I've gathered which explains why my 90 is different, it really was for about 3-4 yrs. All increases my gratitude to Dr. P for getting mine working again.
1. The Script Logo Era (1974 – 1977)
The original "Script" pedals (named for the cursive logo) are the most subtle.
The Circuit: Used six single 741 op-amps and had no feedback loop.
The Sound: Very smooth, "3D," and liquid. It doesn't "honk" or jump out at you; it blends into your guitar tone.
Identification: No LED, no power jack, and the famous "Script" logo on the front.
2. The Early Block Logo Era (1977 – 1980)
This was the first major shift toward a more aggressive sound.
The Change: MXR added the R28 feedback resistor. This sent a portion of the phased signal back through the circuit, creating a much more vocal, "chewy" resonance.
The Sound: This is the classic "Van Halen" era sound—more mid-forward and intense than the original Script units.
Identification: The logo changed to the bold "Block" font, but internally, they were still often using the six single op-amps from the 70s.
3. The Late MXR Block Era (1981 – 1984)
This represents the final design of the original MXR company.
The Circuit: They swapped the six single chips for three dual op-amps (TL062CP). They also added a DC power jack and a red LED.
The Sound: The most aggressive and non-subtle of the vintage units. It has a specific "growl" and grit because of how the early 80s chips interact with the feedback loop.
4. The Dunlop Era (1987 – Present)
After MXR went bankrupt, Jim Dunlop bought the brand and restarted production.
The Modern M101: These are the standard "Block" pedals you see in stores today. While they use the 80s feedback logic, they use SMD (surface mount) components and modern buffers.
The Conflict: These often sound "sterile" or too bright because they lack the organic component drift and the specific harmonic saturation of the vintage 80s chips.