DIY Pedal Mods: MXR Phase 90 - Sounds Samples Now Added: 14/02/2022

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DIY Pedal Mods: MXR Phase 90 - Sounds Samples Now Added: 14/02/2022

Post by FuzzMonkey »

I took a little break from my Uni-Vibe project to sort a few mods on a family of MXR Phase 90 pedals I've been collecting over the last wee while. Most of them were purchased as is and were in need of some sort of repair, with the exception of one that was basically brand new and I've kept stock for comparison purposes.

Below: My family of MXR Phase 90s.
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Below: The lone stock MXR Phase 90.
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The first mod is my Superscript mod that overhauls the stock Phase 90 circuit to be as close as possible both electronically and sonically to the original Script version from the 70s while working within the confines of the current production circuit boards.

In order to archive this, a total of 28 components are removed or replaced due to either being the wrong value/type of the component (sometimes both) or because they simply do not belong in the original circuit. From the input stage to the power supply section, and everything in between. No stone has been left unturned in recreating that classic phase tone you know and love.

The only external difference between the stock version and this one is the status LED is orange rather than red.

Below: The Superscript modded MXR Phase 90.
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Probably the most common and easiest mod to perform on the Phase 90 is the so-called R28 mod; sometimes referred as the Script mod. This removes the feedback resistor known as R28. This, along with a few other changes to the circuit made the phase-shifting much stronger and more pronounced. Think Script logo versus Block logo at the expense of boosting and even causing distortion in the midrange.

Removing this resistor helps to mellow out the effect and while this is incorporated in the Superscript mod, it offers a slightly different yet subtle voicing to a more modern phase-shifting tone. Putting it on a switch, while a bit more involved, means you have the option of one or the other.

Below: The MXR Phase 90 modded with a switchable R28 mod.
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Next up is the Depth Change mod that adds a dedicated Depth control to adjust the overall intensity of the effect. As part of this mod, the status LED now blinks in time with the speed and the intensity of the modulation using a blue LED. This provides a visual representation of where the pedal is sonically set.

Below: The Depth Change modded MXR Phase 90.
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For this one, I designed a cleverly-shaped daughterboard that sits perfectly in the battery compartment to accommodate the extra circuitry. The only downside is that you can no longer use a battery with the pedal. If you can call that a downside.

Below: The Depth Change daughterboard.
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Lastly is the Uni-Phibe mod started out life as another common Phase 90 mod called the Uni-Vibe mod in which the four capacitors at each of the phase-shifting stages are simply changed to match that of the original Uni-Vibe. While this does add a little extra wobble to the sound, it is nowhere near the sound of a true Uni-Vibe. The LFO is the wrong type (triangle vs. sine wave) and doesn't have the unique character of the one found in the Uni-Vibe. And of course, the phase-shifting is done via Jfet transistors acting as variable resistors instead of the light bulb/LDR combination of a Uni-Vibe.

This quickly lead me down a bit of a Uni-Vibe rabbit hole. One of which I have yet to fully emerge from.

Below: The Uni-Phibe modded MXR Phase 90
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Like the Depth Change mod, I designed a daughterboard that contains the same LFO as you'd find in the original Uni-Vibe and the miniature light bulb and LDRs. Like the original, it has an Intensity and Volume control and the ability to switch between Chorus and Vibrato modes. And once again the status LED flashes in time with the speed and the intensity of the modulation; this time using a white LED.

Below: The Uni-Phibe daughterboard.
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The audio section uses the exciting op-amps and the whole thing runs at 9-volts kind of like the old Voodoo Labe Micro Vibe.

As always, enjoy!

~ Blake.
Last edited by FuzzMonkey on Mon Feb 14, 2022 7:47 pm, edited 5 times in total.
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jeremyb
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Re: DIY Pedal Mods: MXR Phase 90

Post by jeremyb »

Way cool Blake!!! Would love to hear some sound comparisons!
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Re: DIY Pedal Mods: MXR Phase 90

Post by rickenbackerkid »

Those daughter boards are very nice. Really excellent work. Where are your boards being made?

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Re: DIY Pedal Mods: MXR Phase 90

Post by MiniForklift »

Very cool, looks like this will be another deep dive for me. That’s a great collection you have there Blake! Also reminds me I need to get one of the ‘vintage’ ones of these on my board. Pretty much the only pedal that Kossoff ever used
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Re: DIY Pedal Mods: MXR Phase 90

Post by FuzzMonkey »

jeremyb wrote: Wed Feb 09, 2022 7:58 pm Way cool Blake!!! Would love to hear some sound comparisons!
Thanks Jeremy. I need to do some recording for the Uni-Vibe project so will do some A/B'ing of each mod with a stock pedal.
rickenbackerkid wrote: Wed Feb 09, 2022 9:49 pm Those daughter boards are very nice. Really excellent work. Where are your boards being made?
Thanks. I think so too. I get made PCBs made by JLCPCB.
MiniForklift wrote: Thu Feb 10, 2022 6:50 am Very cool, looks like this will be another deep dive for me. That’s a great collection you have there Blake! Also reminds me I need to get one of the ‘vintage’ ones of these on my board. Pretty much the only pedal that Kossoff ever used
I personally prefer the Superscript modded version of the Phase 90. Some of the mods can be stacked. For example, the Depth Change mod can be added to the switchable R28 mod or the Superscript mod. The Uni-Phibe mod is pretty much the only stand-alone mod.
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Re: DIY Pedal Mods: MXR Phase 90

Post by FuzzMonkey »

I totally forgot to add this video demo that Jason Herbert did for me comparing the stock standard pedal to a Superscript modded one when I created the original post. So better late than never!



Plus, here are a few audio clips comparing the stock standard version of the MXR Phase 90 with examples of each of the mods. Each example starts off with a clean sound of a quick loop recording into my Ditto Looper followed by the stock pedal, then the modded pedal. Each clip cycles between the stock and modded pedal a couple of times. The only exception is the Depth Change mod where I have just slowly swept the entire range of the depth control up and down a couple of times.









Enjoy!

~ Blake.
"There's no creativity without vulnerability."

www.fantailaudio.com

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