Posts Tagged ‘ Phase 90 ’

Phase 90

there’s been a hole on my pedalboard these last few months and I finally got around to fixing it. I usually don’t use many modulation pedals at all. My board is usually an excessive amount of dirt pedals, one or two delays, and maybe a compressor. If I’m feeling extra adventurous I’ll add a tremolo. The only modulation pedal I really truly enjoy is a good ol’ phase shift. After selling my Black Russian Small Stone several months ago (due to it’s horrible bypass, enormous size, and failing capacitors) I decided instead of replacing it with another Small Stone I’d try building it’s arch nemesis, the MXR Phase 90. Using the Vero layout I found at Sabrotone.com and several tips from Tonepad.com I set out to build a Phase 90 with a couple tricks up its sleeves.

A little bit of an oversimplified background: The Phase 90 has 2 variations; The “Script” model, and the “Block” model. Basically, IIRC, the Script model was the first model released by MXR back in the 70’s. A few years later, they made several modifications to the circuit and thus the “Block” version was born. One of these modifications is the addition of a feedback resistor (the infamous R28) which adds a bit of distortion to the signal, but also makes the phase effect a bit more pronounced. The Script version lacks this feature, and is a bit smoother, more subtle, and cleaner. From previous experience with both models I decided to forgo the mode switch on the vero layout and just have a script mode.

Modifications: One of the first modifications I wanted to add was an “Intensity” or “Depth” knob. There are two schools of thought regarding how to do this, and they both involve replacing the 1M resistor (R17 on Harold’s layout) with a resistor and a pot. I’ve heard of people using a very small valued resistor (like 680 ohms) with a 1M Linear pot, but for those of us who don’t want that much versatility you can swap in a 500K-ish resistor with a 500K pot. I opted for the latter, and it still gives me quite a range of sounds that I can fine tune.

Modified Phase 90 Gutshot

Speaking of fine tuning, the other modification I added was a quasi volume pot. Removing the 150K resistor located between the base and collector of the 2n4125 (R_____ on Harold’s layout) and replacing it with a 100K resistor in line with a 100K pot acts like a very small range volume pot that will help greatly in matching the bypass volume with that of the effected signal. I always hated the volume drop on my Small Stone, and while the volume pot doesn’t have a great amount of “range”, it certainly helps in achieving a balanced level.

Matching JFETS: The most important part of building a phase 90 (or 45, or 100) is ensuring that you use matched JFETS. Instead of buying a pre-matched quad I decided to buy a bulk set (50) and match them myself. Using the following schematic and my trusty multimeter I was able to find multiple matched quads for the same price as buying one pre-matched set.

JFET Matcher Vero

JFET Matcher Vero layout

Now if I decide to build another phaser down the line I’ll already have the JFETS matched and ready to go.

JFET Matcher

Hooked up to my DMM.

Finish: Here’s a fun idea. I haven’t yet decided on artwork for this pedal, so feel free to comment and let me know what you think I should put on the pedal. It has a bright green LED, if that helps with inspiration.

Phase 90

 

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