The Maestro MPS-2 Mini Phase is a phaser from Maestro. Owner reports describe it as a pedal with a pretty big effect on tone, one that "reminds me a bit of the treble cut you'd get from an LPB-1" and that lends itself to overdrive1. The pedal's status is currently listed as on hold pending verification1.
The circuit: JFET stages with no drain-source resistors
The defining feature of the MPS-2 is its topology. As the Guitar FX Layouts technical write-up puts it, "The effect is slightly different from similar phasers in that it has no resistors between the drain and source of the JF"ETs1. In most JFET phasers each stage has a resistor between drain and source, so leaving them out changes how the stages are biased and how the sweep behaves.
That difference cuts both ways. One owner who built and played the circuit was blunt about it: "the resistor missing between the drain source and the JFET is really a poor idea. Even for the 2N4303s, some tweaking is" needed, and the lack of a resistor gives a strange turnaround in the phase sweep1. The same owner suggested the missing resistor may be offset by the devices themselves, calling it "a benefit of the 2N4303 JFETs used in the original"1. For builders, the practical takeaway is that JFET selection and biasing matter more here than in a conventional phaser design, and 2N5952 or similar substitutes have been suggested, though they may require circuit tweaking1.
Controls
| Control | What it does |
|---|---|
| Speed | Sets the rate of the phase sweep; on the original the pot runs upside down1 |
| Bias | Used to tweak the sound; an owner reports hearing things change when turning it alongside the Oscillator Amp pot1 |
| Oscillator Amplitude (trimmer) | Internal trimmer on top of the board; a recommended mod replaces it with an external pot controlling depth1 |
The speed pot quirk is worth knowing before you use an original: an owner describes the potentiometer indicator as upside down, "ie instead of 12 o'clock being about 1/2 way through the" range, with 6 o'clock as the halfway point1. The Oscillator Amplitude trimmer sits on top of the board, and the most obvious mod recommendation from builders is leaving it out and adding an external pot to control depth instead1.
What it does to your tone
An owner of the original pedal, posting a demo clip at several speed settings in April 2014, reported that "the pedal has a pretty big effect on tone. It reminds me a bit of the treble cut you'd get from an LPB-1. I think that i"t lends itself to overdrive1. In other words, this is not a transparent phaser: expect a noticeable tonal shift when it is on, with a treble cut character, and pairing it with overdrive or distortion is where the owner felt it worked.
Building one
Guitar FX Layouts published a stripboard (vero) layout for the MPS-2 on 17 February 2014, marked on hold pending verification, with a corrected layout posted the same day after a misplaced 150K resistor at IC3 was caught1. On enclosure size, the layout "should fit comfortably in a 125B mounted sideways, and maybe even a 1590B if someone has the patience"1.
The published layout was posted marked on hold pending verification, and a corrected version replaced the first one after a misplaced 150K at IC3. Builders should work from the corrected layout1.
Videos
Jimi Photon
Adam Smith
Questions
What JFETs does the original Maestro MPS-2 use?
The original MPS-2 uses 2N4303 JFETs. Builders discussing the circuit note that even with 2N4303s some tweaking is needed, and that 2N5952 or similar JFETs have been suggested as substitutes, which may require circuit tweaking to get the phase sweep right.
Why does the MPS-2 speed pot run upside down?
An owner of the original pedal describes the potentiometer indicator as upside down: instead of 12 o'clock being about halfway through the speed range, 6 o'clock is the halfway point. It is a quirk of the original control rather than a fault in a build.
What does the bias pot do on the MPS-2 Mini Phase?
The Bias pot is used to tweak the sound. An owner reports being able to turn the Oscillator Amp pot and the Bias pot and hear things change, so it shapes the character of the phase effect rather than acting as a set-and-forget trimmer.
What size enclosure fits an MPS-2 Mini Phase build?
The published stripboard layout should fit comfortably in a 125B enclosure mounted sideways, and maybe even a 1590B if someone has the patience for tight packing.
Does the MPS-2 Mini Phase change your tone when switched on?
An owner of the original reports that the pedal has a pretty big effect on tone, comparing it to the treble cut you would get from an LPB-1, and thinks it lends itself to overdrive.
Sources (1)
- Guitar FX Layouts: Maestro MPS-2 Mini Phase, Guitar FX Layouts (tagboardeffects.blogspot.com), 2014-02-17
