MUSITRONICS: Mu-Tron Phasor II

A classic phaser from the mid-seventies - the Musitronics Mu-Tron Phasor II.  It has the same six-stage phaser as the Bi-Phase, with less happening in the oscillator section.  

This is the hardest layout I have ever done and took some time to finish and check...  including some time wasted after I started using a layout with errors in the oscillator section, which was very annoying.   

Being a six-stage phaser using DIY vactrols - what could possibly go wrong?  Unverified at present / build at your peril.

This should just fit in a Hammond 1590BB enclosure (landscape),  The dual rail power supply will need to be done on a separate board.  

I have not wired as per the original, as it was not true-bypass, and it has a relatively low input impedance of 390k.  If you really want to wire it up as the original, just check an old schematic, and you should be able to modify a layout to accomplish this.  

MU-TRON PHASOR II - VERO LAYOUT

Pedal PCB values/schematic were used for the layout above, which is slightly different to one of the original schematics that I've seen.  This is running at +/- 9v.

MU-TRON PHASOR II - VERO LAYOUT - LFO WAVE SHAPE MOD

After looking in awe at the Mu-Tron Bi-Phase schematic, I realised that most of it are actually the same, or at least quite similar to the Phase II.

The mod below adds a sone / square wave selector.  I'm surprised they never included this on the original, as it's pretty easy to add (bearing in mind that this is currently an unverified layout).

MU-TRON PHASOR II - VERO LAYOUT V2

This layout is based on an original schematic, it's running at +/-12v, and it has very minor differences here and there - mainly in the LFO.  

MU-TRON PHASOR II - CALIBRATION

Note this refers to the older AC-powered unit, which is the layout directly above - please don't connect anything to mains power, or you may die in the process.  

In the instructions, it mentions changing R37 from 68k to 82k if there are flat spots on the waveform.  R37 is the resistor from Depth 2, connecting to pin 3 of IC5.  

MU-TRON PHASOR II VS BI-PHASE

A quick note on the main differences between the Bi-Phase and the Phasor II, as they are actually very closely related.  For starters, there are two phasers in the Bi-Phase - obviously.

Common

• The six-stage phaser

• The input section is basically the same, along with the feedback control

• LFO oscillators have the same basic topology in terms of waveform generation

• Sweep, Rate and Depth controls

Bi-Phase, Main Differences

• LFOs have sine and square waves

• LFO A rate can be controlled by an expression pedal

• LFO B can be inverted or synced to LFO A

• Additional routing controls, cause there are two phasers to play with - as an example, route Phaser A into Phaser B, giving a 12-stage phaser... 

• Switching between Phaser A & B is controlled by a vactrol in the signal path of the output stage (audio section).  i.e. an optical switch.  When engaged, the switches increase resistance in the signal path to the point where audio does not pass.