MOSPhaser: Voltage Controlled Phaser Module

Constructional Difficulty: Easy

The Oakley Sound MOSPhaser is a six stage phaser (five variable all pass filters and one fixed all pass filter) that generates a classic sound. Partly based on a well loved 1970s guitar effects pedal the MOSPhaser offers improved frequency range, a more comprehensive modulation oscillator and external voltage control.

Each of the five all pass filter stages uses one of the MOSFET transistors inside a CMOS invertor chip, the 4009UBE, to control the centre frequency of the stage. By using a single integrated circuit for all transistors means that no matching of devices is required as would be the case if we were using discrete components.

The module is easy to build and requires only a simple voltmeter to set up.

The unit includes a useful internal low frequency oscillator (LFO) with accompanying bicolour LED. The centre frequency of the phase shifter can also be controlled by an external control voltage. This allows envelope generators to be used to sweep the phaser to create filter type effects.

Four front panel pots are provided:

Frequency: this controls the frequency of the two notches created by the phase shift network.

Emphasis: this accentuates the feedback signal within the phaser to create a deeper effect.

Modulation: A simple attenuator to adjust the level of the internal LFO or incoming CV.

LFO rate: adjusts the speed of the LFO.

The internal LFO produces a triangle wave output, +/-5V, which is also available from a front panel socket, LFO OUT. This allows you to use the MOSPhaser's internal LFO for other modulation purposes. The frequency range of the LFO is designed for slow sweeps covering 7.5Hz to 0.01Hz (one cycle every 70 seconds) although this can be changed by using a different timing capacitor. The LFO signal is automatically routed to the modulation depth pot when no jack is inserted into the CV IN socket. The module features an on board jumper to select whether the LFO OUT signal is inverted or non inverted compared to the signal modulating the phaser circuitry.

The unit is designed to work with standard MOTM/Oakley signal levels, although it could be easily converted to be driven from other sources including an electric guitar.

It is possible to purchase the Sock4 circuit board which is available separately. This little board speeds up the wiring of the four 1/4" sockets and reduces the chances of any wiring errors.

Power (+/-15V) is provided to the board either by our standard Oakley 4-way header or header. The board size is 143 mm (high) x 77mm (deep). Current draw is around 35mA.

The MOSPhaser as a MOTM format module in a natural finish Schaeffer panel.

Sound Samples

Pink Noise through the phaser using the internal LFO. First half is with no emphasis and then the emphasis increased during the second half returning to no emphasis at the end.

Two Oakley VCOs set to sawtooth and using the phaser's internal LFO. The emphasis and rate controls are manually altered during the sample. Some light reverb is used for ambience.

Single sawtooth VCO used to play a simple riff. The phaser's frequency control is manually adjusted.

Two VCOs, one sawtooth and the other a triangle wave an octave below. The phaser is being swept with a fairly fast envelope triggered on each note. The phaser frequency is manually adjusted. No filter modules are used but a simple delay is used to add interest.

The optional Sock4 socket board helps keep the wiring to the sockets neat and tidy.


Click here for how to order.

MOSPhaser issue 1 PCB ...................................... 18GBP

Sock 4 issue 2 PCB ................................................. 5GBP
MOSPhaser pot bracket kit ..................................... 0.75GBP

1K +3300ppm/K temp co resistor .......................... 1.50GBP

The optional Pot Bracket Kit contains the three 16mm Alpha pot brackets required to complete one module.

All other parts can be purchased from you usual electronic component supplier.

All prices include VAT at UK rates. Shipping and postage is additional to these prices. See also the FAQ page.


Builder's Guide

Construction Guide Our handy guide to building Oakley DIY projects

Parts Guide Our handy guide to buying parts for Oakley DIY projects.

Schematics are only available to purchasers of the PCB and will be sent via e-mail when the board(s) are shipped.

Front Panel database

A Schaeffer front panel can be made for this module. The database can be found by downloading the following link:

5U format in traditional black

5U format in natural finish

To read these files you will need a copy of 'Frontplatten designer' from Schaeffer. The program also features on-line ordering, so its now even easier to buy your panels from Scheaffer.

The company are based in Berlin in Germany and will send out panels to anywhere in the world. Delivery to the UK normally takes around ten days. For North American users you can also order your Schaeffer panels from Front Panel Express.

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Copyright: Tony Allgood.
Last revised: December 04, 2023.