MOSPhaser: Voltage Controlled Phaser Module

Constructional Difficulty: Easy but with many surface mount parts.

The 6HP wide 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 4009UBM, 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. A switch provides the option of inverting the CV's polarity.

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 5Hz to 0.01Hz (one cycle every 90 seconds) although this can be simply changed to something faster by using a smaller value timing capacitor in your build. The LFO signal is automatically routed to the modulation depth pot when no jack is inserted into the CV IN socket.

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

The module's current draw, with no signal passing through, is approximately +/-38mA at +/-12V.

The PCB set

The MOSPhaser comprises of two printed circuit boards (PCBs) connected together with four 0.1” (2.54mm) single in line (SIL) headers and sockets. The main board on the rear of the module houses the power input and conditioning, and the all pass filter cascade. The pot board has the pots, switch, and sockets that are attached to the front panel, as well as the input and output circuitry and LFO. The main board is a four layer design, while the pot board is a two layer design. Each board is the same size; 29 mm x 107 mm.

The majority of the components are surface mount parts. The ICs are SOIC, while the resistors and capacitors are 0805. All components are standard parts.

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.


Click here for how to order.

MOSPhaser (Euro) main board issue 1 PCB........................................... 10GBP

MOSPhaser (Euro) pot board issue 1 PCB ............................................... 5GBP

Both boards are required to build the Oakley MOSPhaser for Eurorack project.

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

All prices include VAT at UK rates. Shipping and postage is additional to these prices. Click here for full price list and ordering details.

Project Downloads

MOSPhaser (Euro) issue 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 boards 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:

6HP Eurorack format in natural silver

To read, edit and print this file you will need a copy of 'Frontplatten Designer' from Schaeffer. Panels can be ordered via Schaeffer's website.

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

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Copyright: Tony Allgood.
Last revised: July 16, 2019.