MOSPhaser

Six stage phaser with a classic sound

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.

www.oakleysound.com/MOSPhaser-e.htm