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Quadratron Twin Phaser

Based On
Lovetone Doppelganger
Effect Type
Phaser
Build Difficulty
Advanced
Project Summary
A stunningly unique four-stage optical phaser with two LFOs allowing independent control over each pair of stages. Used by The Edge, Johnny Marr, and Ed O’Brien among others.
Quadratron Twin Phaser printed circuit board

Printed Circuit Board

What's included?
PCB, build instructions, and parts list.

$22.00

In stock

Complete Kit

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Project overview

The Quadratron Twin Phaser is based on the Lovetone Doppelganger, a four-stage optical phaser first released in 1995.

As with most Lovetone pedals, it’s a traditional topology at its core, but with a huge twist. In this case, each of the four optical phase stages have different filter frequencies, with each stage having a capacitor twice as large as the previous stage. Each pair of stages has its own depth (intensity) control, called Low Frequency Span and High Frequency Span, and each pair of stages can be controlled by a single LFO, separate LFOs, or a single LFO with each pair out of phase with the other (i.e. when stages 1 and 2 are off, stage 3 and 4 are on). Each pair has its own rate/depth LED, so throughout each of the modes there is always a visual representation showing how the signal is being modulated.

The Doppelganger was updated in 1999 with three new features: a Dry Out jack, allowing for stereo operation when combined with the wet signal in the other output; a Square Wave toggle to give the first LFO a “chop” or on/off effect; and a Slow toggle that modifies the first LFO’s range so the slow speeds are even slower. Both of these features are backwards-compatible, so with Square and Slow turned off, it’s identical to the V1 circuit.

The Quadratron is based on this second version of the Doppelganger, but with a few tweaks to the bypass modes to correct some of what could be considered design flaws in the original unit. You’ll find more on this in the pages that follow if you’re curious. The signal path is unchanged, the only differences are in the bypass configurations.

Special thanks to Ian (LaceSensor / Gigahearts FX), the DIY community’s resident Lovetone expert, for help verifying the Quadratron prototype against an original Doppelganger for accuracy.

Usage

The Quadratron has the following controls.

Potentiometers

  • LFO 1 Rate and LFO 2 Rate control the speed of the LFOs.
    • In single LFO mode, LFO 2 is disabled and LFO 1 controls all four stages.
    • In dual LFO mode, LFO 1 controls the first two stages and LFO 2 controls the last two stages.
  • HF Span sets the depth (intensity) of the first two stages that sweep higher frequencies, with an LED to indicate the effect visually.
  • LF Span sets the depth (intensity) of the last two stages that sweep lower frequencies, with an LED to indicate the effect visually.
  • Color (also called Feedback, Resonance or Regeneration in other phasers) sets how much of the phased signal is fed back into the input to amplify the phasing effect. Since it’s fed back to the 2nd stage, it only impacts the 3rd and 4th stages (the LF pair).
  • Blend controls how much of the phase signal is blended with the dry signal. In vibrato mode, which cuts off the dry signal entirely, it acts as a volume control.

Switches

  • Vibrato cancels the clean signal, which results in a pitch vibrato effect. Note that the Blend knob becomes a straight volume control in this mode, so you’ll likely want to turn it all the way up.
  • Antiphase sweeps the last two stages inversely to the first two, causing the high frequencies and low frequencies to be modulated in an alternating pattern similar to a harmonic tremolo.
  • Slow changes the timing capacitor of LFO1, allowing for extra-long sweep cycles of up to 16 seconds at the slowest speed setting. LFO 2 is unaffected. See build notes for a modification that allows the slow time to be extended even more.
  • Square converts LFO 1 from a triangle wave (ramping up and down) to a square wave (full on & off). LFO 2 is unaffected.
  • LFO Cancel disables the LFO entirely, equivalent to the spectral mode of the original unit. The signal still passes through the phase stages which changes the tone slightly, but there is no modulation.
  • Dual LFO (footswitch) engages the second LFO, which makes the HF and LF phase stages fully unsynchronized with each other and controlled by their own Rate knobs.