Phaser + Dual Audio Mixers

Description

The Low-Gain Electronics LGE090 Phaser + Mixers combines the classic Serge three-stage Voltage Controlled Phaser with two independent audio mixers in a compact 4U module. Based on the original Serge Phaser and Mixer circuits, the LGE090 provides a versatile collection of phase-shifting, audio mixing, and feedback functions for sound processing and timbral experimentation. The phaser provides three simultaneous outputs with 360°, 720°, and 1080° of voltage-controlled phase shift. Each output offers a different phase relationship to the original signal, allowing a wide range of spatial and timbral effects. A dedicated MIX output combines the original input signal with the 1080° phase-shifted signal to produce the characteristic sweeping notches associated with traditional phaser effects. Manual phase adjustment, voltage control, and feedback allow the phaser to produce everything from subtle tonal movement to pronounced resonant sweeps and complex phase interactions. Two independent AC-coupled, inverting audio mixers expand the module's capabilities beyond conventional phasing. These mixers can combine the various phase-shifted outputs, blend processed and unprocessed signals, or introduce external feedback paths for more experimental sound processing.

Circuit Description

Voltage Controlled Phaser

The LGE090 Phaser is based on the original Serge three-stage phase-shifting circuit. Each stage consists of two cascaded all-pass filter sections, providing a total of six voltage-controlled phase-shifting sections. The circuit uses TL074 operational amplifiers and optically controlled resistive elements (vactrols) to vary the frequency response of the all-pass networks. The optically controlled resistance allows smooth phase sweeps while maintaining electrical isolation between the control circuitry and the audio signal path. The three outputs are taken at successive points in the phase-shifting network, providing nominal phase shifts of 360°, 720°, and 1080°. These outputs are available simultaneously and can be used independently or combined externally.

Unlike the dedicated MIX output, the individual 360°, 720°, and 1080° outputs do not produce the characteristic sweeping notch-filter effect typically associated with a phaser. This effect is created by combining a phase-shifted signal with the original, unprocessed audio, resulting in frequency-dependent cancellation. The onboard MIX output combines the original signal with the 1080° phase-shifted output to produce this effect internally. The individual outputs can also be combined using the onboard audio mixers to explore different phase relationships, cancellations, and more complex filtering effects.The phaser also includes manual phase control, voltage-controlled phase adjustment, and a feedback control for increasing the intensity and resonance of the effect.

Phaser PCB Versions: Two versions of the LGE090 Phaser circuit are available, differing primarily in their choice of optically controlled resistive elements.

Version 1 – 5C3/2 Vactrols: The original LGE090 circuit uses three dual-element 5C3/2 vactrols, providing the six optically controlled resistances required by the phase-shifting network.

Version 2 – NSL-32 Vactrols: The LGE090N circuit uses six individual NSL-32 vactrols in place of the three dual-element devices. This version retains the same fundamental three-stage phase-shifting architecture, with modifications to accommodate the individual vactrol configuration.

Both versions provide the same basic phaser functions and output configurations. Differences in vactrol response, resistance characteristics, and component tolerances may affect sweep behavior and control response.

Audio Mixers

The LGE090 includes two independent audio mixers derived from the original Serge R2 Mixer circuit, with some updates based on later designs. Each mixer is an inverting, AC-coupled summing amplifier with four inputs. Three inputs are intended for variable-level mixing, while the fourth provides a fixed-level input suitable for cascading mixers or introducing additional signals. The original circuit used a 741 operational amplifier, although more modern devices such as the TL071 can be substituted. Because the mixers are AC coupled, they are intended for audio signals rather than DC control voltages. This is particularly useful when experimenting with feedback, as the coupling capacitors prevent DC offsets from accumulating through interconnected mixing stages. The two mixers can operate independently or be interconnected to create larger mixing arrangements. They can also be used to combine the phaser's individual outputs, introduce additional audio signals, or construct external feedback paths. When mixing phase-shifted signals, the inverting nature of the mixers provides additional opportunities for cancellation and reinforcement, producing a variety of complex filtering and timbral effects.

Panel Functions

Phaser Section

INPUT 1 – Audio input to the voltage-controlled phaser. The associated attenuator adjusts the level of the incoming signal.

360° – Output from the first phase-shifting stage, providing a nominal 360° phase shift.

720° – Output from the second phase-shifting stage, providing a nominal 720° phase shift.

1080° – Output from the third phase-shifting stage, providing a nominal 1080° phase shift.

MIX OUT – Combined output of the original input signal and the 1080° phase-shifted signal, producing the characteristic sweeping notch-filter effect.

CV PHASE – Control voltage input for the phaser. The associated attenuator adjusts the amount of external voltage control applied to the phase-shifting circuit.

PHASE – Manual control of the phase-shifting network, adjusting the center position of the sweep.

FDBK – Feedback control. Adjusts the amount of phase-shifted signal returned to the phaser circuit, increasing the intensity of the effect. This function utilizes one of the mixer circuits on the PCB.

Audio Mixer Sections

IN 1 / IN 2 – Audio inputs for the mixer sections. The associated level controls adjust the amount of signal introduced into each mixer.

MIX OUT – Summed audio output from the corresponding mixer section. The mixers are inverting and AC coupled, making them suitable for audio mixing and feedback applications.

Patching Suggestions

Classic Phaser: Patch an audio signal into the phaser input and monitor the MIX output. Adjust PHASE to sweep the frequency response manually, or apply an LFO or envelope to CV PHASE for animated movement. Increase FDBK to emphasize the effect.

Multiple Phase Outputs: Patch the 360°, 720°, and 1080° outputs into separate mixer inputs. Experiment with different combinations and signal levels to create phase cancellation, reinforcement, and more complex frequency responses.

External Feedback: Use one of the onboard audio mixers to combine the incoming audio signal with a selected phase-shifted output, then route the mixer's output back into the phaser input. Adjust the levels to explore alternative feedback arrangements and resonant effects.

Cascaded Audio Mixing: Use the two independent mixers to combine multiple audio sources, or patch the output of one mixer into an input of the other to create a larger mixing arrangement.

Short Marketing Description: The LGE090 Phaser + Mixers combines the classic Serge three-stage voltage-controlled phaser with two independent AC-coupled audio mixers. Featuring 360°, 720°, and 1080° phase outputs, onboard phase mixing, voltage control, and feedback, it offers a wide range of traditional phasing effects and experimental audio processing possibilities.

** 4U Modular is a term used for the format most commonly known as “Serge Format” or “Loudest Warning Format”. Out of respect for the ever growing format, 4U Modular is the easiest way to refer to it. More Specifically it refers to the panel height and mounting hole style. 4U Modular will patch up just fine with other 4U "Serge" formats such as "Random Source", but it will not mount in RS boats or Buchla Boats (or power off Buchla power for that matter). An info page about this will be added to the website soon to make this a lot more easy to understand.

Alternative 5C3/2 Vactrol:
Mouser: 846-NSL-37V5C3/2
Digikey: 209-NSL-37V5C3/2-ND

LGE090 Phaser + Dual Audio Mixers DIY Documentation:

LGE090 Main Board V2 (uses NSL32 Vactrols)
LGE090N Main PCB V2 Bill of Materials
LGE090N Main PCB V2 Schematic (NSL32)
LGE090N Main PCB V2 Layout w/ Designators

LGE090 Main Board V1 (uses 5C3/2 Vactrols)
LGE090 Main PCB v1  Bill of Materials
LGE090 Main PCB v1 Schematic
LGE090 Main PCB v1 Layout w/ Designators

LGE090 I/O Board
LGE090C I/O Bill of Materials (Intended For use w/ LGE090N Main PCB V2)
LGE090C I/O PCB Schematic (wiring diagram) (Intended For use w/ LGE090N Main PCB V2)
LGE090C I/O PCB Layout w/ Designators (Intended For use w/ LGE090N Main PCB V2)

LGE090 Panel Artwork

Mechanical Parts BOM generally required for building Low-Gain Electronics Modules

Details:

• PCB Size: 6” x 2”

• Current Draw: TBD