Liquidator Tube Phaser/Chorus Effect

(C) 2008 G. Forrest Cook

Front of the Liquidator, the two unlabeled controls on the right are LFO speed and intensity.

Introduction

This project involves the conversion of a Hammond AO-47 organ vibrato unit (Original Hammond Schematic) into a 4 phase guitar/music phaser/chorus box. The individual phase signals can be mixed for a wide variety of sounds. The Liquidator name comes from the liquidy sounds that the device creates, it wiggles the audio all around. Used Hammond vibrato units can often be purchased for little money on eBay and provide the majority of the circuitry for this device. The Hammond AO-41 box can also be used for this project, it has one extra 7 pin tube socket. The Liquidator's companion project is the FuzzniKator Tube Distortion/Preamp.

Builders should have decent technician and metalworking skills to build this project. A chassis and power supply with input output jacks, power line, knobs and switches needs to be constructed. The AO-47 box needs many soldering modification to convert it to the phaser/chorus circuit. Additionally, an LFO circuit needs to be constructed from an Arduino microprocessor platform to make the appropriate control waveforms. Thanks to the tube circuitry, the audio quality of this phaser is superior to a transistor or op-amp based phaser, the level of hiss is very low.

Warning

This circuit uses high voltages including 120 VAC and 320 VDC. The project should only be taken on by someone who has experience working with high voltage circuitry. The power should always be removed when working on the phaser. The circuitry is designed to discharge the capacitors when power is removed, but it's always a good idea to short out the electrolytic capacitors before working on the circuitry.

Connections

AC Power Input - grounded 120VAC
Guitar Input - High Impedance
Amp Output - High Impedance

Controls

On/Off (on the back)
Clean Signal Phase
Clean Signal Mix
Phase 1 Mix
Phase 2 Mix
Phase 3 Mix
Feedback
Output Level (trimmer on top of AO-47)
LFO Speed
Modulation Intensity

Theory

V4a is used as a High-Z input guitar preamp stage, it is similar to the input stage of many tube guitar amplifiers. A clean signal tap comes from V4a. V2b also produces a clean signal tap on the plate, but with the opposite phase. The Clean Phase switch selects between the two signals for different sounding effects when combined with the phase shifted signals.

The Hammond triple transformer array is the most important part of this circuit. Three identical tube stages (V2b, V2a, V3b) are biased to operate as phase inverters. The two primary windings of each of the transformers is wired in series, but out of phase. This allows the low frequency modulation wave that is applied to the primaries to modify the transformers' reactance without passing the modulation wave through to the audio chain. The 10nF output capacitor on each stage sees a signal that varies somewhere between the cathode circuit (in phase) and the plate circuit (out of phase), depending on the transformer's input current. Each successive stage adds more phase shift to the signal. It *may* be possible to replace the Hammond triple transformer with three standard transformers. A 120V to dual 6.3V transformer would probably work if the 6.3V windings were connected in series and out of phase then used for the modulation input.

V4b is used as a summing amplifier (mixer). It receives its input from the clean signal and three phase shifted signals. The output from V4b is sent to the output jack via the output level potentiometer. The clean signal can have its phase selected by the Clean Phase switch, this gives a wider array of effects. The resistors across the Clean Phase switch discharge the nearby capacitors in order to prevent a loud audio thump from happening when the switch is flipped.

The Feedback control routed some of the third stage phaser signal back to the input of the first phaser stage (v2b). The control adjusts the amount of feedback and varies the circuit's resonance.

Construction

Inside the modified AO-47 chassis, the triple phase shift transformer is on the lower right side.

The Hammond AO-47 chassis was modified (see photo). The tube on the left was changed from a rare 7247 to a common 12AU7. The oscillator circuitry was removed from this tube (V4) and replaced with the preamp and output amp circuits. The two other tubes are also type 12AU7. The tube designations in the schematic match the original design, V4, V2 and V3 (left to right) as shown in the photo. The filament and wiring was left in the original state. A fair amount of re-routing was performed on the ground wires. It is a good idea to add newer electrolytic capacitors across the large electrolytic cap, old electrolytic capacitors dry out and lose their value. The Electrolytic capacitor was left in place to provide some physical protection for the tubes.

The three RCA jacks were rewired to provide the input, output and modulation input connections. The 6.3VAC filament power was connected to the rectangular plastic connector. This keeps the filament wiring mostly out of the chassis in order to reduce the coupling of AC hum into the circuit. The high voltage DC supply and signal wires were run through the bottom of the box into the lower chasis. Be sure to leave the wires long enough to allow the AO-47 box to be opened. The audio input and output wires are routed via RCA plugs to the 1/4" jacks on the main chassis. Both 1/4" jacks are mounted with insulated shoulder washers to avoid ground loop hum. The audio signals to and from the mixer pots should be run through shielded coax cable. One end of the cables should be grounded inside of the A0-47 chassis.

An "M" shaped metal framing section was used for the chassis, this allows the AO-47 box to be slightly recessed. The power transformer was mounted on the back side of the chassis top and holes were drilled for the jacks, switches, potentiometers and power cord. A U-shaped metal frame should be constructed to cover the bottom and sides of the chassis. The two bridge rectifiers and first high voltage capacitor are mounted in the main chassis. The LFO board is mounted on the underside of the main chassis. The power transformer was pulled from some old electronic equipment. A Hammond P-T269EX transformer could be substituted if the rectifier circuit was changed from a 4-diode bridge to a center-tapped 2-diode full-wave circuit. The 390/2W resistor value may need to be changed so that B+1 is around 250V. The P-T269EX can be purchased from Antique Electrical Supply.

There are several ways to make the LFO circuit, the LFO should produce a waveform which falls in the 0-5V range. The most versatile LFO uses an Arduino microprocessor board driving a DAC circuit, see my Arduino LFO project for details. The Arduino LFO allows one to install arbitrary modulation waveforms via a PC's USB port for a variety of effects. The waveform can be a simple sine wave or a more complex wave. The Liquidator's modulation input should be driven by pin 7 of the 1458 IC on the Arduino LFO.

An alternative LFO circuit is shown in the Hammonator 2 amp project, the Liquidator's modulation input should be connected between ground and 1458 IC pin 1 on that LFO.

Use

The Liquidator will work like most guitar pedals, plug the guitar into the Liquidator input and connect the Liquidator output to a guitar amplifier. Depending on the output level of your guitar pickups, it may be worthwhile to use a boost pedal or other preamp device between the guitar and the Liquidator input.

For great stereo effects, split the instrument input between the Liquidator and a "clean channel" guitar amp, then plug the Liquidator output into a second amp. Stereo is the recommended configuration for this effect.

Many variations on the basic phaser effect are available by adjusting the four volume controls, the phase switch and the feedback control. Here are a few basic setting suggestions:


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