the Liquidator in a working but unfinished state
the Liquidator in a working but unfinished state
This project involves the conversion of a Hammond AO-47 organ vibrato unit (Original Hammond Schematic) into a guitar/music phaser/chorus box. These devices can often be purchased for little money on eBay and provide the majority of the circuitry for this device. The Liquidator name comes from the liquidy sounds that the device creates. It is necessary to construct a chassis and power supply and the AO-47 box needs some modification to convert it to the phaser/chorus circuit. Additionally, an LFO circuit needs to be constructed to make the appropriate control waveforms. Thanks to the tube circuitry, the audio quality of this phaser is far superior to a transistor or op-amp based phaser. A wide variety of interesting sounds can be produced with this unit.
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.
AC Power Input - grounded 120VAC Guitar Input - High Impedance Amp Output - High Impedance
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
V4a is used as a 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 primaries of the three transformers are modulated with a low frequency waveform. This causes the transformers' reactance value to vary. 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.
V4b is uses 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.
Inside the modified AO-47 chassis, the triple phase shift transformer is on the right.
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 still 12AU7 parts. The tube designations are as in 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 may be necessary to add newer 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 provided 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 is roughly equivalent, these can be purchased from Antique Electrical Supply.
There are several ways to make the LFO circuit. It should produce a waveform which falls in the 0-5V range. The waveform can be a simple sine wave or a more complex wave. One possible LFO circuit is shown in the Hammonator 2 amp schematic, the Liquidator modulation input should be connected between ground and 1458 IC pin 1. This Waveform Generator project can also be used if the 10K resistor on the LM741 pins 2-6 is changed to 5K.
I am currently experimenting with an Arduino microprocessor board driving a DAC circuit similar to that in the Hammonator2 LFO. The Arduino board can connect to a PC's USB port and be programmed to produce a variety of interesting wave shapes. (more to come).
Plug an electric guitar or other musical device into the input, plug the output into an amplifier or mixing board. For a number of great stereo effects, split the instrument input between this device and a "clean channel" guitar amp, then plug the Liquidator output to a "phaser channel" amp. This is the recommended configuration for this effect. Try it, you won't be disappointed.
Many variations on the basic phaser effect are available by adjusting the four volume controls, the phase switch and the feedback control. For a simple phaser, put the Clean and Phase 3 knobs up and the Phase 1 and Phase 2 knobs down. For a chorus effect, put all four mixer knobs near full and adjust for the best sound. For a Magnatone amp sound, put the Phase 2 knob up and the other three knobs down. The feedback control adjusts the circuit resonance from broad and swishy to narrow and whistly. The LFO speed and intensity controls are counter-interactive. High intensity sounds best with the LFO speed set to slow, the intensity can be lowered for higher LFO speeds.
