Modulation Oscillator (Part 6)
Designed to be used in conjunction with last month's multi-Delay project, this module helps to create chorus and flanging effects.
Continuing the series of build-it-yourself modules for the effects rack launched in the December 1984 issue, Paul Williams describes a CV modulation source whose features include sinewave output, variable duty cycle, key or CV controlled depth, triggerable sweeps and two independently variable outputs.
When it is required to produce effects such as chorus, flanging and true vibrato, the source of modulation plays a vital role in making the effect musical and unobtrusive. For this reason, the modulation function has been given its own separate module in the rack, rather than including it in with the appropriate effects modules, so that its performance and range of facilities are not compromised due to lack of PCB and panel space. The unit described here produces, nominally, a sine wave on two independently adjustable outputs over a wide range of frequencies, from gently swept flanging to fast vibrato. The waveform can be modified using a continuously variable shape control to produce rising or falling ramps, or anything in between, as shown in Figure 1.
The depth of modulation produced by the outputs is variable not only by means of individual depth controls, but also be means of a switchable key/CV input, which automatically adapts itself for either an AC key signal, or a DC CV. This allows effects such as amplitude dependent vibrato and chorus to be easily implemented. Alternatively, driving the control input from a CV source such as a second modulation oscillator will enable very complex modulation waveforms to be produced. And as if that isn't enough, a switchable single cycle mode can be selected which is triggerable again either by an AC key signal, or a DC trigger. An envelope follower output additionally allows the amplitude of the AC key signal to sweep a modulated device directly, this being particularly useful for amplitude swept flanging. An LED indicator is provided to keep a check on the status of the unit.
Fig.2 reveals the several novel features of this design. Oscillation is produced around the loop formed by IC1a, IC2a and IC2b. The integrating capacitor, C1 is charged at a rate determined by the transconductance of the OTA (Operational Transconductance Amplifier), IC1a, which is dependent on the setting of the rate control VR2. IC2a is a JFET input Operational Amplifier, acting as a buffer and low bias current so as to have minimal effect on integration. IC2b is used as a schmitt trigger to control the limits of integration, and toggles the polarity of the charging current produced by IC1a when either limit is reached. Asymmetrical waveforms are produced without significantly affecting the frequency by allowing VR3, the shape control to produce a different transconductance in IC1a for each half cycle of oscillation. VR1 is necessary to null any input offset voltage which could otherwise lead to asymmetry.
The waveform appearing at the output of IC2a is triangular, and symmetrical about 0V. The 'soft-limit' input characteristic of IC1b when overdriven then rounds off the peaks of the triangular waveform to produce a sinewave which is surprisingly free of distortion. The voltage gain of IC1b, and thus the voltage at the output of the buffer IC1c is proportional to the current sourced by the voltage to current converter formed around IC3d and TR1. Normally, with SW2 switched 'out', R19 provides the control current, keeping the output voltage at the potentiometers VR4 and VR5 at 5V peak to peak, with a lower excursion of 0V. TR2 pushes current into the LED D5 in proportion to the output voltage, whilst keeping this 'dirty' current off the 0V line.
The precision rectifier IC3b converts any AC signal on the key input into a DC voltage in proportion to the peak amplitude of the input. This DC voltage stored on C4 is amplified by IC3c with a gain adjustable by VR6 to produce a DC control voltage of 0 to +10V which may then be used, by operating SW2, to control the voltage to current generator and thus the output voltage (or modulation depth). A 0 to +5V control voltage is available on JK2 which follows the envelope of the key signal, and can thus be used for 'envelope follower' applications. R27 bypasses the precision rectifier at DC, and thus allows a CV input to control the output amplitude.
IC3a, when switched into operation by means of SW1, forms a bi-stable which detects when the end of a negative ramp is reached. The output of IC2b, in going negative at this point, sets the output of IC3a positive, allowing the FET TR3 to be biased on by R9. This causes the voltage on C1 to be clamped to the value of the lower excursion, regardless of any charging current from IC1a. This condition will continue until a CV or key signal is injected into JK1, when IC3a will toggle, switching TR3 off to allow a single cycle of positive and negative ramp to occur, after which clamping will again take place.
Building the Modulation Oscillator module using the high quality kit should present no problems especially since, by exclusive use of PC mounting connectors, switches and potentiometers, there is no interwiring to do. The first step in construction is to insert, solder and crop the resistor leads, populating the PCB according to the parts list, and the overlay printed on the PCB itself. Bending the leads outward at 45 degrees prior to soldering will hold the components in place without running the risk of shorting together a pair of pads. Solder the eight links in place using resistor lead off-cuts, at the positions shown dotted on the overlay. Taking care with orientation, locate and solder the diodes and the transistors. The IC sockets come next, making sure that they are pressed down onto the PCB whilst soldering, but leaving the ICs themselves out until later. Now insert and solder the capacitors, taking care with the polarity of the electrolytic types. The bus connector and the four jack sockets can then be soldered whilst holding them firmly down onto the PCB. A piece of foam rubber laid on the bench comes in handy for holding connectors and the like in place on up-turned PCBs during soldering. Mount the two presets now, leaning well back to allow access to them around the edge of the panel.
Trim each pot shaft to 8mm from the bush using a hacksaw, whilst holding the shaft in a vice, or just use a pair of cable cutters. Fit a PC bracket to each pot and locate into the appropriate PCB positions, but don't solder at this point. After determining the correct orientation of the LED, bend its leads down at right angles, 4mm from its body and locate into the PCB without soldering. Screw one nut onto each toggle switch then locate them into their PCB positions, again without soldering. Place shakeproof washers on the pots and switches, then offer the front panel up, feeding the pot and switch bushes and LED dome into the appropriate panel apertures. The panel is then fixed in place by means of the pot nuts, which should be fully tightened. Only finger tighten the front switch nuts however, leaving the final securing to the rear nuts, which must be screwed up against the rear of the panel. The pots, brackets, switches and LED can now be soldered, after making sure that they are all positioned correctly, and that the panel is at right angles to the PCB.
Spend some time now to check over the assembly very carefully, especially on the track side where dry joints and solder splashes are all too common, even for the experienced constructor. When you are completely satisfied with the assembly, load the ICs into their socket, being careful with orientation. Note that IC3 is fitted the opposite way around from the others. Finally fit the knobs with caps so that the marker line of each covers the scale evenly, the push on the toggle switch lever covers.
The position of the Modulation Oscillator in the rack is not critical since it has a thro circuit for the audio link bus. It need not necessarily be next to the module(s) it is modulating either as the CV signal is transferred via jack lead(s). It is probably most convenient to leave the jack leads connected permanently from the modulator outputs to the CV inputs of the two devices to modulated; modulation selection then being affected by the depth controls.
Before installing the unit in the rack, initially adjust the two presets so that they are central. Now slide the module into place and switch on the rack power. With both toggle switches set to the left, the LED should fade on and off at a frequency determined by the rate control. With the rate control set fully clockwise, VR1, the upper preset should be carefully adjusted so that the cycle rate is the same at each extreme setting of the shape control. This can be determined either by watching the LED, or by listening to one of the CV outputs via an amplifier, (take care not to redecorate your room with loudspeaker cone, though!). Access to the presets is gained by removing the module to the right of the Modulation Oscillator. The other preset is used to adjust the sensitivity of the key input, but does not affect the CV sensitivity. This preset can be readjusted at any time to suit any particular level, but if you have a standard operating level, then you may find you can set it and forget it.
The Modulation Oscillator can be used with almost any CV controllable device, and more are planned for this series. Without doubt, the modulator is essential for getting the best out of the Multi-Delay module described previously. Much of the discussion which follows concerns modulating the Multi-Delay, but the operation of the modulation oscillator remains essentially the same for other modulated devices.
The frequency required for producing chorus is around rate 6 and the depth control would typically be set at 7, with the shape control at 0 when used with the Multi-Delay. The Multi-Delay can be used to produce true vibrato by setting delay to 2, regen to 0 and mix to 10 in the echo mode by feeding its CV input from the Modulation Oscillator when set to produce a sinewave CV at rate setting of 9½ to 10. The depth setting for this application would typically be 2. Much slower rates would normally be used when modulating a flanger, and it is here that the shape control might be moved from its central position. ADT is often enhanced by gently modulating the delay time at a mid-range rate.
When the trigger mode is selected, the input signal can be additionally used to feed the key input and thus initiate a modulation cycle which is useful for single sweep flanging, chorus or pitch shifts. A cycle can also be initiated by a trigger from a synthesiser or drum machine by using the same key/CV input. The shape control is quite useful here for modifying the character of a triggered sweep.
The depth of modulation can be controlled by a key signal or a CV from a synthesiser or another Modulation Oscillator. When controlled by the input signal, amplitude dependent chorus, vibrato or ADT is quite straightforward to produce. Whereas the depth controls attenuate the output to a 0V level, the key controlled depth mode attenuates to a +2.5V level so that the mean operating point does not shift.
Although not associated with oscillatory modulation, the provision of an envelope following CV output allows some exciting and unique effects to be produced such as amplitude controlled flanging. With the Multi-Delay set up for long echo, experiment also with amplitude controlled echo. The key sensitivity preset may have to be reduced here to avoid pitch changes. Or, how about winding the sensitivity up to produce a totally unique falling-pitch echo effect - it's weird!
Next Month : Input Module
A complete kit of parts for the Modulation Oscillator is available from Tantek, (Contact Details), for a fully inclusive price of £33.95. Please allow 28 days for delivery.
Feature by Paul Williams