Infinite Flanger (Part 10)
Paul Williams adds yet another module to the modular effects rack.
Last month's project was a hard act to follow, but this month Paul Williams has excelled himself by coming up with a radically new flanger design that is capable of producing wide bandwidth, low noise effects, with an infinite flange ratio.
Whereas most of the modules described in the series so far have both creative and corrective applications, this one is surely 100% creative effect, and probably one of the more obvious effects. There are many flangers on the market today with widely varying price tags. It's curious though, that there is so little difference between either end of the scale that it has even been suggested within these pages that the home recordist could just as well save his money and buy a cheap footpedal flanger. There are after all only two major specifications which push flangers to either end of the scale; namely signal-to-noise ratio (or dynamic range) and flange ratio. Lesser units may only provide in the order of 65dB and 3:1 respectively but the design presented here offers you a signal-to-noise ratio as close to infinity as makes no difference, and a flange ratio of infinity itself, hence the name given to this module; surely earning it a place right at the top of the stack.
Some readers may not be familiar with the term flange ratio, or indeed flanging itself. Flanging was originally produced by using two tape machines arranged in such a way as to produce two versions of the input signal, with a delay of up to a few milliseconds between each, resulting in cancellation of certain frequencies. The synchronisation between the machines, and hence the delay time and notch frequencies was manually varied up and down to produce the familiar sky-riding effect. Modern technology came along and made things a lot simpler and easier in the guise of the Bucket Brigade Device (BBD). A single silicon chip made it possible to produce the required delay directly, and in real time. However, this implementation, which is used almost exclusively in flangers across the complete price spectrum, has two major problems associated with it. Firstly BBDs are inherently noisy, necessitating noise reduction to be used for professional applications, and secondly physical restraints on the range of clock frequency (and hence delay time), limits the ratio between the shortest and longest delay times, ie. the flange ratio. Even Digital Delay Lines are often quite short on flange ratio, and it is now broadly accepted that flanging is best produced by a dedicated analogue device.
The Infinite Flanger uses a combination of pre-emphasis/de-emphasis and compansion to yield a dynamic range so wide that an input sensitivity control is unnecessary. Most significantly though, two BBDs are used in parallel, one of which is set by a 'Shift' control, to produce an offset delay, the other BBD being controlled by the manual sweep control and the CV input for automatic control by means of an external modulation oscillator. The overall effect of this is more akin to the original tape method, where zero delay difference is attainable when both machines are in perfect synchronisation. The perceived difference in being able to sweep though zero delay (and indeed into negative delay!) is difficult to describe, but is akin to the difference between watching black and white or colour TV!
The other controls on offer are Re-generation to enrich the effect, and Mix, which not only provides more subtlety if required, but also enables you to mix in the original undelayed signal as would a more conventional flanger. A tricolour 'Cross' indicator continuously monitors the difference between the two delays; a headroom indicator being unnecessary due to the wide dynamic range. A phase reverse switch allows the flange sound to be further modified.
Do you still have a niggling worry about bandwidth? No problem here either; a full 15kHz is always at hand.
Starting at the centre of Figure 1, we see that the BBDs are fairly new devices, TDA1097, each having no less than 1536 stages so that the clock frequency can really be wound up for an uncompromised frequency response. IC3 is the static or reference delay line, which is clocked by IC4; a purpose designed delay line clock oscillator/driver, at a rate determined by VR2; the Shift control. The swept delay line is IC7, which is clocked by IC8. Oscillation within this device is supported by the current drawn from C32 into the collector of TR5. This current is set up by the voltage at the base of TR5 by the CV input, and the setting of the Manual sweep control, VR3. VR4 sets a minimum frequency of oscillation by biasing the temperature compensation diode, D5.
Efficient filtering is obviously a must in a system in which there is sampling at two different rates simultaneously. IC2 provides the anti-aliasing filter for the incoming signal in the form of a forth order Chebyshev design with a slightly peaked-up second stage to compensate for the droops in response caused by cascading two filters, and by the (sin x)/x losses. IC5 and IC6a form a 6th order Chebychev reconstitution filter for the reference delay and the swept delay is filtered similarly by IC9 and IC10a.
With the Mix control VR6 clockwise, the outputs from the two delay lines (via the reconstitution filters) are mixed together by IC10b. SW2 allows the phase of the reference delay to be reversed. With VR6 anti-clockwise, the reference delay is replaced in the mix by the input signal which appears at pin 7 of IC6b. This stage also provides, via VR7, re-generation around the delay loops. IC1a compresses the incoming signal, causing all audio signals within the delay system to be at a high average level, expansion being carried out by IC1b, which then passes the expanded signal on to the output. Tracking between compressor and expander is assured, even in the presence of resonant peaks by using the compressor control signal fed forward to control the expander. Apparent dynamic range is further enhanced by pre-and de-emphasis affected by C2 and C28 respectively.
IC11 forms an edge triggered flip-flop which effectively compares the frequency of each clock generator, producing an indication on the tricolour LED biased towards either red or green, depending upon which frequency is the higher. When the frequencies are very close, the LED indicates yellow. TR3 buffers the 0V supply for the LED in an emitter follower configuration, supplied by the negative rail.
Careless design or layout of this circuit could easily result in heterodyne frequencies whining away when the clock generators have harmonic products within the audio range. Because of this, not only is a double sided PCB used, but we have also gone to quite extraordinary lengths to keep the supply rails quiet by means of R10, 12, 39, 40, 51, 52 and C14, 34, 42 and 43.
The double sided PCB supplied in the kit not only keeps clock noise at bay, but also eliminates the need for links. As usual in the series, all connectors, switches and pots mount directly on the PCB, so there is no interwiring to do. The first step in the construction is to insert from the printed component side of the PCB the 31-track pins at the positions ringed on the overlay. This is best done by leaving the 'stick' of pins intact until the leading pin is pushed home, when the stick can be broken away at the second pin in line. These are then soldered on both sides of the PCB.
Next 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° prior to soldering will hold the components in place without running the risk of shorting together a pair of pads. Taking care with orientation, locate and solder the diodes, D1-6 and transistors, TR1-5. 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. Next place and solder the three presets, VR1, 4 & 5. The buss connector and the three 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.
Trim each pot shaft to 8mm from the bush using a hacksaw, whilst holding the pot shaft in a vice, or just use a pair of cable cutters. Fit a PC bracket to each pot and locate into their respective 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 and locate into the PCB, again without soldering. Place shakeproof washers on the switches and pots, 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 should 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 fully home, and that the panel is square to the PCB.
Next it's worth spending some time to check over the assembly very carefully, looking especially for dry joints and solder splashes (which are all too common), even for the experienced constructor. When you're completely satisfied with the assembly, load the ICs into their sockets, being careful with orientation. For ICs 3, 4, 7, 8 and 11, additional precautions must be taken to prevent static damage to these MOS devices. There is no need to be frightened of them though: just make sure that they are left in their conductive packing until the last moment. Touch both the conductive packing and the ground plane whilst transferring them and avoid, if possible, touching the IC leads. It's preferable that you are not wearing a nylon sweater, nor is it advisable to go for a walk around on a nylon carpet prior to the transfer!
Finally, fit the knobs and caps so that the marker line of each covers the scale evenly, with equal 'dead-band' at each end, then push on the toggle switch lever covers.
If you have followed the above procedure carefully, there is no reason why the module shouldn't work first time. There are first, however, three presets which need to be set up. Plug the module into a rack position, with the module to it's right removed for access to the presets. Apply the rack power, switch the module 'in' and inject a high level signal of fairly low frequency at the input. With the Man, Shift and Mix controls set clockwise and the Regen control anti-clockwise, adjust the second preset down, VR1, back and forth, where the monitored signal will be found to go in and out of distortion. Leave the preset at a position roughly at the centre of the band which is found to be free of distortion.
Now turn the Man and Shift controls anti-clockwise and adjust VR4, the top preset, gradually clockwise from an initially anticlockwise position until any whistles disappear. Check that the unit is still whistle-free when the Man and Shift controls are adjusted, and when signals of varying level and nature are injected, trimming VR4 again if necessary. Finally, with the Regen control fully clockwise, adjust the lower preset, VR5 as fully clockwise as is possible without introducing whining or howling at any setting of the mix and antiphase controls.
Uses for the Infinite Flanger are really a matter of taste and experimentation, the most noticeable treatment being on signal sources rich in harmonics. An insert point on a mixer is probably the most convenient place to patch the Flanger to, although it could be used directly on the output of your instrument.
The Mix control gives you rather more control over the effect than it's name would suggest. When set fully clockwise, both delay lines are mixed and put on the output so that an infinite sweep is available. With the Mix control anti-clockwise however, the reference delay is not used at all; the output being composed of one delay line signal mixed with the incoming signal. This results in very deep, long delay flanging and ADT effects. With the Mix control close to centre, the output is composed primarily of signal from the swept delay line, allowing true vibrato, or very subtle flanging to be produced.
The Antiphase switch can be used when the Mix control is clockwise to impart a more hollow characteristic to the effect. Because of the unusual structure of this device, the Regen control does not have the same function as on most flangers. With the Mix control anti-clockwise, the Regen control strengthens the effect in the usual manner, introducing flutter echo at lower delay settings. With the Mix control clockwise however, the Regen control effectively adds more widely spaced notches rather than intensifying the ones already present.
Although the Infinite Flanger can be manually swept using the Man control quite effectively, use of the Modulation Oscillator module will certainly make life easier, allowing automatic sweeping of the flange effect, ideally at quite a slow rate; say one sweep in ten seconds or so. To produce an infinite sweep with excursions to wide notch spacing, it is perhaps best to set the Shift control almost fully clockwise, adjusting the Man control and the modulator to produce a sweep ranging from green to yellow on the indicator. Alternatively, the Shift control could be set lower so that the unit is swept from green, through yellow to red and back again, crossing the zero delay point each time. Using the CV input, sweeping could be controlled by a synthesiser for instance.
That's all for this month. Next month's offering will be a Mic Preamp.
The Infinite Flanger is available from: Tantek, (Contact Details) either in kit form for £69.95, or ready to use for £92.95. Prices include VAT and postage within the UK.
Further information on the modular effects system can be obtained from the above address, or by 'phoning (Contact Details).
Feature by Paul Williams
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