Paul Williams outlines the design for a high quality module that can be used as either a autopanner or an autofader.
This month's module for the Tantek rack serves the dual functions of stereo panning and stereo autofading either under manual, cyclic (with the aid of a modulation oscillator), or sync track control. Paul Williams gives you the full low-down on this versatile unit.
Although a Panner may not be the first item of outboard equipment you might think of including in your studio, it is nonetheless a very useful gadget and is not simply limited to hard cyclic image movement. Much more rewarding are the results obtained from panning a reverb or echo signal, or drum track synchronised stereo position movement.
Have you ever put your all into creating the perfect mix on your 30 minute epic piece, only to ruin it in the last couple of seconds with a sloppy fade out? Well, you need never make a similar mistake again with a Fader/Panner module. It's controlled either by hitting the Go button, or under the command of a synchronising pulse from a spare track. Fade-ins can also be automated, with individual rate controls for fade-up and fade-down.
Control options include alternate right/left (fade up/down) toggle action and momentary action. This operation is under command of either the panel Go button, or an external CV, trigger or key signal. The unit is configured either as a panner or a fader by toggling the Pan/Fade switch, understandably enough and panning and fading action is indicated by a pair of LED indicators.
Since the unit is provided with two inputs and two outputs, fading is in stereo. Panning is normally performed on a mono input, although if both inputs are used during panning, then the signals are effectively cross-faded since the level of one channel rises, as the other falls. This can not only be put to good use with stereo signals, but also for cross-fading different effects or sound sources under pushbutton or automatic control, possibly with the aid of a mixer to achieve mono cross-fading from two sound sources or effects.
Just in case you are concerned about sound quality when trusting your entire mix to such a unit, worry no longer since exactly the same type of DBX Voltage Controlled Amplifiers (VCAs) are used in some notable professional automated mixing desks.
Figure 1 shows the circuit diagram for the Fader/Panner. IC4 and 5 are the VCAs which are obviously the most critical part of the module. These are current in, current out devices, so resistors R24 and R35 are needed to set the input voltage gain, IC6a and b being used to convert the current outputs into voltage outputs. The VCAs use a cunning transistor bridge configuration whose manufactured tolerances have to be balanced out using the presets VR5 and 6. R28 and 39 are only used during the setting up procedure since they allow the fundamental signal to be removed so that any remaining harmonics can be easily detected and nulled. IC7a and b present a zero impedance source to the control ports of the VCAs to keep the bridge properly balanced and hence the distortion low. The switch SW4a and b takes the control ports to 0v when the module is switched out so as to set unity gain since the VCAs respond to positive control voltage with negative gain.
Control of the VCAs is initiated either by a control voltage or key signal applied to the key/CV socket JK1 or, when no key/CV plug is present, control is handed over to the push switch SW3. The circuit formed around IC1a and b automatically determines whether the signal being input is an AC signal or a DC control voltage. Any AC signal is rectified by IC1a, the resultant DC signal being fed to IC1b which would also receive any DC input control voltage. The output from this stage is available to SW1 at pin 7 of IC1. The other signal selectable by SW1 is from the flip-flop, IC2 which toggles state on every positive going edge of the signal at IC1 pin 7. Thus, depending on the position of SW1, control of the VCAs is either momentary or held latched in either of two toggled states. IC2a operates as a schmidtt trigger to supply the flip-flop proper, IC2b with noise-free switching edges.
The signal thus selected passes to IC3a and b which are precision rectifiers, allowing C6 to charge positively at a rate determined by VR2, and discharge to 0v at a rate determined by VR3. IC3d buffers the voltage developed across C6 and allows an offset voltage to be added by VR4. The resultant voltage at pin 14 of IC3 is now all that is needed to control the VCAs for stereo fading, as selected by SW2. TR1 and 2 drive the LEDs D9 and 10 in sympathy with the control voltage applied to each VCA. Notice that the LEDs are driven from current sourced from the positive rail and sunk by the negative rail, so as not to contaminate the 0v rail.
During panning, IC3c and IC1c and d are switched into action so that not only are the VCA control voltages out of phase, but also ensure that the pan is smooth without any undue amplitude peaking or dipping.
Building the Fader/Panner 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 20 links in place using resistor lead off-cuts at the positions shown dotted on the overlay, but making the two links marked 'cal' into loops so that they can be easily removed later. Taking care with orientation, locate and solder the diodes and transistors in place. 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 presets and capacitors, taking care with the polarity of the electrolytic types. The buss connector and the five 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 LEDs, bend the D9 leads down at right angles, 3mm from its body and locate into the PCB without soldering, and similarly with D10, but this time with the bend 7mm from it's body. Screw one nut onto each toggle switch and locate into the PCB, again without soldering. Leave the push switch out for the time being. Place the shakeproof washers on the switches and pots, then offer the front panel up, feeding the pot and switch bushes and LED domes 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 LEDs can now be soldered having made sure that they are all fully home, and that the panel is square to the PCB. The push switch SW3 can now be inserted into the PCB and it's keytop pushed on so that the position of the switch can be adjusted to centralise the keytop in it's panel aperture before the switch is soldered.
Now it's worth spending some time checking 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're completely satisfied with the assembly, load the ICs into their sockets, being careful with orientation. Note that ICs 1 and 3 are fitted around the other way from all the other ICs. 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.
Plug the completed module into it's rack position and remove the top dust cover, sealing off the area above the power unit with a piece of card to prevent any shock hazard. Apply the rack power and inject into either of the input sockets a medium frequency signal with a pure tone, perhaps from a synthesiser set to produce a flute-like sinewave. Toggle the module 'in' switch to the left (out) position and monitor the output from the left channel with a fair amount of gain. You should hear a slightly distorted version of the input signal. Now adjust VR5, the rearmost preset and listen carefully to the harmonic content. The correct position is where no harmonics can be heard, and not necessarily minimum signal, since this adjustment also effects the fundamental level. The same process is now applied to the right channel using VR6 to null out the harmonics. Having set the presets, no further adjustment should ever be necessary, so the power should be removed and the two 'cal' links can be removed with a pair of side cutters.
Finally replace the dust cover, and the module is ready for action.
Since this is necessarily a stereo device, it doesn't use the rack's mono linking system. Any link signal passes through it without being affected. The position of the unit in the rack is thus not critical since inputs and outputs are jack lead patched.
When panning a mono signal, either input may be used since the input sockets normalise to each other. Channel changing can be controlled manually using the Go button, at a rate determined by the Rise and Fall controls. In the Hold mode, the channel will remain permanently changed until the Go button is pressed again, whereas in the Momentary mode, channel change is temporary, until the button is released. The stereo position can be biased away from the left channel using the Offset control.
Cyclic panning is effected by using a modulation oscillator routed to the CV/Key input. The oscillator can control panning either in the hold mode, using the Rise and Fall controls to determine the shift rate and stereo width or, probably more favourable in the momentary mode using the oscillator shape and depth controls to determine relative shift rates and stereo width respectively. The Offset control can be put to good use here to centralise the panning effect. It will be found that only a little depth is all that is needed to achieve quite a wide sounding stereo field. Rather than sticking with over-obvious hard image movement, experiment with panning just the reverb or echo signal leaving the dry signal stationary; it can be very effective.
By using both inputs in the Pan mode, two sound sources or effects could be cross-faded either across the stereo field or using a mixer, in mono. You could for instance cross fade between reverb and flanging either cyclically or under push button control.
Fading is usually applied to a stereo input in the Hold mode so that the Go button initialises 'fade up' or 'fade down' at rates again determined by the Rise and Fall controls. If the fade is not required to go to zero, then the Offset control can be used. The Momentary mode can be selected for temporarily fading out unwanted sounds while the Go button is held. A modulation oscillator could also be used here for cyclic fading or tremelo effects.
Drum or sync track control can be affected using the Key/CV socket. The sensitivity of this socket to an AC signal is determined by the front panel accessible Sens preset which is adjusted using a small screwdriver. You could move, say your keyboard sound in sync with the kick drum, or how about moving your toms in sync with the snare for some really fast moving drum work!
A modulation oscillator can really open up new avenues in pan control. Firstly, it can provide triggered single sweeps so that a sound can, perhaps in sync to a drum beat, move slowly from left to right and back again. Secondly, it has an envelope follower output, enabling the image position to be controlled linearly by the amplitude of a signal. You could for instance arrange that the harder you played your guitar, the more the sound would move to the right. The third effect possible when using the modulation oscillator with the Fader/Panner is signal amplitude controlled cyclic panning width by using the Oscillator's keyed depth mode.
The price inclusive of VAT and postage (within the UK) is £59.95 in kit form, or £83.95 ready assembled and tested. Further information on the modular rack system can be obtained from the address below.
The Panner/Fader module is available from: Tantek, (Contact Details).
Feature by Paul Williams
mu:zines is the result of thousands of hours of effort, and will require many thousands more going forward to reach our goals of getting all this content online.
If you value this resource, you can support this project - it really helps!