Modular Effects Rack Project (Part 1)
A major new series describing the design and construction of a high quality modular effects rack. This first instalment deals with the Sub Rack assembly, Power Unit and the first module: the Pro-Gate.
The virtues of a modular studio effects system are obvious in terms of flexibility and economy, and have been expounded in HSR previously. Notable are the cost implications of eliminating the need for a separate case and power supply for each effect, together with the neatness and convenience of internally interconnected effects.
This article marks the start of a major new series of high quality projects which have been carefully designed for flexibility, attractive, professional appearance and ease of construction. The mother unit forms a 4U high, 19" rack-mounting Sub-Rack capable of housing eleven plug-in effects modules plus an optional power supply.
The Sub-Rack and every module in the series will be available in complete easy-to-build kit form directly from Tantek.
If a well regulated ±12V DC supply is already available then this can be connected to the rack via the backplane-mounted DIN socket, as shown in Figure 1. This socket could also be used to extend a DC supply from the rack power unit to other DC equipment, or to another slave rack should eleven modules be insufficient!
The all-aluminium frame, with tough stoved epoxy finish end plates and dust covers, makes the unit robust enough for life on the road, and yet the grained satin black anodised front panels with elegant orange screen printing and matching knob and switch trim makes it attractive enough for the living room!
A backplane PCB on the rear of the unit houses the connectors for the module positions, to which ±12V DC power is distributed and between which a stereo virtual earth bus, a module linking network, a key bus, and a pair of (spare) auxiliary busses run.
Apertures in the backplane additionally allow five jack sockets per module to become available to the outside world. There then become five different methods of interconnection available to the user, any of which may be used in combination:
1. A signal injected into the Left Hand Side (LSH) module and extracted from the RHS module will be linked serially through all modules, processing being dependent on the in-out switching.
2. A signal applied to an optional input module can be extracted from an optional output module, and processed as above.
3. The rear sockets may be used as a patch-bay for interconnecting modules and interfacing external equipment.
4. The rear sockets may be extended to a patchbay proper for convenient routing.
5. The outputs of certain modules may be mixed onto the stereo virtual earth bus and extracted using a special mixer module.
Figure 2 shows the bus layout. As designed, the backplane links each module output to the input of its right hand neighbour. Any configuration can however be 'normalised' by cutting the link tracks and hard-wiring the freed track ends. Although the architecture of the rack is essentially mono, certain modules have the provision for processing stereo signals.
Construction of the Sub-Rack begins with the backplane PCB which forms the rear of the unit. Fit the DC power DIN socket using M3 screws and the Key jack socket with the tags facing upwards, such that the PCB will face foil side outwards. Prepare five 60mm lengths of insulated wire, and connect the sockets to the PCB as shown in Figure 3. The twelve bus connector wafers should then be inserted fully and squarely to the PCB and soldered.
Turning now to the frame itself, attach the four cross rails to one end plate loosely, using large self-tapping screws, and align them so that the slots face outwards to the front and rear. If dust covers are to be fitted, make sure that the fixing holes for these also face outwards. Slide four hex head screws into each of the two rear rails, and a tapped strip into each of the front rails, then screw the other end plate on lightly.
Having put the basic frame together, insert a PCB card guide, card slot upwards, at an angle between the front and rear lower rails near the left hand end. Twist the guide firmly into position so that the cupped ends snap onto the rail lobes. Using a wooden block and small hammer if necessary, knock the guide hard up against the left hand end plate. Repeat this procedure for the upper left hand guide with its slot downwards.
Using a soft pencil, lightly mark out 1.4 inch (35.5mm) intervals on the rails, starting from the centres of the already positioned left hand guider. Strict accuracy is not essential here, it just eases final setting up. The remaining 22 guides should then be fitted and aligned with the pencil marks. After fitting each guide, pinch the rails inwards to ensure that the guide cups are engaged fully onto the rail lobes.
This may all sound a little bewildering, but it really is much easier to do than to explain. The tight tolerances on the parts also goes toward making assembly of the rack easier; it almost falls together. Now locate the backplane assembly over the eight M3 screw studs on the rear of the unit and loosely fit shakeproof washers and nuts.
When you have a fully assembled audio module, slide it gently into the LHS position, adjusting the horizontal position of the backplane so that the bus connector mates reliably, then tighten the backplane nuts and the end plate screws. Before securing the module in place with its panel screws, push the tapped strips fully against the LHS end plate.
Insert your second module into the next slot, adjusting the position of the front ends of the guides so that the module panel fixing holes align with the tapped strip holes, and the rear ends so that the jack socket bushes are centralised in the backplane holes. Final positioning of the rear end of the lower guide may be necessary to ensure good bus connector mating. Repeat this procedure until all eleven audio module positions are set up.
If you are using the power module, this is fitted in the RHS rack position. Gently slide it in place, adjusting the position of the front ends of the guides for panel hole alignment as before. Since there are no jack sockets on this module, the rear end of the lower guide is adjusted for good bus connector mating, followed by the rear of the upper guide to make the unit square and parallel.
The power unit should then be removed (it will be necessary to have the position to its left vacant for this) so that the guides can be anchored firmly in place using eight barbed guide clips pushed onto the rail lobes. This anchorage is not necessary on the other module positions since the guides do not take the weight of the audio modules.
The optional dust covers can now be fitted to the top and bottom of the frame using six screws on each.
Although highly recommended anyway, these are only really essential when the unit is free-standing, for electrical safety reasons. The dust cover kit also includes rubber feet and all the necessary screws. To preserve the elegance of the unit when less than eleven modules are fitted, matching printed blanking panels are also available, again with screws.
The complete Sub-Rack kit is available from Tantek, (Contact Details) at a special introductory price of £39.95. The dust cover kit is £6.95, and blanking panels £2.95 each. All prices are fully inclusive. Allow 28 days for delivery.
Power supplies are really quite dull things, and this one is no exception! It provides its +12V at 0.5A per rail quietly and efficiently with the minimum of fuss in a compact, very attractive package. What is more, it couldn't be easier to construct, with absolutely no interwiring to do.
Apart from the use of a special low profile pseudo-toroidal transformer, the only unconventional feature is the output checking LED D1. The zener D2 causes the LED to extinguish should the negative rail dip by more than a couple of volts due to overloads or other faults, while TR1 will cause the LED to extinguish if the positive rail dips. R1 provides the ground lift connection, which can be replaced by a link if you choose the rack to be your signal ground earthing point. A steel screen prevents any remnants of electro-magnetic field from reaching the audio modules.
Insert and solder the two LED veropins into the PCB, along with the rectifier, BR1. The resistors, zener diode and transistor come next, then the capacitors. Depending on your mains voltage (240V in the UK), provide the appropriate links (using resistor off-cuts) near the transformer position. Trim all the leads close to the joints. Now solder in place the switch, fuse clips, mains connector block and bus connector, making sure that each is pressed firmly down onto the PCB whilst doing so.
Using M3 screws inserted from the foil side, position the heatsink and, with their leads appropriately formed, locate the regulator ICs and secure them firmly with shakeproof washers and nuts. Make sure the heatsinks are nice and square, then solder the regulator leads in place. The transformer can now be positioned and, after turning the assembly over onto a flat surface, fixed in place with self-tapping screws and soldered generously, while pressing the PCB down firmly. Make sure all the leads are trimmed close to the joints, then check the assembly thoroughly for dry joints etc., especially on the track side, with an eyeglass or magnifying glass.
Fix the front panel to the screening plate using two black M2.5 screws and nuts. Slip a shakeproof washer over each of the four M3 studs on the screen, and secure with nuts. Fit a plain washer, then another shakeproof washer on each stud and offer the assembly up to the foil side of the PCB so that the studs locate in the PCB holes, while feeding the switch actuator into its panel pole. Secure this assembly with shakeproof washers and nuts on the studs. Fit the LED bezel into the front of the panel, and the LED into the rear. Solder the leads (the longest one to the 'A' terminal) into the veropins.
Now fit a 500mA fuse, push on the switch button and terminate the 3 core cable at the connector block. Pass the free cable end through the appropriate rack backplane hole and fit a mains plug. Leaving just enough slack to permit removal of the module, squeeze the grommet onto the cable as it exits from the rack, and push it firmly into the backplane hole.
Give everything a final check, then slide the unit into its RHS position in the rack, plug in and switch on. If all is well, the LED should glow and you can secure the panel in place. If not, switch off immediately and look for a mistake somewhere! More likely though, it will work first time. If you are really keen, you can check the voltages appearing on the rack DIN socket, and even try loading each rail with a nice fat 22 ohm resistor.
A complete kit of parts for the Power Unit is available from Tantek, (Contact Details) at a special introductory price of £33.95 (fully inclusive). Allow 28 days for delivery.
Noise gates are rarely used these days for reducing tape noise, and for this reason we have intentionally avoided the word 'noise' in the name of this first audio module in the series of projects for our modular effects rack. Although noise gates now often earn their crust by maintaining separation between drums or other instruments played simultaneously in the studio, the reader cannot help but to have noticed the surge in creative uses for the humble noise gate of late.
Gates have a whole host of other uses, from quietening down an old 'beehiving' organ to keeping a guitar track silent until the lead break starts (see the 'Gating Techniques' article in this issue for a full rundown of creative applications).
HSR's Pro-Gate boasts not only a fine specification, but also all the facilities you could want for producing those creative effects currently in vogue. This twin channel unit features variable attack and release, a wide range threshold control, a switched key input and a variable hold timer to keep the gate open for a predetermined time after triggering. This allows you to produce that popular 'gated reverb' drum sound when used in conjunction with a reverberation device.
A gate is essentially an amplifier whose gain is unity when the input level is above a predetermined threshold level. Below this level, however, the gain reduces dramatically so that any residual, very low level signal, such as noise or leakage from another instrument, is completely silenced. Figure 6 shows the amplitude characteristic of the Pro-Gate when the threshold is set at -50dBm.
Figure 7 shows the complete circuit diagram for the Pro-Gate. IC1a is the threshold amplifier which, with its high maximum gain, can boost very low level signals mixed from each channel via R3 and R4, up to gating levels. IC1b then provides an inverted version of this boosted signal so that D1 and D2 can full-wave rectify it into the slope amplifier, IC2a. This provides a further high gain for signals above the threshold determined by the 'knee' voltage of D1 and D2, so that a small change in level causes a large change in attenuation, resulting in a sharp cutoff slope.
Pin 1 of IC2a, in pulsing negative, charges C7 via D5 and VR2, the attack control. C6 is also charged via D4 so that TR1 is turned on, and hence TR2 off, preventing C7 from discharging. When the input signal falls below the threshold level, however, C6 discharges via D3 and VR3, causing TR2 to turn on, thus allowing C7 to discharge through R14 and VR4, the release control. The hold control thus determines the time for which the release function is inhibited.
IC2b and TR3 form a current generator which sources a control current for the Operational Transconductance Amplifiers (OTAs) IC3a and IC3b, in proportion to the negative voltage on C7. The OTAs operate as current controlled amplifiers whose gain is dependent on the control current. Some positive feedback around the control circuit is provided via R20 and R12 to prevent erratic operation on a dithering signal.
Featuring absolutely no interwiring, the simple assembly centres around the PCB, into which the resistors should first be inserted, soldered and cropped. Only do ten or so components at a time to prevent crowding. Insert and solder the three links using resistor lead off-cuts, then the diodes and transistors.
The IC sockets come next, making sure that they are firmly pressed down onto the PCB whilst soldering. Leave the ICs out at this stage, though. Now insert and solder the capacitors, taking care with the polarity of the electrolytic types. Mount and solder the presets VR5 and VR6 so that they lean back slightly to ease adjustment. The bus connector and the five jack sockets can then be soldered whilst holding them firmly down onto the PCB.
Trim the pot shafts to 8mm from the bush, fit a PC bracket to each one, then locate into the appropriate PCB positions, but don't solder at this stage. Locate the LED with correct orientation, and its leads bent down 4mm from the body so that the dome faces towards the edge of the PCB, but again don't solder. Screw one nut onto each toggle switch, then locate these into the PCB.
Place shakeproof washers on the pots and switches and offer the front panel up, feeding the pot and switch bushes, and the LED dome into the appropriate apertures. The panel is then fixed in place by securing the pot nuts. The front nuts on the switches should only be finger tightened, since it is the rear nuts which are now fully tightened onto the rear of the panel. The pots, brackets, switches and LED can now all be soldered, making sure that the pots and switches are fully home into the PCB.
After having a cup of coffee, check the assembly carefully, especially on the track side, for solder splashes or dry joints, then load the ICs into their sockets, taking care with orientation. Fit the knobs so that the marker line of each covers the entire scale, and fit the switch level covers. Having now completed the module, slide it into place in the rack and secure with black M2-5 screws. Although not essential, screwing the jack socket nuts onto the backplane does improve rigidity if the module is not to be removed frequently.
Remove the module to the right of the Pro-Gate, turn all the controls anti-clockwise and switch on the power. With both toggle switches set to the left, the LED should now light. A signal applied to the inputs will be passed to the outputs unaffected. Switching the unit 'in' will result in silence until the threshold control is advanced, when the signal will return and the LED extinguish.The operation of the other controls should also be checked at this stage.
Having now confirmed that the unit is working correctly, disconnect the inputs and instead, connect a noise source such as white noise from a synth, or a tape machine playing a blank tape, to the key input. Switch the 'key' toggle on and monitor the left output. With all other controls anti-clockwise, adjust the threshold such that the LED flickers on and off. The crackling noise which will now be heard should be minimised by adjusting VR5 (the upper preset). The lower preset, VR6 will null the right channel crackling.
Your Pro-Gate is now raring to go! It has been optimised for home studio operating levels of -10dBV, but has sufficient headroom to allow operation at 0dBm. The rack's linking system will route signals via the left channel, which is switched in/out using the toggle. The right channel, which shares a common control side-chain with the left channel, is accessible via the rear panel jack sockets only, and is permanently 'in'.
For general leakage prevention and residual noise removal, the threshold control is set so that the unit only just gates off (as indicated by the LED coming on) when only the residual signal is present. A lowish attack setting, zero hold and moderate decay will be required. Very low attack settings in combination with very low threshold levels (approaching -60dBm) should be avoided since the very high gain in the side-chain can lead to self-gating.
The 'gated reverb' drum sound is obtained by feeding a (stereo) reverb with your snare drum sound and gating the output of the reverb. The dry/reverb mix should be about 50/50. As a starting point, set attack to 2, hold to 3 and decay to 2.
The key facility is really exciting, as experimentation will prove. Try gating your bass track keyed by the bass drum, or 'string' track keyed by vocals. The key input will even accept a mic signal direct, so try controlling your guitar or synth keyed by voice. Apart from using the module key input, the rack's key bus can be accessed via the jack behind the power unit.
Future modules include: Dynamic Noise Filter 2, Sweep EQ2, Mic Preamp, Input, Output, Mixer and many more. Next month: Comp-Lim 2.
A complete kit of parts for the Pro-Gate is available from Tantek, (Contact Details) at a fully inclusive price of £33.95.
Feature by Paul Williams
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