Output Module Project (Part 8)
Continuing the saga of the Tantek rack, this latest module complements last month's input module.
Following the presentation last month of the Input Module for the HSR effects rack, Paul Williams now describes the complementary Output Module which further eases interfacing with the outside world, and provides a stereo headphone output.
Impedance matching is not something which the home recordist need normally worry too much about since the aim is not to transfer maximum power, but maximum voltage to preserve signal-to-noise ratio. This condition is usually satisfied so long as the input impedance is at least an order higher than the output impedance driving it. It would make sense then for manufacturers to make the input impedances of their equipment infinitely high. Unfortunately, it is not that easy to achieve a very high input impedance, especially where low levels demand a good noise performance. It would perhaps make more sense to keep output impedances as close to zero as possible. This would additionally ensure that the effect on frequency response of capacitance in long leads is minimal.
The problem is that there is no common standard concerning impedances, so in order to allow you to forget about impedance matching completely, we have equipped the Input Module with a very high input impedance, and the Output Module described here with a very low output impedance, together with variable gain on both modules to cover any eventuality, whether the equipment be of the low impedance +4dBm pro variety or -10dBV high impedance budget oriental gear or indeed any other type. The user should not be concerned that these two modules are essential to get the best performance from the rack however, but they are worth having for the metering, headphones output and front panel jack access alone.
Monitoring is often a particular problem for the home recordist since listening levels are usually restricted not only by equipment, but by such things as neighbours and wives! We are not suggesting that you should rely entirely on headphones for monitoring, but they can certainly help, especially when you are listening for something in particular like noise, distortion or the setting of an effect. You may well have a socket for headphones on your mixer, but it is not always easy to patch them in to monitor what you actually want to listen to, such as the output of an effect unit.
The Output Module gives you a high performance stereo headphone output which can either monitor the rack output from the linking system, or can be patched from the output of any effect or other equipment, in or out of the rack. The design allows for a wide range of headphone impedances to be used, all with ample listening level, and without the neighbours hearing a thing!
Nothing unusually exciting here, as you might imagine. Figure 1 shows that IC1a and IC2a invert and buffer the signal collected from the audio link buss, or injected into the input jacks, JK1 and 2. IC1b and IC2b then further invert and amplify the buffered signal, depending on the setting of VR1, the line level control. From here, the signals pass through the coupling capacitors C3 and C6 to the output jacks. R5 and R11 and C2 and C5 ensure stability even with highly capacitive loads, without increasing the output impedance.
VR2 taps off a variable amount of the buffered signals to the headphone amplifiers formed around IC3 and IC4 after being suitably attenuated by R13 and R14 and R20 and R21. SW1 allows either full stereo operation, or for the left channel to be routed to both headphone channels for mono operation. R17 and R22, being in series with the output loads fulfil several functions. They limit the output current to a level which will be safe for both amplifiers, and ears, improve the frequency response by swamping out headphone impedance variations, improve the signal-to-noise ratio by allowing the amplifiers to operate at optimum output signal voltage levels; and allow for a wide range of phone impedances without significant differences in level. C9 and C14 and R18 and R23 form Zobel networks to keep things nice and stable.
Once again, we need to keep the 0v rail clean, so the relatively high signal currents involved in the headphone amplifiers are kept off the 0v rail by providing an isolated 0v system for them, generated from the -12v rail by TR1; effectively an emitter follower buffering a voltage slightly more negative than 0v to negate its base-emitter voltage.
Building the Output Module using the high quality kit should present no problems especially since, by exclusive use of PC mounting connectors, switch 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 seven links in place using resistor lead off-cuts at the positions shown dotted on the overlay. Taking care with orientation, locate and solder the diode, D1 and transistor, TR1. 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, making sure that you notice the polarity of the electrolytic types. The buss connector and the six jack sockets can then be soldered whilst holding them firmly down onto the PCB. Make sure that the 3 pole jack is in the JK5 position. 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 the appropriate PCB positions, but don't solder at this point. Screw one nut onto the toggle switch and locate into the PCB, again without soldering. Place shakeproof washers on the switch and pots, then offer the front panel up, feeding the pot, front socket and switch bushes into the appropriate panel apertures. The panel is then fixed in place by means of the pot and front socket nuts, which should be fully tightened. Only finger tighten the front switch nut however, leaving the final securing to the rear nut, which should be screwed up against the rear of the panel. The pots, brackets and switch can now be soldered, after making sure that they are all fully home, and that the panel is square to the PCB.
It might be advisable to spend some time now 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 are completely satisfied with the assembly, load the ICs into their sockets, being careful with orientation. Finally, fit the knobs with their caps so that the marker line of each covers the scale evenly, with equal 'dead band' allowance at either end, then push on the toggle switch lever cover.
The Output Module as best placed at the extreme right hand end of the rack, where it will collect the linking signal from any module to the left of it. This is only true for mono operation of course, when the left channel is the active one. A jack lead can be used to patch the right channel output of a stereo module to the right channel input of the Output Module. The left channel input is also accessible at the rear on a switched jack, which disconnects the link signal when in use. The output to your mixer, amplifier or whatever can be taken either from the rear jack sockets, or (for mono only) from the front panel jack. The line level control can be set to vary the gain from nothing to +20dB, the setting being 5 for unity gain.
Although still not essential, the user will find the Input and Output modules useful for interfacing with mixer access points, where the access point output can be patched to the Input Module, processed by the rack, and returned from the Output Module to the access point input. The level into the rack is then adjusted by the sensitivity control and displayed on the Input Module LED meter, and the output module line gain adjusted to conform with the mixer, as indicated by the mixer's metering. You can use as many Input and Output Modules as you wish, remembering that both types break the link chain, so jack lead patching will usually be necessary if more than one of each are used, unless you group your rack Modules, each group being sandwiched between an Input and Output Module. Another way of getting signals between your mixer and rack is via the mixer's send/return system, or indeed directly from the mixer group outputs.
Almost any type of headphones can be plugged into the front panel socket, the listening level being set by the headphones level control, completely independently of the line level control. You could therefore set up an effect by monitoring through the headphones with the line level wound right down and thus without anything being output from the rack. The toggle switch associated with the headphone output enables the user to select either full stereo operation when the right channel is patched into use, or mono operation where both headphone channels monitor the left line channel when the mono linking system is in use.
Those home recordists who don't feel too comfortable with a soldering iron, or who simply don't have time to put kits together may be interested to know that Tantek are now making available ready built racks and modules. Details are available from the address below, or by 'phoning (Contact Details).
A complete kit of parts for the Output Module is available from Tantek, (Contact Details) for a fully inclusive price of £32.95.
Next month's addition to the Modular Effects Rack will be a Psycoacoustic Enhancer.
Feature by Paul Williams
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