Input Module Project (Part 7)
Brings signals from the outside world into line with the Tantek rack. Features stereo operation and an LED ladder signal level meter.
This month's project for the Modular Effects Rack allows the user front panel access to the input of the audio linking system, and to the key bus. Paul Williams describes how the Input Module widens the scope of interfacing to external equipment with its variable gain, aided by a built-in LED level meter.
Most of the modules described in this series so far have quite respectable input impedances typically in the region of 50-100K, and this, together with the flexible input level characteristics of most of the modules allows easy interfacing to external equipment. Some of the modules in the series however, notably the Pro-Gate and Dynamic Noise Filter, have fixed input level characteristics optimised for -10dBV. Although the designs could be modified for operation at, say +4dBm, it would obviously be a great advantage to be able to attenuate, or for that matter boost the input level to these modules for more universal application. Adjustable gain is of course of little use without a means of indicating the operating levels.
The Input Module described here provides this variable gain on a pair of stereo channels, together with a ten stage LED level meter which is switchable between either of the channels. The extremely high input impedance of this unit will completely eliminate any matching problems, as well as making it ideal for a DI'd guitar or bass. Front panel access to the (left) link channel input and key bus input will, in most cases also eliminate the need for fumbling around the back of the rack for re-patching. The inputs and outputs of both channels are additionally available at the rear of the rack should the user wish to use an external patchbay, or to implement patching at the rear of the rack.
The circuit diagram shown in figure 1 reveals the quite simple configuration. Each channel consists of a very high input impedance JFET input buffer amplifier followed by a variable gain amplifier. The resistors R5 and R11 and capacitors C2 and C5 ensure high frequency stability even with highly capacitive loads, while preserving a low source impedance. VR1 adjusts the gain of both channels simultaneously over a 100:1 range. SW1 selects which channel is metered by feeding the appropriate output channel signal via C7 to the full-wave precision rectifier and peak detector formed around IC3. The peak voltage of the metered signal is stored on C8 and presented to IC4, the logarithmic bargraph LED driver.
Although IC4 is configured to operate in the dot mode where only a single LED would normally be on at a time, the LEDs are in fact connected in series so that any one output becoming active drives all preceding LEDs, resulting in the familiar thermometer style bargraph. As has become the norm with this series, no 'dirty' noise current is allowed to find its way onto the 0V rail; all LED current flows between the +12V and -12V rails. C9 prevents any oscillation around IC4, while C10-13 provide low impedance supply rails by decoupling, as well as eliminating any possibility of noise from the circuit being put on the rails.
Access to the key bus is made by direct connection between the key input socket and the key bus.
Building the Input Module using the high quality kit should present no problems especially since, by exclusive use of PC mounting connectors, switch and potentiometer, 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 six 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, D1-4. 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 six 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 the 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 the pot and locate into the PCB position, but don't solder at this point. After determining the correct orientation of the LEDs, bend their leads down at right angles, 4mm from their bodies and locate them into the PCB without soldering. Screw one nut onto the toggle switch and locate into the PCB, again without soldering. Place shakeproof washers on the switch and pot, 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 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 pot, bracket and switch can now be soldered, after making sure that they are all fully home, and that the panel is square to the PCB. Before soldering the LEDs, push the domes fully into the panel apertures using a small screwdriver.
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 sockets, being careful with orientation. Finally, fit the knob with its cap so that the marker line covers the scale evenly, then push on the toggle switch lever cover.
The Input Module may be used with a stereo or mono source. When using a mono source, either the left channel input socket at the rear of the rack, or the front panel input socket may be used. The extremely high input impedance means that you don't have to think about the source impedance at all.
The sensitivity control would usually be adjusted for an average operating level of about -6dBm, although a slightly higher level could be tolerated, and may even be beneficial on most modules other than the Pro-Gate and Dynamic Noise Filter. When the control is at 0, the gain is unity, allowing any -10dBV sources to be routed through the module without any further gain trimming. This setting also allows the LED meter to be used for checking levels anywhere throughout the rack system, or indeed on any external equipment, by connecting one of the inputs to the equipment output to be metered.
Both outputs are available at the rear of the rack but, if you are using the linking system, then no connection need be made to these. Any signal presented to the left channel input will thus be gain corrected by the Input Module and sent to the module to its right, and through every other module, where the signal can be extracted at the module on the far right. Any processing that is required can then be implemented simply by toggling the appropriate 'In' switch without resorting to repatching.
Wouldn't it be rather nice if there was a complementary module to collect the link signal, present a low impedance line driving output to a front panel socket with variable gain, and maybe even provide a headphones output? Well there is; the Output Module will be presented next month.
A complete kit of parts for the Input Module is available from: Tantek, (Contact Details) for a fully inclusive price of £32.95.
Feature by Paul Williams