The effects link FX-1 is designed with the guitarist and keyboard player in mind. It provides a facility for selecting one of a number of effects pedals or units but it has application in the studio or on stage wherever multi-routing is likely to be necessary.

With the huge range of effects pedals now available as ready made items, it is perhaps a little surprising that few manufacturers until recently have considered the problems of routing more than one of these devices together.

The first effects pedals were primarily for the guitarist and therefore required a high impedance input (to avoid loss of control and upper response from the guitar) and the ability to handle low level signals. This immediately caused problems for the keyboard player who might well use a pedal (e.g. phase or wah wah) that needed some component change internally to correctly match the 'line' level output of a synth, organ or electric piano to the unit.

Several companies have produced designs that use a common power supply and through-linking, so that the player can switch in or out one or more effects at any time without the need for re-routing cables from units. This generally works well but does not present the most versatile solution. Another point to remember is that the order of connecting several effects can be important to the final multi-processed sound. Many professional musicians use custom designed patch boxes to solve their routing problems and there are only a few switch link boxes currently available (e.g. from MXR and Electro-Harmonix) that simply switch between two or more effects.

The Effects Link should provide the variety and compatibility required by the most discerning musician and will cope easily with guitar and all electronic keyboard levels. The electronic switching makes easy selection of the effects from one place, either on the floor (using your feet) or on a console top (using your hands). The circuit uses CMOS analogue switches to eliminate the noise generated by mechanical switches. LED indicators give an instant instrument check on the effect in use (invaluable for effects units without this facility), and the strong die-cast box recommended will minimise hum pick-up and withstand knocks and handling. Level adjustment is provided on both send and return signal lines to enable each route to be set up so that there is no change in level between routes.

Circuit

Figure 1 shows the circuit diagram of the Effects Link which can be divided into two sections, the audio section and the control section. Let us first consider the audio circuits. IC1a acts as a buffer for the signal input (which can range from 50mV to several volts), and as a driver for the three effect sends. The presets RV1-RV3 can be used to set the send level. R1 sets the input impedance of the buffer to approximately 300k and, with R2 sets the gain of the buffer to unity. Each of the return signals and the straight signal are fed through buffers (IC2) and into the CMOS switches. Each of the buffers for the return inputs has a gain variable between unity and times ten, controlled by presets RV4-RV6. The outputs from the switches are fed to a four-input, unity gain mixer formed by IC1b. The return paths are switched in preference to the send signals so that the outputs of noisy effect pedals do not cause a deterioration of the signal-to-noise ratio. The unit has zero phase shift between input and output, input and send, and return and output.

The selection of the route patterns is carried out by a one-of-eight decoded counter (IC4) and a diode matrix. The diode matrix along with resistors R20-23 form four diode 'OR' gates which control the CMOS switches. IC4 is controlled by the footswitch and the clock (IC5a and b). When the footswitch is depressed the decoder is enabled and the unit steps through the eight possible combinations continuously. When the footswitch is released the clock input is disabled and the last combination remains set. The route pattern is displayed on the top of the box by four LEDs driven by IC5c, d, e, f from the switch control lines. If less than eight combinations of routes are required this can be achieved by connecting the reset pin of IC4 to one of the decoded outputs, i.e. if only four routes are required then the fifth decoded output should be connected to the reset pin.

The unit is powered from a single 9V supply and R30, R31 and C12 provide half the supply voltage to correctly bias the op-amps. The power is switched on and off by the signal input jack socket, JK1.

An additional jack plug (JK9) allows connection of an external power source if the unit is to be in regular use. The Synpac project (see September 1981 issue) is an ideal power source for this purpose.

Construction

All the components except the LEDs and the sockets are mounted on the PCB (shown in Figure 2). In the prototype unit the ICs were not fitted into sockets but there is space to do so if preferred. PCB assembly should start with the links. Next the resistors, capacitors, presets and diodes can be soldered into place and the microswitch mounted on the board. Assembly of the PCB can then be completed with the insertion of the ICs. Note that some of the resistors are mounted underneath the microswitch assembly.

The case should be drilled to the dimensions given in Figure 3. The LEDs are glued into their mounting holes with a suitable adhesive. Connect the anodes of the LEDs together and connect a wire to them. Also connect a lead to each of the four LED cathodes. Next fit all the jack sockets and wire them as shown in the wiring diagram. When all the connections to the PCB have been made it can be slotted into position and secured with the nut on the microswitch assembly.

Testing

Having completed the assembly of the unit, the next step is to fit a battery and switch on by inserting a jack plug into the signal input socket. The unit should then display a random route on the LEDs. If this does not happen, switch off and check the circuit again. If all is well, depress the footswitch and watch the Effects Link step through the eight combinations. The sequence should be as follows: Straight, FX1, FX2, FX3, FX1 and FX2, FX2 and FX3, FX1 and FX3, and FX1, FX2 and FX3.

Having checked that the control section is operational you can now test the audio section. Connect a signal source to the input and connect the output to an amplifier. Set the Effects Link to the straight route and the signal should be audible. Check that the signal output is clean and un-distorted. Next, connect jack to jack leads between the send and return sockets and set the presets for unity gain. Now depress the footswitch and check each route is operational and that there is no change in signal level except when more than one route is selected. Finally, before fitting the bottom of the box, adjust the presets to give the required levels for your effects pedals and ensure that there is unity gain on all routes.

Using the Effects Link

The first thing to do is decide on how you wish to use the selector. Its main use is to select one or a combination of single effects pedals or, to select different chains of effects pedals. Figure 4 shows a few connection possibilities.

The wiring diagram shows only the send jack-sockets connect with an earth. This has been done to eliminate the possibility of earth loops when using standard effects pedals, but in some situations this may cause problems with interference and it may become necessary to earth the return jack as well.

The unit can also be used as a switched input mixer for the three return lines and the straight signal or, as a signal splitter with a straight output and three sends. This gives endless possibilities of use in the studio for route selection to tape recorders and effects and for splitting signals for different effects as well as for dedicated use with an instrument and effects pedals.