A unique and versatile project that distributes, splits and matches signals.
Probably most electro-musicians are equipped with a mixer of some kind that enables several inputs to be mixed down into a single signal, but a piece of equipment that enables one signal to feed several inputs is far less common, although in its way just as useful. In some cases, where a signal source having a fairly low output impedance is driving several inputs having high or medium input impedances, simply connecting all the inputs in parallel and feeding them with the input signal should give satisfactory results. However, problems can arise when a high or medium source impedance is used to drive several inputs. Due to loading effects the source signal can be attenuated to such a degree that there is an inadequate signal level.
The Multisplit has been designed to overcome problems of this type, and the unit has three inputs each of which gives a pair of outputs. The input impedance at each input is 100k, and the output impedance of the unit is low. The unit is not restricted to having three inputs with each one giving a pair of outputs, and if only one input is used an output signal is available at all six output sockets. If two of the inputs are used, one input signal is available at two output sockets, and the other is available at the other four outputs.
The noise and distortion levels of the unit are negligible provided the input level is kept below the clipping level of about 6 volts peak to peak, and a very low (say less than about 30mV RMS) signal level is not used with the unit. There is unity voltage gain through the circuit. Apart from use as a splitter, the unit can also be used as a buffer amplifier where it is necessary to match a high source impedance to a low input impedance.
The unit consists basically of three buffer amplifiers, and the full circuit diagram is provided in Figure 1. Each amplifier uses an operational amplifier with 100% negative feedback from the output to the inverting input so that unity voltage gain is obtained. R1 and R2 form a centre tap on the supply lines, and the non-inverting input of each operational amplifier is biased from this via a separate resistor (R3 to R5). It is these resistors that determine the input impedance of the unit, and the input impedance can therefore be easily altered if desired. C2 is a decoupling capacitor which prevents stray coupling between the inputs through the bias circuit.
Each input socket is connected to its respective operational amplifier via a DC blocking capacitor, and break contacts on JK4 and JK7 connect all three input sockets together. Thus an input signal applied to JK1 appears at all three inputs, but if an input signal is also applied to JK4 this signal is applied to IC2 and IC3, and the break contacts JK4 isolate JK1 from IC2 and IC3. With all three inputs in use the break contacts on JK4 and JK7 isolate the input sockets from one another, and each input signal only couples to its respective buffer amplifier.
A unity gain (i.e., with 100% feedback) operational amplifier gives a low output impedance, and each output can readily drive two inputs. However, when driving equipment with input impedances below 2k or so, it may be necessary to limit the signal level to avoid distortion. Separate DC blocking capacitors are used at each output socket, whilst R6 to R11 ensure that the op-amps remain stable, regardless of any excess cable capacitance. JK10 enables an external 9 volt power supply to be used to power the Multisplit, and it is not necessary to use a well smoothed and stabilised supply. The current consumption of the circuit is only about 5mA, and if a PP3 size battery is used to power the unit it should give many hours of use before needing replacement.
The case recommended for the Multisplit is a diecast aluminium type having approximate outside dimensions of 150 by 80 by 50mm. The six output sockets are mounted on one of the 150 by 50mm sides of the case which then effectively becomes the rear panel of the unit. There is very little excess space on the rear panel and the positioning of the six sockets is quite critical. It is therefore advisable to follow Figure 4 which gives details of the rear panel drilling.
On/off switch S1, the three input sockets, and power socket JK10 are mounted on the front panel of the unit. Note that the power socket's body connects the metal case to the 0V rail to provide screening for the circuitry. The front panel is not particularly crowded and the positioning of these components is not critical, although the unit will be easier to wire up if the general arrangement used on the prototype is adopted.
Details of the Multisplit printed circuit board are shown in Figure 2 and 3, and construction of the circuit board is straightforward. Use Veropins at the points on the board which will eventually be connected to the sockets, on/off switch, and battery connector. If the specified case is employed the printed circuit board slots into the fourth set of guide rails from the rear of the unit with the component side of the board facing forwards. This leaves sufficient space for the PP3 size battery in front of the printed circuit board, and a piece of foam material glued at the appropriate place on the base panel of the case can be used to hold the battery in place. The wiring diagram for the Multisplit appears in Figure 2. It is not necessary to use screened wiring but try to keep leads reasonably short and direct.
If the unit is powered from an external power supply, power is applied to the unit via a 3.5mm jack plug which connects to JK10 at the same time disconnecting the internal battery. The tip of the jack plug must carry the positive supply and the barrel carries the negative supply. Alternatively, a latching connector (e.g. BNC or DIN) can be used for extra robustness on stage. If the unit is then also fitted with an internal battery it is essential that on/off switch S1 is set to the 'off' position when an external power source is used with connectors other than the specified jack socket. S1 becomes inoperative when an external power supply is used, and the unit is turned on and off using the on/off switch of the power supply.
If you wish to use the unit with one input and six outputs the input signal must be applied to SK1. With inputs connected to JK1 and JK4 the signal applied to JK1 is available from JK2 and JK3, and the input applied to JK4 is available from JK5, JK6, JK8 and JK9. An input signal applied to JK7 will only be available from JK8 and JK9, and will not give a six-way, or even a four-way split. Of course, with all three inputs used each input is coupled through to its respective pair of outputs (JK1 to JK2-3, JK4 to JK5-6, and JK7 to JK8-9).
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