This interface system was originally designed to connect two high impedance microphones to a tape recorder with a high impedance input, with a long cable run between them. The system has been tried with up to 25 metres of cable and little degradation of the signal was noted, even in a very hostile environment next to a major thyristor lighting installation. The system consists of two parts; a stereo line driver and a stereo receiver. They are connected by a standard 5-pin DIN plug lead, which is very convenient. A slight penalty is paid for this convenience since the interchannel crosstalk is degraded, but not noticeably so. In the ideal situation separate line drivers and receivers should be used for each channel, and with independent connecting leads.

The Unbalanced Line

The unbalanced line is the usual method of connecting audio equipment (Figure 1a). The audio signal is carried by the centre core, contained within a screen formed by the return path which will always be the 'earthy' side of the transmission. The screen helps reduce inductive couplings of external noise (hum, RFI etc.) to the core, but it's effect is only limited, and over a long cable run the noise picked up will be of a high level. The amount of pick-up can be reduced by running the core from a low impedance source into a fairly low impedance termination. A typical termination impedance would be 600 ohms, although 200 ohms will sometimes be found. Low impedance driving has a further advantage in that treble loss due to cable capacitance is much reduced, as is shock noise which occurs as the cable is moved.

The Balanced Line

The balanced line overcomes all of the problems mentioned above, but the main disadvantage is that two signal cores are required as well as the screen. This naturally makes the cable and connectors much more expensive. Figure 1b shows the general idea of such a line. One core carries the signal as in the unbalanced case, and the other core carries an inverted version of the signal. The screen is connected to ground, but in this case does not act as a signal return path. At the termination the difference of the two signals is taken as the final output, thus the output is identical to the inputs, although a gain of two will be noted. However, any noise induced into the system will be of the same phase on both lines, and will cancel out at the differential point. The rejection ratio is very high indeed, limited only by the common mode rejection of the differential point. The only problem that may occur is the noise (particularly spikes) causing intermodulation distortion at the differential point. This can be avoided with suitable high frequency filtering ON BOTH INPUTS to the differential point. This problem is very small when compared with the overall improvement obtained.

Early balanced lines used transformers, and lines of the highest quality still do. Figure 2 illustrates the operation of a balanced line with transformers. The input signal is split into two phases by the first transformer, and travels to the second transformer which acts as a differential point. Note that the screen serves only to earth the source (if necessary) and screen the cores; it does not carry any part of the signal. Transformers of this quality are very expensive, and are themselves prone to pick up hum, so until the introduction of op-amps balanced lines were limited to studio applications.

The op-amp is an ideal building block for balanced lines, as shown in Figure 3, since both phase inverters and differential amplifiers may be easily implemented.

Balanced lines are never run at high impedance, and 600 ohms is the norm. In some equipment (particularly continental microphones) a 200 ohms system may be found, but as a rule this can be plugged into a 600 ohms system with little or no consequence.

Line Driver Circuit

A stereo pair of drivers was built around a quad JFET op-amp (Figure 4) chosen mainly for its very low power consumption. Each pair of amplifiers forms a phase and antiphase generator. The first op-amp inverts the signal and the second op-amp reinverts the output from the first such that the two outputs are of the same level but exactly out of phase and can be used directly. A 100R resistor is included in each output as a protection against short circuits, and a capacitor is obviously required to block the D.C. level. The op-amps are biased to half rail by R15 and R16 which hold the non-inverting inputs at 4.5 Volts.

A single 9 volt battery is used to power the circuit. This is switched by the right input jack socket, which is stereo and has its screen and centre spring contacts connected together when a mono jack is inserted.

Line Receiver Circuit

The line receiver circuit (Figure 5) takes the out of phase signals generated by the driver and produces a single output from them. In this way the receiver performs an identical function to most mixer input stages.

The in phase and out of phase signals are applied to the non-inverting and inverting inputs of the op-amps respectively. These op-amps are connected as standard differential amplifiers, with a gain of 0.5. This gain loss counters the natural effect of a balanced line of this sort and ensures that the output is exactly the same as the input (Note: the 100R output impedance of the driver and the 560R input impedance give 660R total, hence the 330R feedback is exactly correct). Due to the nature of this circuit a low impedance rail splitter is required, and so IC2 is connected as a voltage follower to achieve this.

The input capacitors are not required if the receiver will always be connected to the driver since that has output capacitors, so they may be replaced by links. Do not be tempted to miss out the output capacitors from the driver as well since the d.c. offsets required are half the battery voltage, and there is no guarantee that the battery voltage will be the same in both units, besides which it is bad policy to leave equipment with a D.C. offset on its output.

Construction

The Line Driver and Receiver are each constructed in a small plastic box with the sockets fitted on the front and the printed circuit board fixed to the removable back with 6BA bolts. When the components have been soldered to the board (IC's last), wire up the sockets using screened cable making sure that the screens are connected at the socket ends only, in order to avoid the possibility of internal earth loops. Connect the earths of the sockets together and wire up the battery connector, paying careful attention to the stereo jack socket connections. Note that Din latch sockets are specified in the parts list. Used with latching plugs these are well suited to this application, but non-latching connectors could be used. As specified the units can still be connected by a non-latching 5-pin DIN lead. Construction is completed by screwing on the back of the case, with the battery held in place by a piece of foam or pneumatic packing material. Each prototype was finished off with rub-on lettering followed by a coat of varnish and four stick-on rubber feet.

In Use

Figure 8 indicates how the units are used to connect a tape recorder and microphones. Since the system will easily manage 770mV (0 dBm) it can be used to connect pre-amps and power amps for group or disco.

Conclusion

This balanced line system offers considerable advantages over the more standard unbalanced system. In most cases the extra cost of using balanced lines will be more than justified.