Analog Audio Cables – III

Connectors and Balanced Connections

Aside from TRS Jack as seen in Part 2 of this subject, cables, connectors and connections used at the professional level to carry balanced signals are the XLR that unlike previous have directionality, a connector is generally used for output connections (fig. 1) and a connector is generally used for the input connections (fig. 2).

Fig. 1 maschio Fig. 2 femmina

The XLR cable is also called CANON named after its inventor, is also called microphone cable because it was invented primarily for the transport of microphones signal (a few millivolts) over long distances.

The section of the internal bearing cores of XLR signal as seen in argument Cables Analog Audio – I is generally less than that of the Jack cables (very similar to that of the mini jack ) as the balancing allows to eliminate noises and electromagnetic interference Loaded along the cable allowing to reduce the section dimensions and thus also costs.

An alternative of XLR is the mini XLR (Fig. 3) used mostly for the transport of balanced signal by microphones of small dimensions (as may be the piezoelectric and the head set) towards devices with special mini XLR input (as they can be samplers, receivers in ear monitors and wireless bodypack) or vice versa according to the signal path and any adapters used.

Fig. 3 PSG02964_MINI.jpg

Figure 3 there is a clear difference between the size of an XLR and a mini XLR.

To date is also used to transport at the level of pro + 4dBu line v = 1,224 signals, (that is the potential difference between the voltage on the line conductor and the mass to 0 V) or consumer 0dBv = 0,770 V, such He is the master output left-right-mono aux master, sub group out, matrix outs, bus out of an audio mixer. It doesn’t exist are stereo XLR cables, for such transport they use a couple (special case for transmitting AES / EBU data that can be transmitted digitally multiplexed stereo signal of an XLR connector). These cables, in addition to having a shielding against electrostatic interference also have a return conductor which serves to balance the signal so as to eliminate the external electromagnetic interference, a total of 3 conductors.

Let’s see how:

A connector and balanced XLR connection has 3 pin (fig. 4), because it is formed by conductor wires 3 (X = ground, generally of copper, L = line, generally of white color in which passes the audio signal, R = return or red, to indicate that it is generally of red color, in which passes the audio signal of inverted phase as will be useful to eliminate interference).

Fig. 4 connessioni

When you connect the (pin) carrier signal cables to the terminals, of which as seen comprises the connector, there is a number to follow to comply with the existing connection in the input or output connection which will be lodged in the connector, so to shift the signal once given the supply and avoid certain phenomena that we analyze.

1 = ground       2 = line       3 = return

Sometimes you can find reversed 2 with 3, but there is no problem for the audio signal, however, it is looming in a phase reverse, so be careful especially when mixing several signals.

The important thing is however that both conductors are wired, as the signal return that in balanced cables is given by the mass, in balanced cables is given from pin 2 (line) for the signal in phase opposition and from pin 3 (return) for the signal in counter phase. If you try to remove one of the two pins, the signal will disappear and will make its presence only background noise. While in this case if the noise from the crowd loaded are excessive and hardly attenuated you can try lifting the ground. Lifting the ground in a balanced connection the signal continues to be there but you have to be careful, however, that the interference loaded on the way out from the crowd instead of being downloaded from the devices (as we unloaded the mass) to which the cable is connected will not be discharged into other directions. A classic example is the take a slight shock by touching the microphone or electric musical instrument if the problem is the mass which is probably not connected, has internal deformations along the route of the cable or do not have the right section to bring the interference properly to the mass of the devices to which it is connected.

Not vary the position of the mass, otherwise go on all the interference signal creating high background noise.

There are two balancing methodologies, PASSIVE and ACTIVE mode.

n.b. Not all manufacturers of connectors and/or devices that presenting balanced analog use this standard in the association of the conductors to the relative conduction pins, for this it’s always good to inquire for not determining counter phase or malfunction of the audio line (especially if there’s the necessity of the phantom power transport to + 48 V which if not opportunely sent to the correct pin can also damage possible devices connected to it).

 

Passive mode

To achieve a balanced connection just send our unbalanced signal (because in the electrical signal source is always unbalanced) in the input of a transformer to the center tap, so that at its output each of the signal polarity is positive or negative will also present a polarity reversed on the opposite conduction pin (in the case of xlr, pin 2 in phase and pin 3 in counter phase) (fig. 5). The balanced output dynamic microphones is always achieved passively through the transformer, while in other devices such as the audio output of condenser microphones or pro-level audio mixer it can be found even with electronic components such as amplifiers that as we’ll see define a type of active balancing, more qualitative of the transformer especially in the frequency linearity and wave distortion (although mainly in microphones with active balancing is often placed before the transformer device to prevent overloads on the “weak point of the balancing circuit active”, and make it more compatible the output impedance of the same microphone with the input of the microphone pre-amplifier internal to the audio mixer or outboard, which in turn, and for the same reason almost always a transformer).

Fig. 5 2015-12-26_22-40-45

In dynamic microphones in particular, we prefer to keep their stamp and their characteristic, very useful for the recovery of certain instruments, and it is for this that the active balancing is not used. While in the condenser, as will be seen when we speak of microphones, there being an amplifier inside them, useful for increasing the low voltage output and not only, can be exploited to also balance the signal without the introduction of a further component, for the most part non-linear as the transformer.

When both signals (the one in phase and one in counter phase) for example, output from a microphone, arrive at the input connection (e.g. mic input of the audio mixer), the same device will present if analog, the reverse process, and then an transformer at center tap (fig. 5).

At this point, the signal in counter phase will be reversed polarity (put in phase) and mixed with the one already earlier in phase so as to regain the original signal (within of the analog or digital equipment’s the work on the electrical signal is always to unbalanced level). From these sums, all magnetic interferences that have passed through the cable will be void. Because the signal will return in phase, while the interference will go in counter phase and will be void.

Dividing the output voltage by a balanced device, you will have a halve of the voltage (then 6 dB loss), while when it puts everything into phase, the resulting voltage will be that of departure (6 dB gain).

n.b. If you go to unbalance a balanced signal and increases the level of gain of 6 dB you will not have the original signal as that electromagnetic interference loaded along the way will be well present and amplified in turn as they have not been deleted.

If you want to unbalanced a balance signal, because the circuit functions will need to connect to the ground one of the two carrier signal cables (generally the 3, so it can be find the signal in phase of the pin 2).

The graphical representation in Figure 4 is indicative for the understanding of the operation, in reality, the circuit is much more complete of electrical and electronic components. The principle of balancing applies to both microphone signals of that line, while for the power signals will never balance, both because the signal has high voltage values and therefore less affected by external interference is strong to prevent phenomena of self-induction and capacitive.

We see in detail the functioning of a passive balancing:

The signal output (unbalanced in nature) from the generator enters in the center-tapped transformer in which it is created a copy in counter phase, so that the output from the transformer will have 2 cables in which a signal circulates in one direction and one in another on different conductors. When these signals arrive at the central transformer to the input connection, they will be picked up and that in counter phase will be reported in phase and added to that already in phase, so as to compensate for the attenuation of 6 dB from the voltage helve as seen before, and eliminating the interference load during the path, as they, not undergoing the action of the transformer, will be always loaded in phase and then summed inverted phase. The third conductor is ground, useful for downloading electrostatic interference generated and uploaded from the transformer, generator, and the cables themselves which as we know are connected the shield that surrounds the conduction cores as an additional protection against magnetic interference.

Figure 6 shows a graphical representation of the passive balance.

Fig. 6 2017-04-29_20-23-19

n.b. If the shield is disconnected from the mass is very likely that the interference loaded on it, are transferred to the signal, so it is good to periodically check the condition of cables.

As seen in balanced connections the return signal is not given from the mass as in unbalanced connections, but the return or pin 3.

It is no coincidence, but such circuit was purpose built and thought in this way, such that it is possible when there are strong electrostatic interference from the loaded mass that create signal disturbances, detach it. It should always good to detach only one, to one side or the other of the two connectors at the end of the cable, making objective evaluations, such as those that you will see in this topic when we speak of Ground Loop. Generally, it is always the one in which the device with its input or output connection, has in addition to the mass of the conductor also that of their own ground.

The presence of the transformer is a critical point of this circuit, but which, thanks to the strong reduction of external noise and the good maintenance of the frequency response and dynamic for long cable, enabling it to master as the standard of use in professional audio connections.

The critical point of the transformers is mainly in the fact of generating background noise and give distortion in the response, especially on high frequencies, and for large voltage swings even on low (and even the weight). Excellent transformers which allow to minimize distortion of these factors are also very expensive. The transformer has a high input impedance and this tougher the overall impedance of the conductor favors the onset of problems seen previously, such as capacitive and inductive reactance. Transformers with ratios of variable coils, are a source of greater disturbances than those unit ratios.

The balanced cable is a type connector that can carry long distances, even tens of meters, without losing the quality of the signal itself. The modern professional connections no longer use a balancing physically wired (passive), but electronically ( active ).

The Return conductor then, is also used to send the Phantom to + 48 V or similar voltages as + 24, + 12 V in some cases, from the mixer or phantom generators to devices that need power for its own any components of amplification (D.I.Box, condenser microphones and other active systems). This will not affect the audio signal passing on the same conductor as it’s a direct current of opposite working direction.

n.b. If the phantom does not come it is likely that the cable is not working, it could be that the cable is unplugged to pin 2 or the signal is unbalanced (the return cable makes contact with the ground).

 

Consideration on the transformers (fig. 7)

Fig. 7 2015-12-26_23-31-11

As seen from figure 7 a transformer (generic) presents an input circuit said primary and one of output said secondary, the number of coils determines the impedance of input and output, and then also the value (voltage) which will have the signal. If the primary coils are equal to those of the secondary then it is said that the transformer has a 1: 1 ratio that mean the voltage that enters is equal to that leaving (generally the jack XLR adaptors have this type of transformers since their main objective is to balance a signal and not to amplify or attenuate it).

n.b. A transformer is also useful against any DC interference because it does not let pass this type of current (when you have tools with strong interfering noises often along the signal interposes a passive D.I. Box that present a transformer and helps to reduce the noise).

In a transformer of the coils ratio should not necessarily be 1: 1, since this value strongly depends on what you want to achieve and the type of tone you want to hear (as by varying the coils also varies the dynamic and frequency response). However, there are rules to follow also in relation to what is the difference between the output impedance (e.g. a microphone ) and the input (such as a pre-amplifier microphone) for not to oversize or undersize the circuit (if Out > In the signal will struggle to move and be properly transferred to the amplification process generating strong background noise and interference, if out < too much of in, the signal would travel too fast, creating changes in response, for better signal transfer is considered a good compromise to have relationships transformation of from 1: 10 to 1: 20).

n.b. If a microphone line located at the input to a battery of valves can be of help to interpose before a transformer that raise the load impedance as the input impedance of valves is very high (even in the order of M ohm). In an active line, built as we will see with the help of integrated circuits and amplifiers the relationship between the impedances in / out can be much lower because the amplifier suffers less energy dissipation and visa problems for the transformer but unlike it has further problems in the balance process of the signal.

It should be said that a link between transformers, variable ratio (different between input and output), leads to the increase of symmetry problems, caused by the different impedance between devices, also it leads to the emerge of more capacitive effects along the conductor potential differences on the masses of the two transformers with consequent disturbances on the signal, so the more the ratio of the transformers is high and much more quality will have to be the transformers.

Another problem not negligible, is the resonant frequency of the transformer that it is a passive inductor. In the project phase, such frequency is good the highest possible, usually can get to 100 Khz – 200 Khz, such that can allow proper load applied without damaging the useful audio range (20-20000 Hz). This is reached by reducing the number of coils used, their section and interacting with the magnetic field values. To give an example, if the input transformer having a resonance at 200 kHz, we apply a load (connecting the output of a device with the input of another) with much lower impedance, the frequency response resulting in output will be easily filtered (low – pass), for example with cutoff to 10 Khz frequency, this is because as said, the high impedance input of struggle to make pass the faster output voltage with low impedance and so the higher the frequency of the transformer resonance in which enters the signal and the higher will be the cutoff frequency. The 200 KHz, are still considered as a limit as report between quality, then disturb in high frequency loaded and quantity, then the audio band useful with the load variations applied.

In the case in which the output impedance is much higher, just the opposite happens, and then a high-pass filtering, much more serious problem, as it is located immediately in the audio band and is not compensated by the resonance frequency if not with particular rejection circuits that tend to re-stabilize the impedance. If the two loads are approximating the resonance frequency is compensated adequately and thus is obtained a much larger bandwidth, but as previously seen is not useful to the transfer of the signal to be able to amplify properly. It is basically for this reason, that when it has a pre-amplification circuit with passive transformer is used to insert a low-pass filter, to eliminate eventual interference of higher-order on the 200 Khz (useful band for good sound quality), so that in case of load poorly adequate, as low value, filtering low – pass seen before result more limited. The ideal, though little used to date to qualitative issues would be the use of a device which allows to vary the impedance.

The passive method, in spite of the distortions and the noise generated, allows unlike some active mode to get 100% of yield, and thus to eliminate the external interference completely.

A passively balanced signal can easily be unbalanced for the load transfer of unbalanced connections. By contrast there is the fact of obtaining two different impedances between the output and the input of which can generate an incorrect load transfer and potential differences along the circuit generating the usual signal disturbances. The advantage of using a balancing transformer is the ability to disconnect the mass even when the balanced signal is connected to an unbalanced input, which as we have seen in a purely unbalanced signal cannot be made and as we will see even on an actively balanced signal even if active it’s the only input or balanced output.

Below we see 2 mode of passive circuit with output balanced and input unbalanced (Fig. 8 – Fig. 9), of which the first helpful to solve the problem of the disturbance loaded on the mass thus being able to disconnect.

Fig. 8 2015-12-27_14-18-06

In this case (Fig. 7) the mass can be disconnected as the signal return occurs in any case for the conductor used to transport signal in counter phase as balanced signal. They still manage to eliminate low-frequency disturbances as part of them are able to be discharged to ground, while always present in high for impedance differences between the two circuits and for the imbalance of the signal. The induced current for the potential difference between output and input given the different impedances can create overcurrent that if loaded from the shield would be unbalanced on the two conductors, and then summed with the signal with less intensity than in the next case.

Fig. 9 2015-12-27_14-21-31

Instead in this circuit (Fig. 8), the cable used to return is not brought to the unbalanced connection but is grounded immediately after the balancing, for which the mass cannot be disconnected because the signal return occurs precisely on it. And then, all loaded interference will come at the input stage. As for the overcurrents loaded from the screen should be also added to the signal with greater intensity as loaded only by the sole carrier signal conductor.

It is therefore useful if you need to unbalance a balanced signal passively, to always set to mass the connection toward the unbalanced circuit bringing both the connectors (line. return) from mountain to valley.

n.b. Some connectors have pin 1 (ground) slightly longer than 2 and 3 in which the signal circulates, this to allow you to connect and disconnect the cable even with the powered equipment, without the need for over voltage on the circuit (the chip which feels) resulting in the possibility of damaging the equipment itself. This is because at the time of insertion, the first cable that connects is the mass and then the two signals ( fig. 10 ).

Fig. 10 2017-05-16_12-13-40

 

More About Analog Audio Cables:

Analog Audio Cables – I (Technical Features, Shielding, Operating Environments)

Analog Audio Cables – II (Types of Connectors and Connections, Unbalanced Connectors and Connections)

Analog Audio Cables – IV (Active balancing)

Analog Audio Cables – V (Differences between Jack and XLR, Bantam, Speakon, Powercon)

Analog Audio Cables – VI (RCA, MiniJack, BNC, Midi, Starquad, Edac, D-Sub, Socapex, Euroblock, Tipologie di Adattatori )

Analog Audio Cables – VII (Connection Types, Ground Loop, Solder A Cable, Acoustic Pollution)

 

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