Analog Audio Cables – IV

Active mode

In the passive balance transformer has the task of balancing and unbalancing the signal to then be amplified if necessary by an amplifier device, the active audio balance instead exploits the use of electronic components such as amplifiers to balance-unbalance and very often also amplify the signal itself.

In terms of listening, the answer given is very often erroneously defined cold as opposed to the passive (hot) (because of interferences usually introduced by passive circuits but that does not mean more quality). The active balancing as well as all digital audio devices incorrectly defined of poor quality, because the feeling of cold is associated most often to a poor sound. Through studies done instead, it can be stressed that this application allows to generate a large percentage of distortions inferior to that passive. Starting from very low background noise, contrary to today in those of greater size the noise is mainly due to the instability of the electrons of which these components are composed, then natural phenomena and negligible listening. Also generate a minor distortion value went to the wave passing through it.

Always through psychoacoustic studies, it is believed that such association of cold sound often negative, is derived from the fact that for decades was the sound engineer or audio technician acoustically used to work only with a certain type of devices (passive ones) as the only on global market, such that sound totally different from the usual ones are not so pleasant if not with the subsequent re-hearing habit. To date over that for technical confirmations that facilitating the use of active systems, it’s bringing us to maintain this kind of (cold) associated with still and always perceived sound, but so on more pleasant way. With the passing of years and the increasingly technological evolution that will at some point to outperform the passive systems, there may be persuaded to 100% of the technical potential – practical of these systems.

In an ideal context, the signal passing through the balancing process and amplification as to come down as such to the original one only possibly amplified and processed. In this regard, the active balancing comes closest to this ideal context than passive.

Considering all three types of signal transfer between professional audio equipment, then unbalanced, balanced passive and active balanced, the active one is in the midst of the final sound quality obtained.

 

Historical background and objective differences between active and passive balance.

We start from the birth of electronics around the 60′, but where else the whole thing was just being tested. Only around the year 90 ‘the active balancing systems took root in the professional market and also simultaneously in the consumer market, thanks to the advent of digital systems in consumer, such as audio mixers and outboard.

Very often even today mainly for the connection of the microphones to the pre-amplifiers microphones it’s preferred to use circuitries passive as the microphones almost all have a transformer in their output stage against induced currents and that the impedance level is better with pre- interface amplifiers that have input impedance a least 1: 10 easily accessible from transformers. The active balancing is instead far more prevalent in the line circuits (eg the main out of the audio mixer and the input of the power stages) in which the voltage is much higher than the microphone and in which is simplest to treat the various interference.

As today thanks to the continuous evolution of this type of active circuitry, the so-called “servo balanced”, you can still find some models with active balance both input (microphone – line) and output (microphone – line).

The first active balance was realized through semiconductor-type transistor, such as BJT/ JFET for the outputs and the microphone inputs and line, trying to maintain an optimum impedance ratio, not to create too much resistance on the signal with the consequent growth of the noise.

To date, thanks to miniaturization, are becoming increasingly popular systems in Integrated Circuit (IC), and small hardware containing thousands of these transistors. These circuits are very popular as defined before to realize devices with fully active inputs and outputs, as well as physical full digital domain. Having very high calculation capacity and efficiencies around 100%, to allow balancing level to carry out a completely transparent process, then the minimum of background noise and wave distortion and signal level, can be exploited as A / D converters or D / a for a use of the same in the digital domain, in addition to a smaller footprint, a much more reductive weight.

Below, we see some images (purely indicative) representing qualitatively the difference between passive and active balanced.

fig. 1 a     Fig. 2 b

Fig. 3 c     Fig. 4 d

 

In Figure 1, we have a sinusoidal waveform which we will go through the various types of balanced views.

In figure 2 we have the output signal from a passive balanced.

In Figure 3 we have the output signal from an active balanced with BJT amplifiers.

In figure 4 we have the output signal from a balanced integrated circuits.

It ‘clear that an active balanced is clearly superior in quality to that passive.

It should also be said that the active balanced require from one to two amplifiers (you will then see in what way). Considering the chips being made up of millions of these transistors, one could argue that with a single circuit, you are able to make millions of balanced, more than necessary to cover even the biggest of the world audio mixer. In fact, a circuit, is used for an input this to take full advantage of the potential of such products, especially if they must also be used as a converter, in addition to the fact that, for example, would still be quite difficult, connect tens of wires are balanced, this small hardware of a few mm in diameter.

This type of circuits, as will be seen, and especially the first (single amplifier), suffer from the input voltage and / or rear driven towards the output circuit, such that it is recommended before turning on any type of amplifier with active balance of the signal, to activate in which case it makes useful the phantom power so that it is not caused an overload, however little, if any long-term interest expense. However, there are compensation circuits as protection, but these circuits are more sophisticated and the less will be the final quality sound, even at nominal voltage values, and especially at high frequencies, while in the opposite (too simple circuits), will risk of burn easily the amplifier.

 

Balancing with Single Amplifier

The first active balancing is that in single amplifier (Fig. 5).

This system can be used both to balance that to amplify the signal.

Fig. 5 2016-01-17_00-08-44

Taking into account an unbalanced microphone signal, it enters the electronic circuitry where it meets amplifier with unit gain (see the circuit in Figure 5) then no amplifying, in the case in which it has the sole purpose of balancing, or with different or variable gain from 1 in the case where it is used to amplify. In both configurations, there is to say that such a system, is the poorest compared to the circuits that will be then to see later, so it is used more than in low-end consumer systems. Its weakness lies in the bad balancing of the signal, as defined as for passive ones should reach 100% so as to eliminate all interference loaded up at higher frequencies.

This circuit tends to exceed 90-95% in the audio band and drops vertically at 30-40% over 20 kHz, so it would need a further filter circuit with the resulting drop in performance over which this device was thought. Since there is only one unit gain amplifier, the balancing signal also causes a loss of 6 dB. While with gain greater than 1, the circuit can be specially constructed to compensate for the loss but as also defined for passive ones it might create a problem of compatibility between the circuits used. The input stage is exactly the opposite of that output, in which an amplifier accepts a balanced signal with mass, and brings in an unbalanced output with same mass recovering the unbalance of 6 dB.

In the active circuits, it is not possible to detach the mass, as being part of it and not isolated as in passive ones you should be to generate strong interference induced with the risk of damaging the components themselves, for which in case of hums is good to connect or the output device or the input to a passive circuit so as to disconnect the mass on the side of the transformer. Special attention should be made also in the case where the conductor has a shield, also this component cannot be disconnected if not with a present transformer along line since all the magnetic interference should be loaded to the input circuit or output depending the posting.

Although Being generated a lower dissymmetry between the output and the input than those passive ones causes different impedances, the balance of the signal will not be optimal, especially in high frequency.

The same argument as to the impedance compatibility between output and input of a passive circuit with transformers applies in theory also to active-active systems, so that different components used to make the same circuit can have different impedances.

Below is a simple chart of a balancing circuit connection between passive and active, useful for the detachment of the mass (Fig. 6)

Fig. 6 2016-01-17_00-08-44

Then we open a parenthesis to this kind of problem which is that the disconnection of the mass, since mostly in live events and always present but limited in the study, there are always induced interferences on the masses that can cause signal disturbances, cause malformations of external electrical circuits, or of the same. In theory, the mass could also be disconnected from the circuit even in an active system – active provided that the potential difference between the signal carrier connectors and the mass itself is equal to 0 V, so that any return current can damage the amplifiers. But, in reality, the same disturbances present on the mass create varying potential variables in time and independent between the input and output circuits, and that is why disconnection is strongly advised. It is also true that in order to prevent these phenomena, condensers are interrupted just before the output or input stage to amortize overvoltage, but the problem is not resolved.

The difference of not stable potential for this type of circuit, can be even more problematic in the case in which an output, be doubled, tripled, etc. of more inputs. And that is why today, especially in live events it is preferable to use microphones with extended output (transformer), especially when they have to be connected to the splitter.

In the case of microphones, being more delicate, more often are constructed with passive balance, the only problem that may be is that of the usual impedance incompatibility, in this case between the active and the passive, with a result of an incorrect signal transfer, phase distortion and frequency. There are, however as will be seen, the circuits that compensate a little so these factors favor the use of an active circuit with the shortest possible interference and improve the transfer of load (the servo balanced).

The Balancing error is not caused by wave distortion if the amplifier is also used for variable gain purposes and for voltage variations both of the input signal and output, the so-called symmetry problems due to ‘ Alteration of the response to the variation in the gain. Although, to stabilize the whole, there are built-in compensating circuits with capacitor and parallel.

It must be said, however, that the compensation, at least nowadays is fixed and not variable, so it is calibrated to optimally balance a certain voltage at the input, generally the nominal 0 dBV (always depends on whether it is an Amplifier for microphone or line signals). Thus, a unit gain as well as a higher than 1 can be discretely linearized at least for the aspect of phase and frequency distortion. But whenever the amplifier is at variable gains, the usual distortion problems are beginning to occur, though less than as if the compensation circuit was not present. This problem can be found in any amplifier.

It can also be found a circuit with unity gain amplifier for balancing and one or more later stages of amplification, for example the first amplifies + 1dB to + 10 dB and the second from + 10 dB to + 20 dB in order to improve the symmetry.

This circuitry, as defined, is almost never used unless it has consumer level due to its low yield but at a not excessive price. Or you can also find for the line level balance, as at higher voltages than those microphone is able to maintain a good dynamic so low background noise and a good linearity in frequency and phase.

The output of this type of active balance can be connected to an unbalanced input, by grounding one of the two outputs or inputs. Generally the reverse of their own stage not to find themselves a phase-shifted signal.

 

Balancing with double amplifier

Output Circuit (balance an unbalanced signal) (Fig. 7)

Fig. 7 2015-12-27_18-38-10

Input circuit (unbalance a balanced signal so as to eliminate interference) (Fig. 8)

Fig. 8 2015-12-27_18-40-10

The second type of circuit is that in double amplifier (Fig. 7 output circuit, Fig. 8 input circuit) much better than the previous one in that it has a percentage of 100% of the balance of the entire audio band and excellent tonal quality of the signal, much more linear and very low impedance. It is based on the previous circuit only with more amplifier inverting input to create a copy of the signal inverted phase. Also in this case, the mass cannot be disconnected, and there are present the compensation circuits, for both overcurrent that for symmetry. The problem is in fact always to be subject to external interference, surges and capacitive effects induced on the conductors themselves. Its strength lies in the fact of being made up of 3 stages of which 2 unit gain amplifiers for balancing and subsequently a greater than 1 or variable gain amplifier, so that the works are well separated and the input stages are maintained low impedance so being able to work at will on the gain and have an optimum balance of symmetry. In addition, thanks to this circuit, the variable gain amplifier can be constructed with additional stages so as to promote best amplifications with less distortion always phase and frequency, unlike the previous one, as being also used to balance cannot be separated if not by adding an additional amplifier part but with loss of quality. For the input stage is the same output circuit only in the opposite direction and then with the inverting that re-balance the signal in phase.

Another exception is made for the capacitive effect induced on the cables, especially when they carry long distances. In the case of a passive circuit, the only even if not negligible problem was that of a natural filtering low – pass, while in active systems as there is no transformer which blocks the return current all induced interference end up inside the amplification circuit, creating a kind of loop, and then the signal oscillations although lower than when you will see later, since these two types of circuit have an impedance of lower output.

In facts is much used as also the previous one, in circuits in which the external interferences are minimal and short distances, but generally in a studio environment and even more live in which there are many sources of disturbance are using another much more type of circuit safe in terms of problems of overcurrents and various interferences, called Servo balanced. Although it must be said that the transformer despite its flaws is still the favorite of all as it solves many problems.

The output of this double balanced amplifier circuit, cannot be connected to ground as it would be short-circuited because the mass is common to both the amplifiers and is not isolated. It is however possible to remedy the problem by disconnecting one of the two outputs so as to have an unbalanced signal though attenuated by 6 dB. Although leaving a disconnected cable it is charged by all the interferences that may affect the signal.

Servo Balanced (fig. 9)

Fig. 9 2015-12-27_18-49-25

The most widely used circuit, is precisely the servo balanced, which allows to obtain a perfect balance of the signal with greater protections against over voltages and interference due to the presence of particular counteractions realized with resistors in series and parallel, as seen in Figure 9. On the other hand, the fact that all of these passive components used with feedback and create higher output impedance of the previous ones, resulting in a worst tonal quality and putting the circuit to suffer most from the capacitive effects induced on the cables. For this it is dimensioned placing two resistors in series to the outputs that counteract the effect but consequently worsening the quality. For capacitive values of low value the circuit is able to compensate the effect but for high values suffers from sharp fluctuations, higher than the previous cases.

The circuit is similar to the previous one with a pair of identical features amplifiers such as to optimize the symmetry and subsequently for the input stage or previously for that output a multi-stage amplifier for the amplification of the signal. Unlike the other, this type of circuit is built specifically to be able to unbalance the signal when it is deemed necessary.

n.b. In the digital audio mixer or digital outboard of last generation signal balancing is mostly servo-balanced directly from A / D converter, instead of the transformer, electronic circuits are provided to prevent overloads and return currents. If the input circuit is not intended for these reminders, be careful not to send an inverted signal (pin 2 with pin 3 of the XLR connection to avoid in shortcircuit ), the same goes for unconventional connections, so they can have the pin 2 and 3 inverted.

 

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 – III (Balanced Connectors and Connections, Passive 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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