Analogue interconnections

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To maintain the high sound quality of Atlas, it is important to follow some basic guidelines when making analogue connections to the unit.  This section discusses some things to watch out for.



Cable quality


Use of good-quality, heavy duty audio cables is recommended.  For microphone use, quad-twisted cables may give best results.  Cables with heavy screens are recommended, especially for unbalanced use.  Owing to mechanical differences between connectors from different manufacturers, it is advised to use cables with identifiable connectors from reputable manufacturers.  This is especially true for jacks, where unreliable tip connection can occur owing to the slightly non-conforming shape of some manufacturers' parts.  Neutrik connectors are used in Atlas, and these are recommended to ensure reliably-mating cables.



Balanced versus unbalanced connections


Where possible, balanced interconnections should be used, since the audio signal is represented as a voltage difference between two dedicated conductors (neither of which is ground-coupled), which are usually closely-twisted to ensure that any interference pickup is cancelled out.  In unbalanced connections, the signal is represented as a voltage difference between a single signal conductor and an accompanying ground conductor.  Where dynamic ground-potential differences exist between the source equipment and the receiving equipment, this difference is effectively added to the unbalanced audio signal.


This effect has long been familiar in audio systems as 'hum loops', where the variation in ground potential occurred at line-frequency, and was developed by the flow of line-frequency currents to linear power supplies. Hum loops were usually resolved by either steering the currents along non-critical routes by re-arranging the topology of the system ground interconnections, or by mass-interconnection the system grounds using heavy gauge cable so as to minimize the hum voltage resulting from the current.


Obviously many items of analogue audio equipment only have unbalanced connections; this is especially true of consumer equipment, which is often used for monitoring even in professional studios.  If you must use unbalanced connections, keep them as short as possible and use good-quality cables with substantial screens.  If you have a choice, keep the signal level as high as possible on the interconnection, since this will make any interference proportionally less noticeable.


Instrument connections are often particularly vulnerable to hum and other interference, since they are usually unbalanced and low-level,  and frequently employ a long cable not selected for its interference-immunity qualities.  Also, the source impedance is often high, making the connection particularly vulnerable to interference.


Some digital audio and computer equipment with switched-mode power supplies can cause particularly troublesome interference problems, especially for low-level, unbalanced signals.  This is discussed in the following section.





The increasing use of low-cost digital equipment and computers in the audio production process results in various potential problems for the remaining analogue devices.  It is well-known that the hostile power and EMC environment inside a typical computer is likely to be the limiting factor governing the audio quality of an internal analogue sound card. A solution to this is the use of external 'sound cards', such as Atlas, with their own enclosures and power supplies allowing adequate space, power and electromagnetic peace and quiet for the well-being of studio-quality analogue circuits.


However, even the sound quality of external devices can be compromised by the proximity of some types of digital equipment.  Many low-cost switched-mode power supplies emit interference which can compromise system audio quality even at a distance.  The hostile mechanism is usually 'conducted interference', wherein the high-speed switching action of the power converter results in voltage and current transients being conducted back down their power cords.  If the equipment is connected to mains safety-ground, transients can also be conducted down the ground connection.  Radiated emissions (airborne radio interference) can also be a problem, but it is less common that this will have such a serious effect on audio quality.


Conducted power-line interference can cause problems in analogue equipment within the installation if its own power supply allows the transients to pass through to the audio circuits.  However, conducted ground interference can be even worse since, if the ground connection of the analogue equipment is modulated by switching interference, there is nothing that the designer of the equipment can do to combat it.


How much any conducted ground interference affects audio quality depends on many factors, mostly to do with how the various analogue boxes in the system are interconnected and grounded.  Where possible, high-level balanced connections should be used, just as in the case of hum-loops as discussed in the previous section.


Where ground-potential variations are caused by switching power supplies, the effect can be more difficult to resolve, since the signals can occur at more noticeable frequencies: although the supplies usually switch at frequencies too high to hear, the frequency is often modulated by variations in the load current over time, resulting in a continuous modem-like chirping in which can be heard particular events such as computer screen updates, disk activity etc.).  Another problem is that even heavy ground cabling may not reduce the effect of the interference, since high-frequency currents may not see much less resistance in a thick conductor than a thin one.


How do the equipment manufacturers get away with this?  Surely there are stringent regulations covering conducted and radiated emissions?  Well that's true, but the level of emissions which can result in audible degradation of low-level, unbalanced audio interconnections are well below legislation levels.  Unfortunately, computer power supplies (and especially the switching wall-warts and line-warts which power notebook computers and other small items) are amongst the worst offenders.



Vinyl decks


Atlas is equipped with an RIAA de-emphasis filter in analogue inputs 1 and 2 to allow direct connection of a vinyl deck, as described in the Analogue inputs section.  Since vinyl decks usually have a low-level, unbalanced output it is important to minimise interference as discussed above when connection a vinyl deck.


Since most magnetic cartridges require a higher input impedance than that of the Atlas microphone preamplifier input, it is usually best to connect a vinyl deck to the instrument inputs using a pair of phono-to-mono-jack cables.  The instrument gain controls can then be set to an appropriate level for the particular cartridge.  The 1MR input impedance of the instrument inputs will work satisfactorily with most magnetic phono cartridges (which are 'moving magnet' types), but with some cartridges, improved frequency response and noise levels can be achieved by fitting the cartridge's required load resistance (usually 22kR or 47kR) across the instrument input terminals; this is best achieved by soldering it inside the jack.  Moving coil cartridges have a lower output level and require a lower preamplifier input impedance.  These are best connected to Atlas's mic inputs, or may require a dedicated preamplifier.


Most vinyl decks have a ground wire separate from the audio connectors.  Connection of this wire for lowest hum is often a matter of trial and error.  Ideally this should be connected to Atlas's analogue signal ground (the outer of the instrument input jacks, or pin 1 of the mic input XLRs).  Since no dedicated terminal exists on Atlas, it is usually easiest to connect the wire to the outer of one of the deck's unbalanced output connectors.  In some situations, a direct connection to local mains ground may work better.



In summary


Use good-quality cables with reputable connectors;


Use balanced connections where possible; if you must use unbalanced connections, keep them short;


Ensure that signals passing between equipment do so at as high a level as is practical;


If switching interference is heard, try to identify the source equipment by unplugging things one by one.  When you find the culprit, either re-plug it a long way from the audio equipment, or use a power filter, or both.