Industries, companies, technologies and manufacturing is always changing and what connector is “best” today could change tomorrow. Also, what connector is “best” in one use might not be “best” in another. I would like to give an explanation of connector attributes that anyone can use to evaluate the suitability of connectors of all types for their specific use, not just XLRs.
The first set of attributes that are important for connectors that are designed to be “hot plugged” (Such as XLRs) and are unimportant for connectors that are not inserted live.
Sequenced Contacts: Sequenced contacts are a difference between the contact mating point of some conductors, such as ground, shield, or power, which are fully mated before the signal conductors. In a properly designed XLR the female contact for pin 1 is mated before pins 2+3. This prevents the often more sensitive signal circuitry from being damaged by large parasitic currents.
Sacrificial Contact Features: In some circumstances the initial mating of the contacts in a connector causes a large current to flow before the contact resistance is low. This can create a large current flow in a small part of the contact which damages it, then the fully mated contact will not make a reliable or stable connection. Properly designed connectors will have contact features that are not integral to the performance of the fully mated connector that can be damaged sacrificially (on purpose). In XLRs this is usually accomplished with multiple contacts that are not engaged simultaneously, so that the first contact mated is damaged but the others are not, enabling a good fully mated connection.
The next set of attributes are integral to contacts that carry small signals but “may” not be necessary for contacts that carry power.
High Energy Mating: The mating contact should have a large contact pressure, sliding force or other locally highly disruptive force to clean contaminants, adsorbed liquid and gas molecules, and/or tarnish from the surface. If the energy is not sufficient the contacts will have a higher than normal resistance that will quickly drop as electrical current flows through the contact. This lowering of contact resistance will often revert if the current is reduced causing a modulation of the contact resistance based on the current flow. In XLRs this is usually accomplished by the sliding force of the connector insertion and the concentration of that force by having some “sharp” feature on the female contact which increases the local contact force. Connectors with large effective contact areas should be avoided unless the mating force is very high. For contacts carrying large currents the energy from the current itself cleans the contacts so physical energy is not necessary and large area contacts are applicable in these circumstances.
High Mated Contact pressure: The contacts should have a very high point pressure. If the mating force is to be reasonably low this means that the contact area should be small. This ensures that the contact stays stable electrically and does not allow contaminants such as gas molecules or organic compounds to intrude into the contact area.
Redundant contacts: The world is not perfect so redundancy is needed…
Stable Contact Resistance: The contact resistance itself, if low enough (lower than a few tens of milliohms for the majority of signals sent through XLRs), is not important. However that resistance should not change with time, environmental conditions or signal level.
A Low Impedance Shield: EMI is everywhere now. Without a shield you are potentially injecting large amounts of high frequency noise into circuits that may, or may not be able to deal with it gracefully. Even when the equipment can deal gracefully with this out of band noise, it will nearly always have better performance if not forced to do so. In one location or time of day the EMI may be inconsequential to performance but at another time or location it almost certainly will be. EMI is invisible to humans (without measurement equipment) so you won’t notice the change in conditions.
High quality dielectric: The contacts need to be separated by a physically robust, electrically insulating material that will hold them apart. Today this is not much of a problem and any well designed plastic dielectric will do. It does need to survive the limits of temperature the contacts will experience and be resistant to physical damage. In some applications the geometry of the dielectric, shield and contacts needs to be carefully engineered to prevent high frequency signal degradation (BNCs are one example).
The next set of attributes are needed for connectors that stay mated for long periods of time.
Non-reactive Contact Surfaces: The surface of signal contacts should be non-reactive so the mated contact properties do not vary with time and environmental changes. Gold, Platinum, and Rhodium are ideal. They are listed from lowest cost -> highest cost and smallest number of mating cycles -> largest number of mating cycles. Performance for a given contact design when mated is nearly identical for all three choices. HOWEVER, NEVER USE A CONECTOR PLATED WITH ONE METAL WITH A CONNECTOR PLATED WITH ANOTHER METAL FOR PRECISION SMALL SIGNALS. Silver is excellent for power contacts because of its extremely low contact resistance but is not ideal for signal contacts that are not exercised or cleaned regularly.
Contacts that account for movement, vibration and thermal cycling: As contacts move relative to one another in a cyclic fashion they can “fret”. This is a degradation caused by the repetitive movement of the contacts relative to one another and the microscopic scratching that occurs. One solution is to have such a large force between the contacts that they do not move relative to one another (requires very high pressure, like in a screw terminal or crimped terminal), welding (cold welding or current induced welding like in power relay contacts) or to have “floating” contacts (IE, contacts that are not rigidly connected to one of the housings). In XLRs the latter is the best method since it allows for a reasonable connector insertion force while still eliminating contact degradation.
Consistent Spring Force: For the springy contact, usually the female, the contact material should be permanently springy and not take a “set” even if a connector is mated with a male pin that is out of specification. Usually only one contact (the female) needs to be springy. Ideal materials for the springy contact are beryllium copper, tellurium copper, or various electrical bronzes that combine high electrical conductivity with high plastic yield. Pure copper, pure silver or brass should be avoided for the springy contact because the can easily “set” and fail to provide the correct contact pressure.
Power connectors have special requirements.
Low Contact Resistance: Power connectors need a very low contact resistance to avoid contacts overheating or wasting too much energy. This often means contacts that are much larger than signal contacts, have large contact areas or have silver plating, because silver offers the very lowest contact resistance. Contacts do not necessarily have a low contact resistance on initial mating, they may be designed to use the current flow to lower the contact resistance. Contacts that use current flow to lower contact resistance may just need to be solid copper, brass, or have an inexpensive tin plate because the current flow will provide the recurring energy necessary to keep the contact resistance low. Examples include electrical outlets and amplifier binding terminals. Since amplifier binding terminals carry signals of widely varying energy and it is critical for them to have a low resistance even when the signal is very small it is better for them to be plated with an inert metal and have a high pressure large area contact.
And finally attributes that are necessary for specific use cases.
Durable contact surfaces: Durable contact surfaces are necessary for connectors that are expected to have a large number of mating cycles. This means Solid Contacts, Thick Silver Plating, Thick Gold Plating, Thick Platinum Plating or Medium to Thick Rhodium Plating. A smooth surface on the larger area contact (usually the male) is also advantageous to limit wear.
Environmental Protection: Connectors that are intended to work well in their environment should take that environment into account. Examples include circumferential spring finger shield contacts to limit EMI interference, Gasket sealing for connectors used in wet locations, durable housings for connectors expected to withstand physical damage (XLRs are in this class), and cable strain reliefs for frequently moved connectors and cables (XLRs as well).
