Connectivity for Beginners

One of the most basic aspects of a sound system happens to be one of the most misunderstood.  That of connectors, operating levels, electrical scheme, etc.

Let’s start with the most easily understood: levels.   There are a few terms that are roughly descriptive when talking about levels you will need to deal with in hooking up a sound system.

The easiest is “speaker level.” This is pretty obvious.  It’s the output of a power amplifier, and connects to a speaker cabinet.  It is relatively low voltage, but can be VERY high current in high power amps.  Since it is high current, it is very important that you use large enough diameter wire between the power amp and the speaker.  If you use wire that is too small, the wire itself will cause a voltage drop, heat up, and lower the damping factor.  I’ll go into those things later, but for now, just realize that those all are Very Bad Things.

Speaker wire should not be shielded, but should be stranded, both from a flexibility standpoint, as well as the fact that stranded wire just conducts better.  Use connectors that are capable of handling the current of the connection. In high power subs, 30 AMP Twistlocks are a good choice.

Line level would seem to be descriptive, but keep in mind that there is no single “line level.”  There are actually several standards that can be used. Home stereo equipment uses a “line level” that is -20dBm.  Remember that “dB” is a reference, so -20dBm is 20 dB below 0dBm. So-called “semi-pro” gear is -10dBm.  This was a trend that was started by the early cheap MI (Musical Instrument market) grade gear such as Fostex, Tascam, Ramsa, and others.  It is less susceptible to noise than equipment operating at -20dBm due to its higher operating level, but still can not approach pro-level gear, which operates at +4dBm.

Operating level has nothing at all to do with whether a connection is “balanced” or “unbalanced,” or whether it’s high impedance or low impedance.

Balanced or unbalanced is like black or white.  It either is, or it’s not.  An example of an unbalanced connection is a guitar cord with its 1/4″ plug.  The tip of the connector is the electrical “hot” (+) connection, and the sleeve of the plug serves as both the “negative” (-) and the ground (shield).  The shield prevents electrical noise from being induced in the “hot” center conductor and ‘drains’ it to ground.  However, since the shield is also the negative (-) conductor, some of that noise ends up being introduced into the next stage of the audio circuit.

A balanced connector and cable requires 3 conductors.  Examples of 3 conductor connectors are XLR connectors, such as on the ends of a mic cable, and “TRS” 1/4″ plugs. “TRS” stands for Tip, Ring, Sleeve. The tip, or end of the plug is the hot (+) connection, the ring is the negative (-) connection, and the sleeve is the ground/shield.  These two connectors are the most commonly used ones for all line level connections, and you will most likely never encounter anything else in the audio field.  The cable should be 2 conductor (twisted pair) plus shield.  If you see something called “line” that has an RCA connector (like on your home stereo), it is almost guaranteed that it is really a -20dBm or possibly -10dBm connection, and you must be careful so you don’t run into distortion problems.

The third of the traits of a connection you need to consider is impedance, which is something like resistance, but is not static (it can chance because of several factors, the most common being frequency).  High impedance values are VERY high.  A high impedance line input or guitar input will have an impedance of around a million ohms (1M ohm).  Generally, the output impedance of the device which will feed this line input will have an impedance of 1/10th that value (or less), or 100K ohm.

Low impedance is considerably lower.  Generally, anything below 10K ohm can be considered to be low impedance.  Again, the source device should have a much lower impedance than the ‘destination’ device (load).

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