The Pro-Audio Guide For People Who Know Nothing About Pro-Audio Part 3


Our last “episode” was all about transferring electric signals to a mixing console or recording device. Now, it’s time to talk about the input section of a console.

Jack Be Nimble

When connecting to a console input, it’s important to choose the correct connection point. Some consoles make this very easy by offering only one physical input, usually a simple female XLR – or, a female XLR/ female TRS “combo” jack. In other cases, you might have separate connectors for microphone-level signals and line-level signals.

When the choice is necessary, the major deciding factor is primarily based on the amount of gain (that is, voltage increase) necessary for the signal to work well with the console. We’ll talk more about this later.

Another kind of jack you might encounter in a console’s input section is the insert. Inserts are actually a combination input/ output point that, electronically, comes after the preamp or trim. An insert is meant for pulling a line level signal out of a console entirely, processing the signal, and then putting the signal back into the console so it can be mixed as normal. This is often accomplished by using a cable with a TRS male on one end, and two TS males on the other. One TS male connects to the tip and ground of the TRS, whereas the other connects to sleeve and ground. One TS, then, is the “send” to the external processing, and the other is the “return.”

PAD

Some consoles, as well as some DI boxes, come equipped with a PAD switch. “Padding” an input means running the signal through electronics designed to reduce voltage before other devices are encountered. PADs are used to avoid overloads from a following gain stage. For instance, in the case of a console with only one input point for each channel, the input stage itself might not be able to handle certain voltage levels. With a PAD engaged, that incoming voltage can be reduced to something more manageable.

Preamps…Or Just A Trim?

Many audio processing devices work best when the audio level presented to them is between roughly 1 volt and 10 volts, also known as “line-level.” Signals from microphones and DI boxes can often be well below this general area, perhaps down in the tenths or hundredths of a volt. These lower-voltage signals are commonly categorized as “mic-level.” Signals at mic-level may require large, positive gain changes to correctly drive downstream electronics, and so a jack that can be connected to a microphone preamp is needed in that case. Mic pres are important and specially designed, as they have a difficult job: They make very large changes to input signal voltages while being as noiseless as possible. They are the “highest” gain stage in a console; Nothing else can change the ratio of input to output voltage like they can.

Signals already at (or very near) line-level may need only a small gain adjustment, if any, and so they can be connected to a “trim.” A trim is a gain control with a more limited range…and, often, the ability to reduce a signal’s level.

If only one input point is available, it’s very likely connected to a gain control that can trim down line-level signals, while also having enough positive gain available to work with mic-level inputs. (Or, a mic pre with an available PAD).

Digits

As a quick aside, let’s discuss the difference between analog and digital systems. In an analog console, signal voltage is passed directly from the input stage to everything else. In a digital system, the signal is passed to a converter, which then sends data along. In either case, an appropriate drive level from the input stage is necessary – and although the “technology bases” are different, the general behavior of the console regarding signal levels is unchanged. A good, healthy, line-level signal is necessary, whether that signal will be passed as analog voltage or data that represents a voltage.

Leveling Off

So, when setting up your level from the preamp or trim, what should you look for?

A basic rule of thumb is to shoot for 10 – 20 dB below overload. This gives you room for the input to get louder without clipping, while avoiding being too low. This also gives you some room to make changes with processing and other level controls later. It is NOT necessary to “get as loud as possible without clipping,” especially because trying for that tends to lead to levels that are too hot. On the contrary, in most cases you’ll be just fine if the signal is reaching the middle of whatever metering you have available. If all you have is a “Signal Present” light, you’re probably in decent shape if the light is continuously illuminated during louder passages.

Please do read up on the manufacturer’s specifics for metering on your console. Knowing what all the lights and numbers mean is very helpful for proper operation of your equipment. Also, be aware that digital consoles often use a different dB scale than analog mixers.