Guitar Connections

An article by instructor Ivan Milenkovic

We all need to connect our gear in various ways, and there are always more than one piece of gear that we can connect, wetter it's only PC and amplifier line out. Many people connect these devices, and get noise problems, or even fried cards, and are not sure why. It turns out not all the connectors work on the same "level", as I will explain soon.

LEVELS (VOLTAGES)

As far as analog signals go, there are several different standards that had to be made, relative to the strength of the output signal from the source. Today, we use 3 of these standards: Line-level, Mic-level and Instrument-level.

Line Level:

One that is most commonly in use is "Line Level". It's used to standardize the voltage/strength of the input/output signals, in order to make connecting of various audio gear possible. Most gear that has Line Level inputs are CD and DVD players, consumer electronics like MP3 players, or integrated/consumer PC audio cards.

The standard for Line Level signal is around 750 milivolts. What this means is that all the outputs or inputs that are marked as "Line", are usually operating at this voltage.

For example, with intergrated sound cards you can see the arrow sign, or "Line In" or "Line Out" sign. This means that you can connect this line level output to another device that is operating at line signal voltage at their connectors.

Mixing consoles usually have various inputs available, because of their function, so they usually have labeled out line level inputs as well. On most mixers, the last 4 or 8 channels (ones without XLR connectors, are usually operating only at 750mV (line-level).

There are two types of line level:

consumer line level is at -10dBV
professional line level is at +4dBu (or dBm)

In order not to get you confused with dBV and dBu values, it is just important to understand what goes where. Here are some common connectors and what line level they operate at:

RCA (phono) connectors are usually -10dBV.
XLR connectors are usually +4dBu (Mic Level).
TRS quarter inch jack plugs can be either -10dBV or +4dBu, depending on the use.

Mic Level:

Microphones are operating at much lower voltages then line level devices, around 1mV - 80mV. This signal is too weak and prone to external interference. This means that a preamp is being used for microphones in order to get the mic-level low voltage signal to line-level high voltage signal.

Instrument Level:

Instrument level is not a standardized voltage, but can vary depending on the instrument's output, and it's usually somewhere between mic and line level voltages.

Here's a hi-res picture of a mixer that has mic-level and line-level inputs:

As you can see, XLR inputs are low voltage mic-level inputs, and they can handle lower voltage power, which means that you can "burn" the inputs by using the higher voltage sources like instruments or line-level devices. Preamps are very sensitive, and serve to amplify the low voltage mic signals to appropriate 750mV line-level.

Also, notice that there are lots of line-level inputs available on the mixer. All these serve for standardized 750mV equipment.

Impedance

A high impedance circuit tends to have high voltage and low current. A low impedance circuit tends to have relatively low voltage and high current. Impedance is usually labeled as "Z" and it is measured in "Ohms" (Ohm).

If you connect a low-Z source to a high-Z load, there is no distortion or frequency-response change caused by this connection. But if you connect a high-Z source to a low-Z load, you might get distortion or altered response.

When you connect an electric bass guitar or an electric guitar (a high-Z device) into an XLR-type mic input (a low-Z load). The low frequencies in the signal will roll off, so the guitar will sound thin. We want the guitar to be loaded by a high impedance, and we want the mic input to be fed by a low-impedance signal. A direct box or impedance-matching adapter does this.

- Microphones and line outputs are usually low Z.
- Electric guitars, synthesizers, and line inputs are usually high Z.
- XLR mic inputs are low impedance; phone jack mic inputs are high impedance.

BALANCED/UNBALANCED audio:

Balanced audio is a method of interconnecting audio equipment using impedance-balanced lines. This type of connection is very important in sound recording and production because it allows for the use of long cables while reducing susceptibility to external noise.

Balanced connections use three-conductor connectors, usually the XLR or TRS connector. XLR connectors, for instance, are usually used with microphones because of their durable construction, while TRS jack plugs are usually used for mixer inputs and outputs because of their smaller profile.

TRS and XLR cables are basically the same, except having different connectors, here you can see the picture for XLR male/female and TRS connectors.

All these have ground wire, and two wires that carry the same signal, hot & cold. These two wires are phase-reversed, so they aren't susceptible to external noise.

- Microphones usually have low-voltage, low impedance signal that is very susceptible to interference, and because of this, they are usually featuring balanced connectors.
- Electric guitars and bass guitars have "mid" voltage, high impedance sources, and they usually have unbalanced connectors
- Consumer audio devices usually feature unbalanced connectors
- Prosumer audio usually feature both unbalanced and balanced connectors