There often arises among people confusion over different types of audio signals and their levels. This tutorial aims to give a basic guide to the various signal levels that you are liable to encounter in the world of audio. Hopefully this will help you to understand the terms commonly used, where to plug what, and also what sort of cabling you need to carry the audio signals in their various levels.
Signal Levels and voltages
a) Microphones and Mic Level
The voltage level generated by a microphone is typically about 2 millivolts. Levels can vary between 2mV (0.002 volt), to as much as 1V or more depending on how loud the source is, and how sensitive the mic is. A loud guitar amplifier or drum kit can generate a signal of several volts.
A millivolt is 1/1000 volt, and 1 volt = 1000 millivolts
A preamplifier is required to bring a mic level signal up to usable Line Level. It is important that a preamp has a large gain range to accommodate the widely variable signal levels that can be fed into it from a microphone. Many preamps have a pad switch or gain control pot to prevent overloading as can happen with the large voltages generated when recording amplifiers, drum kits etc.
Another way of stating the measurement of mic level signal sometimes found is the dBu. Mic levels can vary from about -52 dBu to +2.2 dBu
b) Line Level
Line level has a much higher voltage than mic level. This stronger signal enables it to be carried over longer distances than mic level. It is a medium level output ( less than speaker level), and is the common output level for compressors, Equalizers, reverbs, delays and other signal processors, as well as microphone preamplifiers.
The two normal line level standards used are:
Consumer or semi-pro (-10 dBV) .316 volts, this is the level commonly found in such equipment as CD players.
Professional (+4 dBu) 1.23 volts, found in mixers, signal processors etc
Two reference voltages are common. These being decibel volts (dBV) for consumer applications, and decibels unloaded (dBu) for professional applications.
c) Instrument Level
Instrument level is generally somewhere between mic level and line level, though it can vary greatly. Typical values are between 0.1V and 1Volt for passive guitar pickups, and up to 1.75V for active pickups. Expressed in dBu these are from -17.7 dBu and +7 dBu. Therefor there is no absolute standard for instrument level, as it can range from a few millivolts where passive or piezo pickups are found, to several volts on instruments with active pickups and on-board preamps.
d) Speaker Level
Speaker level is the strongest signal. It can vary between 3 volts to 90 volts or more, depending on the power output from an amplifier. To produce a listening level of 85 dB in a theatre would require a speaker output level of about 4 volts.
An often overlooked factor that greatly affects the sound and performance of devices is impedance compatibility and impedance matching. Impedance, measured in ohms, is the resistance of an electrical current in a circuit such as an audio signal circuit.
The input impedance of the device you are plugging into should be at least 7 to 10 times higher than the output impedance of the source of the signal (electric guitar, keyboards, microphone, etc) in order to obtain the optimum sound quality and the highest voltage transfer.
To illustrate this, if a mic’s impedance is 200 ohms, the input impedance of the device that you plug the mic cable into should be 1400 to 2000 ohms. Typically the input impedance of a mic preamp as found in a mixer is around 1500 ohms to 2400 ohms. The impedance of an electric-guitar pickup is High-Z at between at 20K to 40K ohms (20,000 to 40,000) ohms, so to match input impedances, the guitar amp, DI box input, or instrument input that you plug into should be at least 280K to 400K ohms. Examples here, are a Peavey 5150 guitar amp Hi-Z input is 470k ohms, a Mackie Onyx instrument input or Countryman DI input is 1.0M ohm. ( 1 million ohms).
Microphones with a rating of 150 ohms to 300 ohms are classed as low impedance. Above 600 ohms and up to 2000 ohms is considered medium impedance, while above 10,000 ohms is high impedance. Most microphones are low impedance( or low Z), and all mics that have XLR (3-pin) connectors are low Z. Low impedance microphones can be used with long cable runs of up to hundreds of feet without picking up hum or losing any of the high frequency content of a signal. A high-Z mic will lose treble frequencies and pick up hum if the cable is longer than 3 metres or 10 feet or so .
Balanced or Unbalanced?
Unbalanced cables use a single centre conductor, and a shield. Audio signals travel along the centre wire and the shield of the cable. This means that any interference that the shield picks up, will find it’s way into the audio signal. This noise picked up on unbalanced cables can range from hums to radio interference.
Balanced cables consist of two conductors that are twisted together and are covered by a shield wire that is usually braided. Each of the conductor wires are connected to an impedance at either end. The impedances of both conductors are the same. The audio passed on the two conductors is identical, except that on one of the conductors the phase of the signal is inverted at the source. The input of the equipment connected at the destination puts the inverted signal back into phase, and sums the two channels together. The shield of the cable does not carry the audio signal and should be grounded or earthed to the chassis of each audio device.
Unbalanced cable runs should be kept to 3 metres (10 feet) maximum length if possible, whereas balanced cables can be used for short runs or runs of several hundred feet. Guitar and instrument cables are usually unbalanced, as are the outputs of guitars etc. Professional Microphones usually have balanced Low impedance outputs, and professional signal processors commonly have balanced inputs and outputs. Where you have a choice between balanced and unbalanced generally choose the balanced option.
Getting Good Audio into Video Cameras
Many non-professional video cameras have a single 3.5mm mini-plug mic input. If you need to plug two microphones into a camera that only has a single mini-plug mic input, using a simple Y-cable will not work properly. The mic input on the camera is only designed to handle the impedance of one microphone, so attempting to patch two microphones into this input with a Y-cable will mis-match the impedance and result in really lousy audio quality. In this case the use of a camcorder XLR adapter which doubles as a mini-mixer and impedance matcher, or a field mixer is the answer. These use transformers to convert the audio from two or more mics into something usable by the camera’s mic input. The best camcorder adapters have the ability to supply phantom power to condenser mics and also have a mic/line switch that enables you to switch between mic and line level signal inputs.
Professional video cameras usually have 2 XLR mic inputs and deliver phantom power to condenser microphones. Many cameras still only record at 16 bit however, so use of a high quality audio recording device and good mic preamps recording at higher bit rates and separate from the camera is the most desirable option where possible.
Phantom Power: What is it and when do we need it?
Condenser microphones require the use of phantom power in order to operate. Ribbon microphones and dynamic microphones do not require phantom power. Tube condenser microphones normally have their own separate power supply and therefore also do not require phantom power to be supplied from a preamp, mixer or battery.
Phantom power is a DC voltage, usually +48V, but sometimes less, that is supplied down pin two and pin three of an XLR cable to power condenser microphones, active DI boxes and effects pedals.
Some Points to Note When Deciding Which Input to Use
The instrument inputs on audio devices are high impedance (Hi-Z), designed to match the impedance of a guitar/bass amplifier. An important point to note is that, signals fed to the instrument inputs go through the preamps where their low-level input signal is amplified to bring it up to line level. These inputs are usually unbalanced. Any coloration inherent in the preamp will therefore affect the sound of the signal.
The line inputs on audio devices are lower impedance than the instrument inputs, and are not usually amplified by the preamps, although in some circuits they do pass through the preamps without actually being amplified. Therefore, one would expect less coloration from the preamps than would occur when using instrument inputs. Line inputs are usually balanced inputs.
Use of DI boxes
Direct boxes are typically used where instruments or other devices that have only unbalanced 1/4″ outputs need to be connected to the XLR inputs of another device. The Direct Injection box or DI as they are commonly called, takes the high impedance and unbalanced signal from the instrument and converts it to a low impedance, balanced signal that can then be connected to the XLR input of the ensuing device.
The signal can be sent over long runs via microphone cable, with little or no signal loss due to it’s low impedance, and with greater rejection of interference than would be found using unbalanced cable. The low impedance of the signal sent from the DI (which is usually around 600 ohms), allows the mixing console or preamp to accept the signal at it’s XLR inputs which have been designed to cater for microphone and their low impedance outputs.
DI boxes are available in either passive or active models.
Passive models use a transformer to convert the high impedance signal from an instrument to the low impedance expected by a mic input at a mixer. They do not impart gain, and do not require powering. Because they are simple, the good ones are generally reliable and quiet.
Active models contain a preamp and impart gain to the signal. They require powering from a battery, phantom power, or sometimes even an AC outlet. Instead of a transformer, an active DI box usually uses a FET (Field Effect Transistor) which has a very high input impedance. Active DI boxes usually incorporate switches to pad or control gain, ground lift, external power or battery source selection, and sometimes a mono or stereo mode selection. They are therefore more versatile than passive units.
Hopefully this tutorial article will be of some benefit to those of you baffled by audio signal types and their associated jargon. There is much more than could be written about this topic, but I hope this gives you a useful overview on the subject.
Written by Tony Koretz
© copyright January 2012