This class is designed to provide enough knowledge to setup and operate a basic sound system, using readily available components. The instructors are Joel Lord and Paul Kraus.
The first is the signal level. There is a whole continuous range of signal levels, but generally they fall into a few categories. Audio signals are measured in both volts and decibels (dB) relative to 775 millivolts.
The other important characteristic of an audio signal to worry about is whether it is a balanced signal or an unbalanced signal. This does not refer to the relative strength of the left and right signals of a stereo pair!
Balanced Audio refers to a signal that is composed of both the main signal and its opposite. Cables that carry these signals will have at least three conductors, as will the connectors. The most common connector for balanced audio is the XLR type connector. This is one of the quickest ways to make an educated guess as to whether a signal is balanced or unbalanced.
Be careful, as intercom systems also use XLR connectors, and connecting an intercom line and a balanced audio line can be disastrous to both. At TechnoFandom events, intercom cables are usually a distinctive color (green), but it pays to be careful if audio and intercom are both in use.
It has become common to find balanced audio making use of 3 conductor 1/4" plugs. This connector is frequently used for a stereo pair of unbalanced signals, or two discrete mono unbalanced signals, or a single balanced signal. It is important to know how any particular 1/4" cable is wired to insure that the right signal goes to the right place.
Professional microphones are almost universally balanced, with a male XLR connector for their output. If you run across a microphone that has a 2 conductor 1/4" plug on it's output, it is probably a high impedance microphone. The signal from high impedance microphones is unbalanced and can only survive 10 to 20 feet before seriously degrading, therefore these should be avoided.
Many pieces of equipment use 1/4" jacks for both inputs and outputs. It is very hard to tell just by looking if these jacks are 2 conductor or 3 conductor, and therefore if they are unbalanced or balanced. You can often tell by feeling very carefully when inserting a plug, if you feel the plug `locking' into place before it is all the way in, the jack is probably 3 conductor. Sometimes you get lucky and the panel is marked "Bal." or "Unbal.".
Be aware that 3 conductor 1/4" jacks are also used for both input and output of unbalanced signals at the same time for insert points on a mixer. This is a way to insert some additional piece of equipment, like a compressor or equalizer, into the signal path within a mixer.
Speaker level signals are almost universally unbalanced. There are four different connectors used for speaker signals; Speakon, dual banana plug, 2 conductor 1/4" plug, and spring clips for bare wires.
An important note about the banana plug connectors, they were not designed for permanent installation. The action of plugging and unplugging them cleans the connector, so they are very well suited to sound systems that get setup and taken down on a regular basis, such as at conventions.
1/4" plugs make poor speaker connectors, but are commonly used. They are inexpensive compared to Speakon connectors, the best choice. It is important when using 1/4" plugs for speakers to only plug and unplug the speaker cables when the amplifiers are turned off. If 1/4" speaker cables are plugged and unplugged with an amplifier on, damage to the amplifier may occur. It is safe to plug and unplug both Speakon and banana plugs with a live amplifier (just don't touch the metal parts of the banana plug, they represent a potential shock hazard).
The most basic source is one or more microphones. Cassette decks, CD players, computers, and video equipment are also sources that you might run into. In general, professional equipment will have XLR connectors, with some professional gear using 1/4" jacks. All consumer equipment will use RCA or phono jacks for input and output. Your normal Hi-fi hook up cable uses RCA plugs.
Some way to control both the overall volume and the relative volumes of the various sources is needed. The device that accomplishes this is called a mixer, because it mixes the various sound signals. Mixers range from very simple devices with a few (three to four) inputs and one output, to extremely complicated mixing consoles, called desks in the UK, with hundreds of inputs and tens of outputs.
The mixers that TechnoFandom typically uses range from a basic Shure M-67 with four inputs and one output, to a Mackie SR24-4 sound reinforcement mixing console with 24 primary inputs, seven primary outputs, and six secondary outputs. The secondary outputs are called Auxiliaries or Monitors or Effects. The best way to think about them is as an addition simple mixer built into the bigger console.
For this class we are using a Soundcraft Spirit Notepad mixer. This mixer has four mono inputs, two stereo line level only inputs, a primary stereo output, and one auxiliary output. Additionally, this mixer has a headphone output and an output for connecting a monitor amplifier and speakers (either stage monitors or recording monitors).
For each of the four mono inputs there is a column of knobs. The red knob at the top adjust input gain to allow for different level sources (microphones to line level). The two grey knobs below the red one are hi and low frequency equalization, think of these as bass and treble controls just like your stereo at home. The light blue knob adjusts the signal level for the auxiliary level, this level is also affected by the main level control for that input. The yellow knob is the pan control. It divides the signal between the left and right primary outputs, think of it as similar to the balance control on your stereo. Finally the white knob adjusts the overall level for this input.
The next two columns are the two stereo inputs. These inputs use RCA jacks on this mixer, on bigger mixers stereo inputs typically use 1/4" jacks. Red is gain adjust, blue is auxiliary level, yellow is balance, and the bigger blue knob at the bottom is overall level. [Note that there is a difference between a Pan control and a Balance control, if you really want to know the details, ask me at the end of this session]
The four red knobs to the right are the master levels for the primary stereo output, headphone output level, FX Return input level (a secondary input used for signals from effects equipment like reverbs), and Tape input level (another secondary input, used for either tape playback in recording situations or background taped music during breaks between musical performances in live reinforcement).
Power amplifiers have inputs designed for line level signals and output the correct speaker level signals (both voltage and current) to drive loudspeakers. Output power ranges from 30 watts per channel to over 1,000 watts per channel.
For this class we are using a Crown D-60 amplifier rated at 30 watts per channel. This is an old professional amplifier, designed in the early 1970's. It has 1/4" unbalanced inputs and binding post outputs. The binding posts will accommodate either bare wire or banana plug connections.
The loudspeakers are the last technical component of the sound chain. These convert the electrical representation of sound into actual sound waves that people can hear. Loudspeakers range from small single component systems to very large four and five component systems, often with dozens of systems working together to provide sound coverage for a given audience area.
TechnoFandom uses mostly small speaker systems, the most common being a system with a single 12" woofer and some type of horn midrange / tweeter. We also use very small speakers for things like video theatres, and have one large horn based system with bass, midrange, and treble horns and a custom built amplifier rack. This is an old EV Sentry IV system that has been specially modified and is usually used at Arisia and Lunacon. [We are looking to possibly sell this system as it is a pain to transport... make us an offer.]
For this class we are using a pair of Radio Shack Minimus 7 speakers. These were an old standby in any sound person's kit as they are very rugged, were relatively inexpensive, and they sound pretty good.
Now that we know what signals and equipment we have, how do we know which ones can be successfully connected to each other? The best rule to follow is connect balanced outputs to balanced inputs and unbalanced outputs to unbalanced inputs. Unfortunately, that goal is not always achieved and signals of differing types must be interconnected.
Generally speaking, you can connect any type of signal to a balanced input, either balanced or unbalanced. Balanced outputs, on the other hand, either need to go to balanced inputs, or get fooled via an adapter into thinking the input is balanced. An exception to this are balanced outputs that are available on 3 conductor 1/4" jacks. You can sometimes connect them to unbalanced inputs just by using a 2 conductor 1/4" cable.
Now we figure out what we need to string together to get a sound system up and running. For starters we need a source of sound or two, in this case we have a microphone and a tape deck. Then comes the mixer to control the sound levels (volume) and equalization (tone) and possibly an additional feed for effects or monitor speakers. Since this is a simple setup, we go straight into the amplifier and then to the speakers.
In a more complicated system there would be many more microphones and other sources, such as CD Players, Video Playback, and additional (backup) cassette decks, but the concepts are the same. There may be one or more snakes, cables that carry multiple XLR balanced cables in one overall jacket.
On the output side of the mixer, there may be equalizers, compressors, feedback filters, and electronic crossovers; but they are all wired the in the same manner. There may be many amplifiers, each with their own input wiring from crossovers and output wiring to the various speakers.
TechnoFandom (TF) is a loose association of people who enjoy doing technical work in support of Science Fiction Conventions in the Northeastern United States. TF members are not compensated for our time beyond what the convention already provides for committee, staff, or gophers; depending on what role we take with the convention. We do this because we enjoy the challenge of putting on a good show.
Almost all TF communication is handled via email. There is an email list for those interested in the technical aspects of Science Fiction Conventions. This list is maintained by Alex Latzko (firstname.lastname@example.org), email him if you would like to be subscribed. Since this is all volunteer work, please allow at least a week for a response.
TF is trying to make TechnoFandom University (TFU) a regular occurrence, with various different classes offered at different conventions. We have tentatively planned to continue TFU at Albacon in October and Arisia in January.