Stage Practices
Michael Drolet -- 2018

Sound -- The Block Diagram

In a sound system,  a source of sound is referred to as an output.  A destination for sound is referred to as an input.  

Building a sound system involves connecting sources to destinations  --   an output to an input to an output to an input ... and so on.

The sources might be the outputs of microphones, CD players,  effects units or the sound outputs of a computer.

The destinations might be the inputs of the House speaker system, onstage monitors, headphone amps or effects units.
The Block Diagram is a  simple drawing showing:
  • the interconnection of the various pieces of equipment in a sound system
  • the  flow of signal through the system
Sample Block Diagram:

Below is a rough sketch of a Block Diagram for a simple sound system with the following requirements:
  • microphones on stands for 2 performers,
  • 2 FOH  (Front of House) speakers for the audience, 
  • 1 Stage Monitor (or foldback)  for performers, 
  • 2  tracks of computer playback (SCS), 
  • reverb processor on vocal mics.
Sample Block Diagram
Equipment List:

 We can  use the Block Diagram of this sound system to make an Equipment List
  • 1 x Soundcraft Signature 12 MTK Console (12:2:2 -- 12 inputs, 2 group outputs, 2 master outputs, 3 auxilliary outputs)
  • 2 x Shure SM58 vocal microphones (cardioid, dynamic)
  • 2 x K&M microphone stands
  • 2 x Mackie SRM450 Powered Speakers
  • 1 x Mackie SRM150 Powered PA System
  • 1 x Roland SRV330 Effects Processor
  • 5 x 3XLF - 3XLM microphone cables, 25 ft. (3 pin, XLR female to XLR male)
  • 3 x TS-TS cables, 3 ft. (Tip-Sleeve to Tip-Sleeve)
  • 1 x TRS male - 3XLM adapter, 3 ft.
  • 1 x 3.5 mm stereo TRS to RCA male (x2)
As we set up the sound system, we can highlight each connection as we complete it.  When all the interconnections are hightlighted, we're finished.

If something doesn't work , we can follow the flow (path) of a signal through the Block Diagram, going from left to right. We can plug in a set of headphones to monitor (listen to) the signal a each point of interconnection.  

When we reach the point at which the signal disappears, we've found the location of the fault.

A connection error, or the turn-on (or turn-off)  "thump" from some other piece of equipment could damage the speakers.  (The thump also sounds very unprofessional.)

To avoid this, we don't turn on the power amplifiers (or powered speakers) until we know everything is working properly.  And we turn them off before turning off or disconnecting anything else.

A steadfast rule in setting up a sound systerm --
"Power amps -- last on -- first off"!