CHURCH TECH
SIGNAL PATH | Bill Gibson
It ’ s advantageous for every sound operator to understand a relatively simple block diagram . It ’ s not my mission here to teach circuit design ; however , it is helpful anytime we face a new mixer or hardware processor to be able to understand the signal flow diagram . And there are a few electronic circuit symbols that are helpful to recognize and understand .
The study of a signal path doesn ’ t necessarily provide in-depth insight into how to create a great-sounding mix . However , it does shed light on what ’ s happening to the signal as it progresses from the input , through the channel , and out the main console outputs . That has value . The following is an excerpt from The Ultimate Live Sound Operator ’ s Handbook , 3rd Edition .
INPUT TO THE PREAMP First , we ’ ll study the block diagram from the inputs to the preamp . This section of the channel / block diagram contains several important controls , such as the XLR and TRS inputs , switches , transformers , preamplifiers , potentiometers , and so on .
Notice that throughout this article , each part of the diagram is labeled . The labels will help you learn some of the basic symbols used to indicate electronic components . However , a block diagram assumes the reader knows what the symbols mean , so labelling is minimal . An accompanying illustration of the channel strip and many connecting lines is provided so it ’ s clear which parts of the diagram refer to which part of the channel .
FOLLOWING THE SIGNAL PATH ( Image on next page )
1 . XLR input . The XLR (# 1 ) input receives the signal from the microphone or direct box at mic-level ( between
10 and 100 mV ). Some microphone inputs can be switched on the mixer surface to accept mic- or line-level signals . A simple look at this diagram shows that this mixer is not switched and that the XLR inputs are not suitable to receive line-level signals . This is an important realization because many outboard processors connect using XLR cables ; however , this diagram reveals that connecting a line-level output to the XLR inputs would probably result in distortion immediately at the input , no matter how low the source output level was set .
2 . Line . The symbol at Line In (# 2 ) shows a 1 / 4-inch tip , ring , sleeve connector that receives balanced line-level signals between 0.5 and 2 volts . By tracing the connecting lines , we see that the tip connects to the same destination as pin 2 on the XLR connector and that the ring connects to the same destination as pin 3 of the XLR connector . Also , pin 1 of the XLR and the sleeve of the 1 / 4-inch TRS jacks connect to the shield .
3 . Phantom power switch . Notice that the phantom power connection (# 3 ) is the first stage the mic signal encounters . Phantom power must be switched on and applied to condenser microphones and active direct boxes , so it makes sense that 48V phantom power is sent immediately to the mic or else the active DI or condensor mic signal would be dead at the start of the signal path .
4 . Insert jack . Like the line in jack , the insert jack (# 4 ) is also 1 / 4-inch TRS . However , an insert jack connects the tip of the TRS connector to the unbalanced input of an inserted device and the ring accepts the unbalanced return from the inserted device . Notice that the insert in this diagram inserts just after the preamp , receiving the signal from the output of the preamp and then returning the signal from the ring of the TRS jack to the input of the equalizer .
5 . Small solid black squares . Lines that intersect at a small solid black square (# 5 ) are connected together at that point .
6 . Semicircle cross points . Lines that cross another line with a small semicircle (# 6 ) are not connected . Some diagrams don ’ t use the semicircle and assume that lines that merely cross without the small solid black box at the intersecting point are not connected .
7 . Arrowheads . When a line has an arrowhead (# 7 ), the point of the arrow indicates the direction of the signal flow .
8 . Mic / Line switch . In this signal path , the Mic / Line switch (# 8 )— immediately after the phantom power switch — selects between the mic and line inputs . In the case of this channel strip , the selection is either the XLR mic-level input or the 1 / 4-inch TRS line-level input .
9 . The Pad switch .