Mixer

| Audio Systems Index |

Our sound board is a MPL 2242 Samson 22x4x2 Mixer.

 
• Concept: The mixer is the chief sound destination in the entire audio system (for example, look at the 2nd row of the patch bay, which are all mixer inputs). The mixer is a routing device, a Grand Central Station for audio signals.

   

• Faders: The lower row of sliding controls are called faders, and control the mix of the various sounds sent to the mixer.

   

• Input Modules: Each gray fader corresponds to the column of knobs above it, and each column--called an "input module"--represents one of the mixer inputs on the second row of the patch bay. That is, the knobs and buttons on any input module all control the sound sent to the corresponding mixer input of the patch bay (from mics, ADATs, switcher, cassette or CD).

   

• Mono input modules: The first 10 input modules are mono channels, where each module (or fader) controls a single channel of sound:
  • 1-4 Microphones
  • 5-10 RD8-A outputs

  RD8-A pan buttons (see below) are set alternating left and right, left and right for first four sound tracks; the dialog track is panned straight up.

   

• Stereo input modules: Note that after input module 10 the modules are labelled with two numbers, from 11/12 to 21/22. These stereo modules are each the equivalent of two columns in the space of one module (constructed by layering two potentiometers beneath each control knob in the module, wired from two input jacks on the rear mixer panel, and both controlled by the single fader). This compact arrangement conveniently compresses our mixer into a smaller size than would be the case if left and right channels were always controlled by separate mixer channels. The stereo input modules are assigned as follows:
  • 11 RD-8 A (actually monoaural; 12 is not used)
  • 13 RD-8 B (actually monoaural; 13 is not used)
  • 15/16 Lexicon Reverb unit outputs, left and right
  • 17/18 Switcher outputs, left and right (which could derive from any of the switcher inputs)
  • 19/20 Cassette deck outputs, left and right
  • 21/22 CD player outputs, left and right

   

• Mixer Subgroup or White Faders: Besides the gray faders, which control the various input modules, the four white faders in the lower right control the level of sound sent to the four main speakers, in the order of front left and right, rear left and right. These white faders send signals directly to the Mixer Subgroup outputs of the patch bay (301-304), which are normalled to the fourth row of the patch bay (401-404), sending sound on to equalizers, amplifiers, etc., and finally to the four speakers.

   

• Input Trim: When a sound signal comes from the patch bay to the mixer, a pre-amp inside the mixer strengthens the signal to an appropriate level. This pre-amp is controlled by the top knob in any input module, called the "Input Trim" knob. Turn any input trim knob up or down to change the volume of sound in that channel of the mixer. Note the Peak Indicator light (also called a Clip light), which is located just above each fader. If the Peak light comes on, turn down the signal strength with the Input Trim knob.

   

• Equalizer knobs: The next four rows of blue knobs (the 2nd through 5th knobs of any input module) comprise an equalizer section. Instead of controlling the tone with just treble and base, these knobs provide four bands tone equalization: high, high-mid, low-mid, and low. When an equalizer knob is set straight up at 12:00 o'clock it is said to be "flat." This means that the signal strength in that frequency range is neither cut nor added. To set it at 10:00 would be to cut it, or to set it at 2:00 would be an addition. Most of the time, equalizer knobs on our mixer may be run flat, since the room has been tuned by setting the equalizers in the equipment-room rack.

   

• Auxiliary Sends: Rows 6 through 9 on any input module are the "auxiliary sends," which send the signal for that module to some destination other than the four main speakers. These auxiliary sends constitute a mixer within a mixer, because they function much like an input module with its own fader designated specifically to the auxiliary sound destinations. Since the fader at the bottom controls the house mix, these four knobs control the signal strength that is sent to their designated destinations. Auxiliary destinations might include radio broadcasts, the monitor in the foyer, the intercom, etc., but on our mixer they are currently set up as described below.

   

• Aux Send #1 (reverb): This aux send goes to the Aux #1 output on the patch bay (), which is normalled to the Lexicon reverb input. Therefore, unless cable patches are used, the default destination of Aux Send #1 is the reverb unit. Turn the Aux #1 knob up to add reverb to the sound of any input module. For example, suppose Mic #1 is put on a cello, and comes into the mixer input module #1. Turning the Aux #1 knob on for the first input module would add reverb to the cello. Using the mixer fader #1, pure or "dry" cello sound would be sent to the house. Pushing the mixer fader 15/16 up, however, would send reverb cello sound to the house (since mixer channels 15/16 are the input modules for the reverb unit outputs, left and right). One may use the combination of the dry fader and the reverb fader to control the house mix of dry and reverb sound.

   

• Pre- and post-fade sends:
  • Aux #1 is a pre-fade send, because the signal sent by the Aux #1 knob is not affected by the fader for that input module. Can you see why the reverb channel needs to be a pre-fade send?
  • Aux Sends #2, 3, and 4 are all post-fade sends, in that the signal strength is affected by the faders, and therefore reflects the house mix.

   

• Aux Send #2 (Hearing Enhancement): The Audex Hearing Enhancement system sends sound via infrared signals to handheld units, which visitors hear through small headphones. Usually Aux #2 knobs should be open on every mixer channel, so that what is heard reflects the house mix. However, one might set sources like Mic #1 higher, and sources like the CD channel lower.

   

• Aux Send #3 (Subwoofer): The 5th channel of sound, after the four main front and rear left and right speakers (controlled by the mixer subgroup faders), is the subwoofer channel for low frequency sounds. Obviously, the subwoofer needs to be a post-fade send, so that its volume is affected by the fader position along with the four main speakers. But since the human voice trails off at a frequency higher than the cross-over point for the subwoofer, the Aux Send #3 knob for microphones may be set to zero for vocals (or be turned up for a cello).

   

• Aux Send #4 (Dialog): The 6th channel of sound is for narration, and is sent to the Dialog speaker (located above the dome, front center). This knob should be on for mixer channel #1 (wireless mic), but turned off for many other channels where no narration is carried.

   

• Aux Sends #5 and 6: Pushing this button changes Aux Sends #3 and 4 into two additional aux sends (thus compressing our powerful mixer into a more compact size, eliminating two additional rows of knobs). These aux sends are not assigned at this time.

   

• Pan or Balance knob: Sends sound left or right in the "audio panorama." Normally we set these straight-up in stereo mixer channels, and all the way either left or right for mono channels.

   

• Buss or Assignment button: Sends sound front or rear, depending on whether it is set up or pushed down. The first two white faders control front sound, including sound from any input module where this button is up. The second two white faders control rear sound, including signals from any input module where this button is down.

   

• PFL mode: This mode allows headphone monitoring of single-channel input sound, without altering the house mix. Use it to isolate a sound problem, selecting the channel heard through headphones with the Solo buss button.

   

• Phantom Power: The mixer can send power to some components so that they do not need to be plugged in to their own separate power source. For example, Phantom Power drives our microphones (other than the wireless, which needs batteries). It should be left ON.

| Top |

---

OBU Planetarium:
http://www.okbu.edu/academics/natsci/planet/index.htm