Using The Subwoofer

This is a technical article discussing the operation of Burien Little Theatre’s audio equipment. It will probably be mostly interesting to audio operators and sound desingers who want to make use of it. If this is of interest to you, click on the link “Continue reading” below to see the article.

After spending some time getting the JBL MRX 518S subwoofer working with the BLT audio system, I thought I’d try to explain how it seems to work, and what are some of the eccentricities of the equipment we are using.

Perhaps first I should mention that there is one very important principle to remember when trying out a new piece of electronics – start as simply as you can. The first question that most often occurs is whether the component being installed is working or not, and whether it’s working as it is supposed to. This is difficult to determine when the component is placed in a complex system of electronic devices that may or not be able to work with the new component.

That’s why it’s best to use the simplest setup possible to check things, then gradually get more complicated. It’s a good rule to follow in most efforts of this sort, not just audio systems. In audio, it’s best to start with components that are known to be functioning, and hooked up correctly with reliable connections. The fewer changes made to accomodate the new component, the better.

BLT audio equipment

Figure 0 – Audio equipment for BLT’s sound system. Image by Craig Orsinger for Burien Little Theatre.

To drive the new subwoofer, we decided to try using an amplifier that was in a rack of equipment we had inherited from the previous theatre administration, a Crown D-150A Series II. There was no manual for it, of course, though I found one online later. We didn’t even know if this amp was functional, much less if it could drive the subwoofer. Initially, somoene had connected the amplifier to the subwoofer output of the equalizer, and could get nothing to work.

Determining if the amplifier worked was the first step. I connected a pair of monitor speakers to the amplifier’s outputs, then provided an input from a computer’s headphone output. This proved to be difficult to hear, because the amplifier is meant to work with a preamplifier, and apparently requires a strong input signal to reach full volume.

That’s the first lesson. The amplifier requires a substantial input signal. Since that signal will come from the mixer in most circumstances, that means that the output of the mixer can only attenuated a little before the subwoofer is almost inaudible.

The amplifier has two volume knobs on it, but as a particularly informative forum discussion on the amplifier noted, these attenuate the incoming signal. Once again, this means that the input signal needs to be pretty strong.

Once I’d figured out that the Crown amp was working, the next step was connecting the subwoofer back up. The complete subwoofer circuit is illustrated in Figure 1:

Circuit for the BLT subwoofer, as of July 30, 2012

Circuit for the BLT subwoofer, as of July 30, 2012. Diagram assembled from various manual pages by Craig Orsinger.

Once I’d confirmed that the computer’s signal was playing through the subwoofer as it had been through the monitor speakers, it was time to see if we could get the equalizer’s subwoofer output to drive it.

There is a separate set of controls on the front of the equalizer for the subwoofer channel, which is shown in Figure 2.

Equalizer setting used for BLT subwoofer, as of July 30, 2012

Equalizer setting used for BLT subwoofer, as of July 30, 2012. Note that the red indicator is on, meaning the subwoofer signal is active, and the volume wiper is nearly all the way clockwise, meaning maximum output. Image by Craig Orsinger for Burien Little Theatre.

As Figure 1 shows, the output is through an XLR connector, so an adapter is needed to route that output into the Crown amp’s phone jack inputs. There is an output knob there that controls the strength of the subwoofer signal from the equalizer. Since we need input to the Crown amp to be strong, this doesn’t leave much oppurtunity to use that knob as a way of balancing the subwoofer with the main speakers, but that is one of two ways this can be done. Generally, this will be nearly all the way clockwise.

The other lesson learned here came shortly thereafter. Using the computers’ output again, I found that only the left channel was playing out of the main speakers. While this was true, there was no output from the equalizer to the subwoofer channel. This all was shown by the VUE meters on the equalizer, by the way. The right channel (Channel 2 on the equalizer, actually), was strong, but the left’s (Channel 1) and the subwoofer showed no signal. Figure 3 shows the equalizer’s VUE meters glowing.

VUE meters on BLT’s Ultragraphics equalizer

VUE meters on BLT’s equalizer, July 22, 2012. Photo taken and enhanced by Craig Orsinger for Burien Little Theatre.

Wiggling the input connection from the mixer to the equalizer’s channel 1 restored the left channel signal. It was at that moment that the subwoofer output began working as well.

So, it’s clear that the equalizer’s Channel 1 is required for the subwoofer channel to work.

Of course, this still leaves the amplifier itself. To maximize power output to the subwoofer, I put the amplifier in mono mode, and bridged the speaker channels as the Crown manual described. Figure 1 shows how this is done. The one other important thing to remember is that the control for amplifier Channel 1 controls the volume, but unless Channel 2’s knob is at full attenuation (fully counter-clockwise, in other words), the speaker output will be attenuated. Figure 4 illustrates this idea:

Settings for the Crown amp used to drive the BLT subwoofer. July 22, 2012

Settings for the Crown amp used to drive the BLT subwoofer. Set Channel 2’s volume knob to 0 (fully counterclockwise), and use Channel 1’s knob to control subwoofer volume. Image by Craig Orsinger for Burien Little Theatre.

Once that is straightened out, the procedure for balancing the subwoofer sound with the rest of a production’s sound program involves:

  1. Pushing the mixer’s Main output to ‘U’, or nearly there. Any less, and there’s just not going to be much signal.
  2. Keeping the mixer’s input channels at the highest volume possible.
  3. Cranking the equalizer’s subwoofer volume knob nearly all the way clockwise for maximum volume there.
  4. Using the Crown amp’s volume (attenuator) knob to balance the subwoofer with the mains.
  5. Low frequency volume controls on Mackie mixer, July 22, 2012

    Low frequency volume controls on BLT’s Mackie audio mixer. Image by Craig Orsinger for Burien Little Theatre.

    Using each mixer input channel’s Low Cut wiper to balance or maximize the low frequency gain for any sound sources that is expected to be coming through the subwoofer. Figure 5 shows the mixer’s low frequency gain controls.

In short, balance the sound of the subwoofer with the main speakers by using the equalizer’s subwoofer volume knob and the subwoofer amplifier’s Channel 1 attenuator knob. Then use the individual low cut wiper controls on the mixer’s input channels to balance the individual sound sources, and to emphasize those that are to be especially audible.

Do all that, and you can get a sound out of the subwoofer that is roughly balanced with the main speakers, and appropriate for the sound program for a production. Setting up the subwoofer-related controls with these ideas in mind will probably make audio levels more manageable during a show.

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