The Service Call That Didn’t Need Any Service!

Many of are accustomed to dealing with clients that do not have dedicated audio/video people on staff. I’m sure you have received a call from an upset customer that is convinced your system in malfunctioning and causing them embarrassment and headaches. More often than not these items can be resolved with a little TLC.

Distortion, impedance & the 10:1 rule.

PROBLEM: I am troubleshooting an audio system installation in a large conference room that can be divided into three separate rooms. Each room has a dedicated powered mixer with distributed ceiling speakers and the source equipment is in a roll around portable rack. The idea obviously is that the customer can take the source equipment to, whichever room is in need or it can be shared between any combinations of the three. The only problem is if one of the powered mixers is turned off because a room is not in use it causes distortion in the other areas. From what I can tell who ever did the original installation has simply paralleled the sources to all three mixers. How can we alleviate this distortion.

Speakers; A cluster of polarity desparity.

PROBLEM: We just installed two speakers in a small auditorium. They are hung in a tight cluster at about a thirty-degree angle to each other. The problem is that we are experiencing drastic phase cancellation right down the center of the auditorium. It starts as an approximately three foot wide area and slowly expands to about nine feet and the rear of the auditorium. With an SPL meter we are reading about a 12-15db difference in and out of the area of cancellation.

Powered stage monitors make me buzz…

PROBLEM: We are using powered stage monitors and we are experiencing a loud buzz. We’ve switched cables and speakers to no avail. The monitors are fed from the FOH up in the organ loft. Are we picking up noise somewhere along the way?

SOLUTION: The stage and the main equipment location in the organ loft are probably not on the same circuit. This is causing two problems;

Pin 1 Problem?

What is the “pin 1 problem”? How do you fix it if your gear has it?

The Pin 1 problem is when pin 1 (or sleeve) of a balanced input or output on a given device is not connected to chassis ground. Instead the pin 1 conductor is tied to the signal ground. The best way around this is to tie the pin 1 conductor directly to a point on the chassis that you know is grounded. If you’re dealing with a XLR or a TRS ¼” connector it’s best if you have a test cable with alligator clips that you can insert into the jack and test for problems.

Speaker Cluster Gone Arigh

Q: A year ago we hung two full range cabinets with aircraft cable packed in a cluster above an alter in a church. Since then they have twisted towards stage right just ever so slightly. There is now no way for us to get back up to the ceiling to make any adjustments. What can we do?

A: This is a fun one. Assuming you have some sort of DSP in this system this will be rather easy. If not it will be more complicated. If DSP is available and you are using a separate output for each cabinet you should be able to resolve your problem using delay. If the cabinets have turned towards stage right then delay the cabinet closest to stage left. The interaction of the two cabinets with the delay will steer the image to the left. You simply need to experiment with the amount of delay on the stage left cabinet to re-center the audio image. Now if you do not have DSP available or you are using the same output for both cabinets we will need to get up to the cabinets themselves. Disconnect the rigging cables from the cabinets. By twisting the cables counter clockwise they will have a natural tendency to untwist. This will require quite a bit of trial and error but the tendency of the cables to return to the resting state may be just enough to direct the cabinets front and center once again. Please note that it is very important to not twist the aircraft cable excessively. This will compromise the integrity cable putting undue stress on the individual strands diminishing its’ strength and load capacity. So this solution is to be approached conservatively with your first priority always being safety. Never stray from the standard safety requirements when doing overhead rigging under any circumstances

My 18″ Subs Sound Like 8″ duds.

Q: I recently installed a new system with two dual 18” sub cabinets. No matter how much I increase the gain on the amps or in the DSP they are barely audible even with the others cabinets turned off.

A: Double-check the settings in your DSP. When setting up your high pass and low pass filters for the sub cabinets it can be easy to get them crossed essentially eliminating all low frequency information from the signal path. For example if you had intended for you HP filter to be set at 45hz and it was mistakenly entered as your LP filter then the only low frequency information being allowed to pass on to your subs is that below 45hz which doesn’t amount to much. So the result is no matter how much you increase the gain at any point in the signal chain the low frequency is still being filtered out.

Mic Inputs vs. Line Inputs

Q: All of the sources coming into my mixer are extremely loud and distorted except for the microphones.

A: Most likely your line level source is overdriving an input configured for a microphone. All mixers that use the same physical input for both line level devices and microphones will have some sort of input gain adjustment. On some it may be an adjustable pot with a typical adjustment range of 20db to 30db. Others may use a simple push button or dipswitch that inserts a pad into the signal path. Again this pad will be somewhere in the range of –20db to –30db. Simply activating this pad should resolve your problem. If not it’s highly possible that you have a wiring problem such as incorrect pin configuration.

Balanced or Stereo Input?

Q: I have a CD/Cass combo with unbalanced outputs going to a balanced input on my mixer and the sound is extremely low and very distorted.

A: Look at the input on your mixer. You need to verify whether it’s an actual balanced input or is it a left / right input with a shared common. Balanced inputs use a principal known as common mode rejection in order to prevent noise. When sending left and right information to pins 2 and 3 the input is receiving voltage on both pins of the same polarity. Because of the fact that the left and right channels aren’t identical they will not completely cancel each other out. Instead the result is a low and distorted signal. Balanced inputs are looking for the same voltage on pins 2 and 3 but opposite polarity. If noise such as RFI is induced on to the cable it will be induced on to both conductors equally as a result of the twist in the cable resulting in the same voltage and same polarity on both pins. If you are dealing with a left/right input it is expecting positive voltage on both pins 2 and 3 whereas pin 1 is used as a common for both channels. So this is consider an unbalanced input even though there is three pins or terminals. This leaves you with two options. If you do have actual balanced inputs you can either combine the left and right positive on pin 2 with the left and right negative on pin1 or used two separate inputs/channels. So one channel would have left(+) on pin2 and left(-) on pin1 another channel will have right(+) on pin2 and right(-) on pin1. For the first option there are careful considerations to be made. You must consider the ratio of the output impedance of the source device verses the input impedance of the input it’s going to. If the ratio is not high enough you must use some sort of combining device or summing transformer. A good rule of thumb is a 10:1 ratio but the higher the better.

DSP & Disappearing Audio

Q: I was programming a new DSP unit and everything was working fine until I compiled the file and now I have no output. The input meters on the front faceplate are still lighting up and I have several places in the signal chain where signal present LEDs indicate that the signal path is intact.

A: The answer to this will vary depending on the equipment manufacturer. Every manufacturer’s DSP will have it’s own unique quirks. What you are most likely experiencing is a built in safety feature where the unit will mute it’s outputs after compiling. Others will mute their outputs in the event of a power failure. I would begin your investigation by opening the window for your outputs to see if they are muted. If the outputs are NOT muted then open each “device” in the signal chain to check for mute switches or virtual connections that were missed. It is easy to overlook incomplete connections especially when working with a system where you are performing a lot of switching, mixing and presets as opposed to simply using the DSP for just dynamics. The reason behind the muted outputs is to prevent costly mistakes after you have made changes to the DSP parameters. Fore example when you meant to enter a +2db gain increase for the horn in your bi-amped cabinet you inadvertently entered +22db. Or if you mistakenly routed low frequency information to your horn with that massive 1000w per channel amp behind it that was intended for your low frequency drivers.