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.

Re-using Existing Wiring

Q: I have received numerous questions that involve installations using existing wiring during retrofit projects. Most of which are too involved for simple brief answers and almost all contain the same variables. So my answer is a broad answer on how to address these types of issues.

A: Whenever dealing with existing wiring there are a few cardinal rules that you must follow. Never, ever believe what others tell you concerning the current status, condition or configuration of the existing wiring. It’s your reputation on the line and until the project is complete that’s your equipment you’re working with. Meaning it’s not the customer’s problem if you damage it. So with this in mind always, always, always test the wiring before connecting it to any of your equipment. No matter how sure you think you are that you know what is on the other end. It is crucial that you check all wiring for shorts between the conductors (i.e. – between red/black, red/shield, black shield, etc.) and also shorts to ground on all conductors. When working with speakers you need to be checking the impedance (with an impedance bridge not a multi-meter) of the speaker line to ensure you have the proper load on the circuit. Another element to check for is stray voltage both AC and DC. This covers the “big” monsters that can stop you project dead in its’ tracks. The brutal truth is we could go on and on further investigating the wiring with oscilloscopes and all sorts of fancy equipment that the majority of you will never see or have a need for.

No Audio – Push to Talk Mic

Q: I have a push to talk mic that I can’t get any audio from. It has a pre-terminated cable attached from the factory. What gives?

A: First things first let’s never assume that we know how any device is wired from the factory. Especially a push to talk microphone! I’ve seen manufacturers that didn’t use consistent wiring schemes throughout their own product line. Refer to your manual to check for any unusual wiring practices. If it appears to be wired in accordance to what has become the accepted norm for these days (Pin1 shield, Pin2 hot and Pin3 negative) then we will proceed to the next step which is the switch for the push to talk feature. PTT mics can come in any number of configurations. Usually the switch is pre-wired and is selectable between ‘normally open’ and ‘normally closed’. If it’s wired as “NO” then when you press the switch it completes the circuit and activates the mic. If it’s wired as “NC” then the mic is constantly on and mutes when you press the switch. Some units also provide an extra pair of conductors in the pre-made cable so that you could connect the switch to an external device. For instance in a paging system you may push the switch to talk, but also want to send a contact closure to a mixer to mute background music as well. Some units with this feature may not have the switch internally wired from the factory meaning the mic will not function until you wire it yourself for the configuration that fits your needs.