General Lavalier Microphone Theory
Before we discuss the difference between Bias, we need to review the basics of lavalier microphones. Electret Lavalier Microphones require power to operate. When a lavalier microphone is connected to a wireless transmitter, that transmitter provides power (usually 1.5VDC to 5VDC) to the lavalier for it to work. When you are using a lavalier microphone without a wireless system, you need some way to get power to the lavalier.
You cannot just connect a lavalier microphone to a camera or mixer's input and expect it work, even if your equipment supplies phantom power. Phantom Power is 48VDC, which is way too much voltage for a lavalier microphone, which usually wants between 1.5VDC to 5VDC. The solution is a lavalier power supply. A lavalier power supply looks like a long XLR connector or XLR Barrel. The power supply is about 3/4" in diameter, around 6" long and usually contains a battery. It has a 3pin XLR Male on one side. The other side of the power supply either has a wire coming out that has the lavalier mic attached to it (thus termed "hardwired to power supply"), or has a mating connector on it, so you can plug your lavalier microphone that has a connector on the end of it, into the power supply (thus termed "with detachable power supply"). With the latter configuration (with detachable power supply), you can use your lavalier microphone with your wireless system or have the option to go directly into a camera or mixer using the power supply. With the first configuration (hardwired to power supply) you always have to go "wired" into a camera or mixer.
What is Bias?
Now that we have covered the basics, lets discuss what Positive and Negative Bias is. Bias is basically which way power flows to the microphone. Think of any piece of electronic equipment that requires a battery to work. When you install the battery, you need to insert it the correct way. The positive(+) side and the negative(-) side of the battery needs to be inserted the proper way. If you insert the battery the wrong way it won't work.
Bias Must Match
If you are using a wireless microphone system or a lavalier power supply that unit's bias and the bias of the lavalier must match for the microphone to work. Some wireless system only offer negative bias, some only offer positive bias and some offer both bias options. The same goes for lavalier microphones, some can only be used with negative bias, some only with positive bias and some can be wired to be used either way. The ones that offer both options are of course the most versatile, but also the most confusing. Lectrosonics Wireless Systems and Tram Microphones are two examples that offer both bias options. So whether your Tram is wired for positive or negative bias, it will work either way with a Lectrosonics (applies to older Lectrosonics units only [M185 M187 UM190 UM195 UM200 series] --- newer Servo Bias Lectrosonics Transmitters require Positive Bias Trams [UM400a UM450 and Entire SM Series]). The problem comes into play when you are using a "detachable lavalier power supply". If the power supply is positive bias, the microphone must be positive bias as well. If you attempt to connect a negative bias microphone into a positive bias power supply, it will not work.
Lavalier Power Supplies
Lavalier Power Supplies come in a couple of different varieties. Some require you to use an internal battery, some require phantom power (12VDC, 48VDC or either), and some work both ways either with a battery or phantom
|Tram Negative Bias Power Supplies:||Will operate only with the internal battery
|Tram Positive Bias Power Supplies:||Will operate with either the internal battery or 12-48VDC Phantom Power.|
With Tram power supplies the positive bias version is the most versatile (giving you the option to use a battery or phantom power), but it is also a little more expensive.
Which Bias To Choose?
If you are using the most common components, a Lectrosonics Wireless System and a Tram Microphone, which bias do you choose?
We would recommend Positive Bias (Positive Bias is required for newer Lectrosonics, servo-bias transmitters [UM400a and All SM Series Transmitters]lectro). With a Wireless Application, the choice generally does not really make a difference. However, when the microphone is used in a "Wired" configuration with a lavalier power supply, positive bias is definitely preferred. If a lavalier is wired for Negative Bias the mic element is left ungrounded, which leaves it vulnerable to buzz and hum caused by RFI/EMI interference. Plus, in a hard-wired configuration, with a positive bias power supply you can either power the lavalier mic by the internal battery in the power supply or via +48VDC phantom power (a negative bias power supply will only work with the internal battery in the power supply and will not work with +48VDC phantom power).
One last thing to consider, if you already own Tram Mics
If you already own Tram lavalier mics and/or Tram power supplies and are looking to replace them with the same type, you may want to get the same bias. Using 2 microphones that are of the opposite bias will normally cause them to be out of phase with one another (Easily remedied phase reverse adapter barrel, but is an extra step). If you have 4 Trams wired for negative bias with detachable power supplies and 4 Trams wired of positive bias without a power supply, you will not be able to use the Tram positive bias mics with the negative bias power supplies... and your intern won't know that. They will attempt to mix-n-match mics with power supplies (because they all look the same) and determine that you have a lot of dead mics.
How to determine if the Tram Mics you already have are positive or negative bias?
There is no way to tell by looking at them, you need to disassemble the connector to see how it is wired (do not disassemble the power supply tube if you have one!). After disassembling the connector:
|Tram Negative Bias Microphones:||Red Wire & Shield are tied together and Black Wire is by itself
|Tram Positive Bias Microphones:||Black Wire & Shield are tied together and Red Wire is by itself|
Most Tram microphones manufactured after 2005 have a small piece of red shrink tubing just behind the connector boot to notate that it is wired positive bias. Negative Bias mics do not have any sort of notation.
If your Tram only has a White Wire and a Shield, then you have a really old Tram! Back then, the Tram had to be manufactured either positive or negative bias. In this case you need to look at the actual mic head. On the back of the head (the side opposite of the grille) there will be a circular indentation. If inside that indentation you see a "+" sign then the mic is positive bias... if you see nothing inside the circular indentation, then it is negative bias. Like we said, you have an old mic, so be sure you look hard at the indentation to be sure the "+" has not just rubbed off a little!
Most Tram power supplies manufactured after 2005 (and select earlier ones) have a Red Dot or Red Plus Sign on the plastic cap of the power supply to determine that it is positive bias. Negative Bias Supplies do not have any notation. The only other real way to determine the bias of the power supply is to connect a positive bias and a negative bias microphone to it to see which one works.