Question: What is bias voltage and when is it needed?
Answer: In last week’s blog we discussed what phantom power is and why it is needed to power certain types of microphones. Another closely related powering method is bias voltage. Bias voltage is a low DC voltage, typically somewhere between 1.5 and 9.5V DC, used to power electronic circuitry located inside a condenser (or capacitor) type microphone’s capsule. It is usually a fixed amount of voltage, and it is important to provide the precise amount of voltage for a given capsule design. Too little voltage may not adequately power the microphone, reduce microphone output level and/or reduce the microphone’s capacity for handling higher sound pressure levels. Too much voltage could permanently damage the microphone’s circuitry, making the microphone unusable.
It should be noted that phantom power and bias voltage are not the same thing and they are not interchangeable. What may be confusing to some is that we often say that we power wired type microphones with “phantom power.” In most cases, however, this phantom power, which can be as much as 48V DC, is actually being stepped down to a much lower bias voltage by an associated in-line (or remote) power module (or power adapter). The module is suited to the particular requirements of the microphone model with which it is furnished. For many Audio-Technica brand microphones this lower bias voltage is around only 1.5 to 5V DC. Therefore, bypassing the power module and connecting the microphone directly to phantom power will almost certainly damage your microphone.
In addition to managing phantom power, a power module also converts the audio signal to the proper impedance and balances the output. In some cases the module may provide the option of powering the microphone electronics via an internal battery in the event phantom power is not available or it fails during use for some reason. A power module may also provide other functions, such as a switchable low frequency roll-off filter, which can reduce or eliminate the pickup of unwanted low-frequency environmental noise. Remember, a power module does not supply phantom power, it only manages it by converting it to a low bias voltage. Should the device you will be connecting your chosen microphone to not supply the appropriate amount of phantom power or not supply phantom power at all, plan accordingly. You may need to add an in-line phantom power supply, such as our AT8801 Single-channel 48V Phantom Power Supply, between your microphone’s power module and the device, or you may need to choose one of our models that come furnished with a power module that accepts a single AA battery to power the microphone instead. An example of this is the PRO 70 Cardioid Condenser Lavalier/Instrument Microphone. The provided power module may be operated via phantom power or via a single AA battery.
Most wireless microphone system transmitters, with the exception of XLR-type, plug-on transmitters, provide bias voltage only. This voltage is present at the input connector of a body-pack transmitter and travels from the transmitter to the microphone over the microphone’s cable. A microphone that attaches to a wireless system’s body-pack transmitter is specifically designed to operate off this bias voltage.
If you have any additional questions about powering microphones, please feel free to contact our Audio Solutions Department.