The M2R was designed originally as a personal listening receiver, often called an “IEM pack” (in-ear monitors) and thus, we used a very high-quality headphone amp for the design. The downside is there is no protection from 48V phantom power when the M2R is used as a camera hop receiver and is connected to professional microphone inputs. We have seen several M2Rs (and occasionally other receivers) come into repair with damage from this issue. This List will discuss what phantom power is, why it’s important, and how you should work with it to get the best results and avoid damaging your equipment.
What is phantom power and why is it important?
Phantom power, commonly designated as +48V or P48, is a way of sending DC current through an XLR cable and was designed to power these devices without using external power supplies. The power is sent through the same cable that is carrying the audio signal and leverages the multiple wires in a balanced-XLR cable to provide voltage to mics that need it (condenser mics, which have active electronics and require a voltage for polarizing the microphone’s transducer element) without affecting those that do not (dynamic mics). You can plug either type of mic into a mixer that provides phantom power and the condenser mic will use that power, while the dynamic mic will ignore it. That is why it’s called phantom – it lurks in the background for mics that need it, is invisible to those that do not.
When did phantom power become an issue with Lectrosonics equipment?
With some of our earlier generation receivers (specifically CR-series, which some of you might still use), the signal from the microphone was cut off until the phantom power was removed. Some customers didn't realize what was happening, thought their units were failing and needlessly sent them in. So, we modified our circuitry to add a resistive series into our receiver design to protect against capacitive discharge, along with resistive bleeders to ground to reduce peak voltages and non-polar capacitors for protection against miswiring. This works, although we do lose one or two output stages when outputs are wired to 110VAC.
Can phantom power ever damage my equipment?
As mentioned, although most of our equipment has since been redesigned to account for phantom power, it is important to know how phantom power may negatively affect microphones in older units (and this covers rental equipment or other scenarios you might encounter onsite) before you decide to use it. For example, some multi-channel preamps can only apply phantom power across multiple channels.
The situations that could potentially cause phantom power to damage a microphone include:
- Shorts. Electrical shorting will send the phantom power voltage up one audio conductor rather than both. Even a quick/brief short can cause the DC voltage to enter the wrong parts of the microphone and damage it.
- Sending phantom power to an unbalanced microphone. Phantom power requires a balanced connection to work properly. If phantom power is forced through an unbalanced cable to an unbalanced microphone, the 48 volts on the audio cable may overload and damage the circuit. Examples of unbalanced microphones include DC-biased lavs and karaoke mics. Older ribbon microphones like those made by RCA can also be damaged by P48.
- Power surges and brownouts. Power surges can overload the phantom power circuit. The spike in electrical current can fry certain wires or components within the circuit. Power conditioners are always a good idea, and although we don’t endorse one vendor over another, Furman is one most of you will be familiar with.
6 best practices when working with phantom power
Phantom power is not just a microphone phenomenon! There are some other devices like preamps and direct boxes that require a small amount of power to function, hence the presence of phantom power. These devices can often be powered by batteries, but many of them are designed to function using phantom power. Here are six best practices when working with phantom power:
- There is a warning in the M2R manual: “WARNING: If connecting this receiver to microphone inputs, such as in a camera hop arrangement, 48V phantom power MUST be turned off. Otherwise, damage to the receiver will occur.” We cannot stress the importance of reading your product manuals, especially for warnings of this nature.
- If you are unsure whether or not the device you are connecting with may have P48 present, use a phantom power blocking cable made specifically for this purpose. ACE Cables is one good source. If you choose to use this option, please be mindful of their disclaimer, which reads, “Phanton power block cables are fitted with passive components to protect from phantom power. An initial voltage spoke will occur when phantom power is turned on. Whenever possible, plug the XLR connnector in first, wait 5 seconds for the internal bleeder resistor to reduce voltage, then plug 3.5mm connector in. Components have an estimated 2,000 hour life. Periodically test cables with a voltage meter to ensure correct function.”
- If possible, only plug/unplug all mics when global phantom power is off. “Hot patching” (plugging-in and unplugging patch cables) is not advised while phantom power is engaged.
- Always mute your outputs/channels to avoid speaker- or headphone- (or ear)-damaging pops when plugging/unplugging mics with phantom power on or when turning phantom on/off.
- If you have to use a modern, balanced output ribbon mic with global phantom power, make sure it's plugged in before turning phantom on.
- Don't use a patch bay when using global phantom power, or make certain that phantom is deactivated when patching. Shorting 48V to ground is not a good idea for any mic.