Concepts in Audio Signal Transmission: Part III
Part II of this series noted that condenser microphones require an amplifier circuit which must consist of either a JFET or a tube very close to their capsules, and therefore a power source for that amplifier. Externally polarized condensers also required a power source for their polarization voltage. This entry will consider the variety of power schemes employed for condenser microphones. There are a wide range of power sources used:
- internal batteries: required for wireless microphones, but also seen in a few condensers, including the Naiant X-Y microphone capsule amplifier.
- low-voltage powering: in many non-standardized formats, in the range of 1.5-10V; usually called “plug-in” or “bias” power.
- phantom power: the standard powering scheme for professional microphones, in the range of 12-48V.
- T-power: an earlier 12V powering standard for professional microphones.
- dedicated power supplies for tube microphones, with two or more voltages supplied across a custom multicore cable to the microphone.
- custom power supplies for remote “active” capsule condenser microphones, requiring low-voltage “bias” as well as high-voltage “polarization” supplies.
Internal battery power, whether as a sole power source or as an alternative to phantom powering, was implemented in a number of older electret condenser microphone designs, which was especially useful before phantom power become more common, especially in live sound mixers. Microphones such as the Nakamichi series, Shure SM94, and AKG C1000 all could use internal batteries to provide the bias power supply for their capsule JFET amplifiers. Naiant offers internal battery power as an option, both for the X-Y amplifier as well as the IPA amplifier, which can provide power for both electret and externally polarized condenser microphones. Battery power is still a useful feature when connecting microphones to recorders or instrument amplifiers that do not offer phantom or bias power supplies.
Since there are few standards for low-voltage microphones, each manufacturer has tended to create their own system. Low-voltage power is often called “bias” or “plug-in” power, but should not be labeled as “phantom” power, in order to avoid confusion with the professional microphone standard.
Some low-voltage systems are interoperable—for example, most microphones designed for use with a manufacturer’s wireless bodypack transmitters can be made to run on 3-9V, with the proper connectors or adaptors. Other low-voltage systems may not be as flexible. There are a wide range of connectors used, often with the same connector used in an incompatible manner in two different systems—for example, 3.5mm plugs are used for PC, stereo, and wireless system microphones, each in a different configuration. It’s a bit of a mess, really, but it’s a good business making them all play nicely together! If you want to read more details, have a look at the Naiant PFA product description.
Phantom powering, by contrast, is an official international standard for professional balanced output microphones, using standard XLR-3 connectors. There are three standard voltages, with two current and one deprecated:
P48: 48V supplied to each signal conductor (noninverted and inverted) across 6k8Ω resistors
P24 (deprecated): 24V supplied to each signal conductor across 1k2Ω resistors
P12: 12V supplied to each signal conductor across 680Ω resistors
Condenser microphones will have a specification for required phantom voltage and usually also current. With reference to the Shure® condenser microphones considered in the previous post, here are their power specifications:
SM81: 11-52V, 1.2mA
KSM44A: 11-52V, 5.8mA
KSM141: 11-52V, 4.7mA
Preamplifiers and phantom powering
While microphones almost always have a complete power specification, that is not the case for preamplifiers. Most preamplifiers strive to provide P48 phantom, since many phantom-powered microphones require that, and nearly all will accept it.
The standard for maximum required current from a P48 phantom supply is 10mA per microphone. There are many, many preamplifiers on the market that cannot meet that specification—be careful, because such units often will claim to be in compliance with P48 phantom. This is especially true of USB-powered interfaces, and also battery-powered portable recorders, but is sometimes even seen in AC-powered units—for which there is truly no excuse!
If you find improper performance (such as excessive noise or inadequate headroom) from a high current-draw microphone, it could very well be that the preamplifier does not meet the P48 current specification. That can be uncovered quickly by testing with a digital multimeter set to measure voltage and current: unloaded voltage must be between 44-52V, and short-circuit current from either pin 2 or 3 should be between 5-7mA. A deficiency in either measure is an out-of-specification phantom power supply!
Portable devices and phantom powering
There is no technical reason why portable devices cannot meet the phantom power current specification, but in practice many designs fall short. 10mA per microphone is perhaps an unreasonable expectation for a small battery-powered device, because it will result in a high draw from the battery and thus short autonomy for the device. Thus, the audio technician may want to select microphones that have low current draw as well as a flexible requirement for the supply voltage—the difference between low- and high-power microphones could be a difference in autonomy of a factor of three or more.
Since practically all phantom-powered microphones can accept P48, one might wonder why any device would not be so designed. There is a good reason for portable amplifiers to supply lower-voltage phantom: it’s much more efficient, and thus can result in longer autonomy for a battery-powered amplifier or recorder. Indeed, the power dissipated in the P48 phantom resistors (and thus, not used by the microphone—wasted!) can be the greatest use of power in a portable amplifier circuit. That’s not very smart design! Issues with portable phantom power efficiency were considered at length in this entry.
Thus, many portable devices will provide a low-voltage phantom supply, either solely or as a selectable option. Often, this supply won’t conform to the P12 standard; voltage supplies within the range of 15-24V can be found across various portable device models. This is not critical, because microphones that can run on less than P48 phantom are often capable of running on any voltage/resistance combination that can supply the specified current.
Custom power schemes – tube microphones and remote capsules
As we learned in part II, condenser microphones can be externally polarized or permanently polarized (electret). Nearly all condenser microphones that can run on a low-voltage supply are electrets, although there are exceptions: the Schoeps CMR accessory can generate a high polarization voltage to support its attached externally polarized capsule from a 5V supply.
About 50 years ago, Schoeps® designed the first remote capsule system for externally polarized capsules. The remote system was designed to minimize the size of the microphone capsule for unobtrusive placement near the source. This required a minimal circuit at the capsule, with the bulk of the phantom powering and output drive circuit in the main amplifier “body” extended via a custom cable from the capsule. Unlike with their more recent CMR accessory, their original “active” KC remote capsule scheme requires a supply of both a low voltage for the JFET amplifier and a high voltage for capsule polarization. Several other manufacturers followed, and as with low-voltage electrets, each has their own custom system. These systems are considered in detail in this article. Here is a simplified diagram of a typical scheme (although different from Schoeps, which requires separate power and signal leads):
The earliest condenser microphones used tubes as their amplifier, as JFETs hadn’t been invented yet! Again, here manufacturers each developed their own circuits and thus every tube microphone requires its own specified power supply. Here is one much simplified example:
There have been three tube microphones that did not require a custom power supply, instead running on 48V phantom power—the discontinued Audio-Technica AT3060, Microtech Gefell UM900, and the former Naiant MSH-4 available now in an improved kit version.
The conclusion when considering microphone powering: phantom power is a standard, bias power can be semi-standard; everything else is custom. Many different microphones can be made interoperable between systems, and most can be made to work on phantom power with an adaptor. Again, see the Naiant PFA for a comprehensive solution for interoperation!
Soon to come is the final entry in this series—Part IV: Attenuation; when, where, and how?