This document is a collection of information and circuits for powering electret microphone capsules. This document is written for people who understand the basics of microhone circuits.
Many types of microphones require power to operate, as a general rule these types are described as condenser microphones. The power is used for internal pre-amplifiers and polarizing microphone capsules. If internal batteries are to be avoided then the only solution is to supply the power via the microphone signal cable.
In some cases microphones might be claimed to be "dead" not realising that they require a battery or in other cases phantom power.
An electret MIC is the best value for money omnidirectional microphone you can buy. Electret microhones are used in very many applications where small and inexpesive microphones with good performance characteristics are used. Electret microphone occupies (at a rough guess) the lower 90% of applications, quality wise.
The electret is a modification of the classic capacitor (or condensor) microphone, which exploits changes in capacitance due to mechanical vibrations to produce voltage variations proportional to sound waves. Whereas the condensor MIC needs an applied (phantom) voltage, the electret has a built in charge, and the few volts needed are to power the built-in FET buffer, not to create an electric field.
The electret capsule is a 2 terminal device which approximates to a current source when biassed with around 1-9 volt. Electret microphone capsule routinely consumes less than half a milliamp. Be aware that this impedance is swamped at signal frequencies by cable capacitance so that at 1kHz the assembly will exhibit an impedance of a few 10's of K.
The load resistor defines the impedance and can be matched to the low noise amplifier intended. This is usually 1-10kOhm. The lower limit is defined by amplifier voltage noise and the upper limit by interference pickup (and amplifier current noise). Suitable resistsnce values are typically in the range of 1-10 kohm.
Because the electret itself contains a small buffer amplifier which adds noise, it is common to specify a signal to noise ratio (usually at 94dB SPL) or self noise figure, which is the equivalent acoustic noise level, commonly around 20-30dB SPL.
Electrets need biassing because of the built-in FET amplifier inside the microphone capsule. Bias voltages should be kept clean, because the noise in thiss will get to the microphone output.
Here is another drawing of the same circuit:
Battery powered electret microphone
This circuit can be used with normal tape recorders and sound cards which usually are designed for dynamic microphones. When you build this circuit inside the microphone case (or to small external box) you can make yourself an universal microphone out of an electret capsule.
Super-simple powering circuit
In many cases it is possible to use one or two 1.5 V batteries (depends on microphone type) as a power supply for the mic. Battery is directly in series with the microphone.
If you do not know the right polarity of the battery, try it in both ways. In most cases wrong polarity at those low voltage should not cause any damage to microphone element.
Different powering methods used in soundcards
Sound Blaster way
Sound Blaster soundcards (SB16,AWE32,SB32,AWE64) from
Creative Labs use 3.5 mm stereo
jack for the electret microphones. The micrphone connector uses he following
Creative Labs has given the following specs for the Sound Blaster microphone input in their web site:
Input Type: Unbalanced Low Impedance Input Sensitivity: Approx. -20dBV (100mV or 0.1Volt) Input Impedance: 600 to 1500. (Ohms) Input Connector: 3.5mm Miniplug (Stereo Jack) Input Wiring: Audio on Tip, Ground on Sleeve, 5Volts DC Bias on Ring
Some other soundcards might use the same method or a different one. The soundcards which use 3.5 mm mono jack for microphones have typically a jumper which enable selecting if power for electret microphone is sent to the microphone connector. If the jumper is put on the bias voltage (usually +5V through 2..10 kohm resistor) is wired to the audio wire. The connector has then the following pinout:
This same microphone wiring seems to be used also in Compaq PCs equipped with Compaq Business Audio sound system (I tested this on Compaq Deskpro XE 560 and it worked nicely with Sound Blaster microphone). My measurements revelaled the the bias voltage that the Compaq put out was 2.43 V and the short circuit current was 0.34 mA. This will indicate that the bias voltage is fed through arount 7 kohm resistor. The ring of the 3.5 mm jack was not connected anywhere. The Compaq manual tells that this microphone input is wired to work only with a phantom-powered electret microphone, such as supplied COMPAQ microphone. COMPAQ manual calls the microphone powering method to be phantom powering, but as the measurements revelal the powerin method is not phantom powering in the the sense as understood by audio professionals. COMPAQ manuals tells that the microphone input has 1 kohm nominal impedance and can accept 0.013 volts maximum input level.
Powering three wire electret capsule from soundcard bias voltage output
This circuit is suitable for interfacing three wire electret microphone capsules to Sound Blaster soundcards which supply bias voltage for powering electret microphones.
Powering two wire electret capsule from soundcard bias voltage output
This circuit is suitable for interfacing two wire electret microphone capsules to soundcards (Sound Blaster soundcards) which supply bias voltage for powering electret microphones.
This wiring is used in Fico CMP-202 Computer Microphone.
Powering electret microphones with 3.5 mm mono jack form SB16
The following powering circuit can be used for powering electret
microphones which want the bias voltage through where they output
the audio signal.
Connecting telephone handset microphone to sound card
According some news articles in comp.sys.ibm.pc.soundcard.tech newsgroup this same cirrcuit can be used with Sound Blaster and the electret capsule in the telephone handset. First check that the microphone is electret microphone. Then carefully split a shielded cable for the mic, open your phone handset and confirm the positive side of the condenser mic cartridge. Then wire the cartridge like in the picture above (if you want to use the RJ11 connector in the handset then the microphone is connected to the wires in the outside pair). Differend handsets generate different output levels and levels from some handsets might not be enough for Sound Blaster.
If you want to also make the speaker to work then just wire it to the tip and shield of the sound card plug. Confirm that it has a greater resistance than 8ohms, or you might blow the soundcard output amplifier.
Powering multimedia microphones using external power supply
There's only one type of Phantom powering, and that's Phantom powering. Phantom powering deltais are covered by DIN spec 45596. Initially, it was specificed as 48 volts (P48) provided thru 6.8k resistors. The precise value of the resistors is not too critical, but the two resistors must be matched within 0.4% rof good performance. There are now defined systems at 24 and 12 volts (P24 and P12), but those are more rare than traditional 48V feed. Those lower voltage systems use lower resistance values for phantom powering. Most modern condensor microphones will work off a wide range of phantom power voltages.
Phantom powering is now the most common microphone powering methid due to it being safe if a dynamic (moving coil or ribbon) microphone is accidently or purposefully plugged into a powered microphone channel. The only hazard is that in case of a shorted microphone cable, or certain old microphones having a grounded center tap output, current can flow through the microphone, damaging it. It's a good idea anway to check cables regularly to see that there are no shorts between any of the pins, and the few ribbon or dynamic microphones with any circuit connection to ground can be identified and not used with phantom power.
The name for phantom power comes from telecommunications: A phantom line is a configuration where a telegraph signal is imposed on a balanced voice pair by using the ground return. This same method was also suitable for powering microphones in studios so it was used also there.
Phantom Power Types P48, P24 and P12
There is often a lot of confusion over the differences and indeed similarities of the various types. DIN 45 596 defines that phantom powering may be achieved using either of three standard operating voltages; 12V,24V or 48V. The way that these voltages are presented to the microphone may vary depending upon the type of powering used. The voltage does not usually definitively indicate the way that the power is delivered to the microphone, although 48V is almost certainly P48 powering when it is encountered.
Creating a clean 48V DC supply is difficult and expensive when on location with only 9V PP3 battery is available, partly because of this it should be noted that most modern microphones will work with voltages anywhere in the range 9-54V.
Phantom powering electret microphone from phantom power
The 10u capacitors in the HOT and COLD signal leads should be high-quality plastic film types. The values of these may be reduced to 2u2 if the preamp input impedance is 10k or greater. If you for some reason use electrolytics for the capacitors which couple the audio signal then those these mutst have >50V working woltage and you must bypass them with 100n plastic film caps. The 10u capacitor in parallel with the zener should be a tantalum type, and can have a 10n plastic film cap in parallel if you wish.
The cable to the capsule should be twin+shield. The shield should be connected to ground near the zener diode, and left unconnected at the capsule. The pinout given is the standard for XLR3 mic connectors.
Source: PZM Modifications web page by Christopher Hicks.
Better electret microphone phantom powering circuit
This circuit provides lower impedance output than the circuit above:
Source: PZM Modifications web page by Christopher Hicks.
Phantom power feeding unit for microphone
This is a schematic of external phantom power feeding circuit for
those who don't have mixer with phantom power output.
There should be two 12V zener diodes (wired back to back) between audio wires (HOT and COLD) and the ground to prevent 48V voltage pulse passing through the capacitors going to the mixer microphone input.
Use 1% accurate resistors for those 6.8 kohm resistors for best hum and noise elimination.
Obtaining the +48V power supply for phantom power
In mixing consoles the phantom power voltage is usually made using a separate transformer output or using a DC/DC converter.
If you are operating using batteries then it might be useful to know that many phantom powered micks will work fine on less than 48v, try 9v and work up till you get good results, 27v would be 3 9v batterys and a lot simpler than a DC to DC converter. Remeber that some microphones do not work properly or sound different when run on too low voltage. Five 9v batteries in series is 45 volts which should be enough for any phantom power microphone. If you do use batteries, put a capacitor around them because batteries do make noise. Filtering of battery noise can be done for exammple by using 10 uF and a .1 uF in parallel with the batteries. Another option is to decouple batteries with a 100 ohm resistor and 100uF 63V capacitor.
Does turning on the phantom power could do any damage to dynamic mics ?
Providing dynamic microphones are balanced types and wired with twin screened all the way through the phantom power will do no physical harm. So there should be no problem with most popular dynamics with correct balanced wiring. Modern dynamic microphones with balanced connectors are constructed so that the microphone element is completely floating so adding phantom power voltages does not affect it when you are using proper balanced cabling.
Many older dynamic mics have a center tap of the transformer in the mic grounded to the body of the mic, and to the shield of the cable. This could short the phantom voltage to ground, and could fry the transformer. It is easy to tell if this is the case with your mics. An ohmmeter or continuity checker will tell you if there is a DC circuit between either pin #2 or #3 and the cable shield (pin #1) or the mic housing. If so, don't use that mic with the phantom turned on. Good luck.
Do not try to connect microphones with unbalanced conenctor to a microphone input with phantom power on, because the current from the pahntom power supply will flow though your microphone and can damage it.
Interfacing professional microphones to computers
Typical computer sound interfaces which supply power to microphone supply only +5V voltage (PC soundcards, SUN workstations, Apple Macintosh). Those computer sometimes call this +5V voltage feed to microphone "phantom power", but that power feeding is not real phantom power as used in professional microphones (more details in text above). Professional microphones typically need real phantom power feed which is typically +48V, but many microphones work down to +12..15V. This means that you can't use professional microphones which need phantom power feed directly with the computer soundcard.
Depending on budget and degree of technical skill, you could either go to cheaper consumer grade mics, professional micrphones with option for using battery power or build a real phantom power interface for those microphones microhones. If you plan to build a phantom power adapter then you can use an external power supply for it or plan to take the power for it from inside computer. Many computer have +12V in them and this might be enough for many microphones if wire to them in the right way.
These are the same thing, A-B being the old term for what is now referred to as T powering. T-power (short for Tonaderspeisung, also called AB or parallel power, and covered by DIN spec 45595) was developed for portable applications, and is still common in film sound equipment. It would seem that 'T' power is mainly now only used by location recordists and specialists for specific applications, usually over long microphone cable lengths.
T-power is usually 12 volts, and the power is connected across the balanced pair through 180 ohm resistors. Due to the potential difference between the A and B conductors, a current will flow through a dynamic (moving coil) microphone if it is connected to this sort of phantom power. This is not good and will probably cause distortion to the sound and perhaps longer term damage to the microphone. Only T-power mics may be connected to T-power inputs; dynamic or ribbon mics may be damaged and phantom powered mics will not operate properly.
T-powered microphones behave like capacitors and hence block D.C. current flow. The advantage of T-power is that the shield of the microphone cable need not be connected at both ends, thus this allows the common practice of disconnecting one end of the shield to a microphone in order to prevent hum (earth loops).
Here is my udea for powering T-powered microphones from an external
power supply to be able to connect them to a mixer with balanced microphone
input and no T-powering option:
Note: This circuit idea is based on the information I have read about T-powering. I have not tested this circuit idea.
A balanced mics can often be connected unbalanced connector by just making a proper wiring (this is often used practice).
Unbalanced microphones can be connected to balanced microphone inputs but this does not give any benefits of balanced inputs compared to unbalanced input. An unbalanced (single-ended) mic can be converted to balanced by going through a proper DI box.
Tomi Engdahl <Tomi.Engdahl@iki.fi>