## Some notes on phantom power. | |

- Introduction
- Phantom power
- Phantom supply noise
- Amplifier input capacitors
- Battery powered phantom supplies
- Microphone connector wiring
- References
- Copyright and disclaimer

This is a collection of notes on phantom power for microphones.

They are in no particular order.

Most of the calculations can be found in the zip file for download.

Phantom power is DC power transmitted through a microphone cable to supply microphones or other equipment that requires power to operate.

It was originally designed for capacitor microphones where it supplies power to the amplifier and polarization voltage for the transducer element.

Fig.1: Typical phantom powered system.

The microphone block shows the general principle for phantom powered microphone:

C is the transducer element ( a capacitor that varies with sound pressure ).

RC supplies the polarization voltage for the transducer.

A1 is an amplifier with a very high input impedance and a low impedance ( typically 150 Ω ) balanced output capable of driving the cable.

RM1 and RM2 supplies the the microphone with power from the line.

Microphone amplifier:

VP is the phantom power supply.

RP1 and RP2 are series resistors for VP.

A2 is the microphone amplifier.

Both A1 and A2 must be AC coupled to the line either with capacitors or with a transformer.

Resistors pairs RP1, RP2 and RM1, RM2 must be matched to within 0.4% to maintain good CMRR ( 0.2% resistors ).

If you select them from a tape of 1% resistors it is easy to find pairs with 0.1% matching.

The original phantom power specification was 48 V with a maximum current of 2 mA, but today the IEC 61938 standard defines several different phantom power systems, where P48 is the most common:

Name | Open circuit voltage ( VP ) | Series resistor ( RP ) | Maximum continuous current ( IP ) | Short circuit current (IP) | Maximum power available to load | Maximum power drawn from VP during short | Line voltage at maximum power ( VL ) | Current at maximum power ( IP ) |

P12L | 12 V ±1 V | 3.3 kΩ | 4 mA | 7.2 mA | 22 mW | 87 mW | 6.0 V | 3.6 mA |

P12 | 12 V ±1 V | 680 Ω | 15 mA | 35 mA | 100 mW | 420 mW | 6.9 V | 15 mA |

P24 | 24 V ±4 V | 1.2 kΩ | 10 mA | 40 mA | 180 mW | 960 mW | 18 V | 10 mA |

P48 | 48 V ±4 V | 6.8 kΩ | 10 mA | 14 mA | 170 mW | 680 mW | 24 V | 7.1 mA |

SP48 | 48 V ±4 V | 2.2 kΩ | 22 mA | 44 mA | 520 mW | 2.1 W | 24 V | 22 mA |

Although the standards for phantom powering vary widely, they are, to some extend, compatible.

Many units designed for P12 or P24 are tolerant to 48 V ( no guarantees though ).

Units designed for P48 may work with 12 V or 24 V, but in many cases with ( severely ) reduced performance.

Some low-cost mixers and microphone amplifiers have a phantom supply that is taken from an existing supply.

I have seen 9V ( from a battery ), 15V ( from the general supply ) and 22V ( directly from the rectifier via a RC filter ).

In many cases, you can not turn this phantom supply off.

You can download a spreadsheet with the calculations here.

This is an attempt to get an idea of the noise requirement for phantom power supplies for different microphone / amplifier designs.

Rather than trying to cover all possible combinations, this shows the requirement for a few selected examples.

Fig.2: AC model for typical microphones.

(a) is a microphone ( or DI box ) with electronically balanced output, (b) is a transformer balanced output or ( without RM ) a dynamic microphone.

The output impedance is 150 Ω ( including RM ).

The common mode output impedance is 0 for A1 and ∞ for T1.

Fig.3: AC model for typical microphone amplifiers.

(a) is an electronically balanced amplifier, (b) is a transformer balanced amplifier.

The input impedance is 2 kΩ ( including RP ).

VP is 48 V.

The following tables show the maximum allowed noise on VP for different values of amplifier CMRR and noise figure.

The noise-bandwidth is 20 kHz, dBu is dB referred to 0.775 V.

The worst possible combination is when a dynamic microphone is connected to a phantom powered input.

RP = 6.8 kΩ ±0.2% and RB = 2 kΩ ±1% ( using 1% resistors here does not really make sense, but it is common practice ).

A2 or T2 CMRR | Electronically balanced 3 dB NF | Electronically balanced 1 dB NF | Electronically balanced 0.1 dB NF | Transformer balanced 3 dB NF | Transformer balanced 1 dB NF | Transformer balanced 0.1 dB NF |

40 dB | -74.9 dBu ( 140 µV ) | -80.7 dBu ( 71 µV ) | -91.2 dBu ( 21 µV ) | -91.2 dBu ( 21 µV ) | 97.0 dBu ( 10 µV ) | -107.5 dBu ( 3.3 µV ) |

60 dB | -61.8 dBu ( 630 µV ) | -67.6 dBu ( 320 µV ) | -78.1 dBu ( 97 µV ) | -71.2 dBu ( 210 µV ) | 77.0 dBu ( 110 µV ) | -87.5 dBu ( 33 µV ) |

80 dB | -58.0 dBu ( 980 µV ) | -63.8 dBu ( 500 µV ) | -74.3 dBu ( 150 µV ) | -51.2 dBu ( 2.1 mV ) | 57.0 dBu ( 1.1 m;V ) | -67.5 dBu ( 330 µV ) |

100 dB | -57.5 dBu ( 1.0 mV ) | -63.4 dBu ( 530 µV ) | -73.8 dBu ( 160 µV ) | -31.2 dBu ( 21 mV ) | 37.0 dBu ( 11 m;V ) | -47.5 dBu ( 3.3 mV ) |

This is for a capacitor microphone is connected to a phantom powered input.

RA = 75.9 Ω ±0.2%, RM = 6.8 kΩ ±0.2%, RP = 6.8 kΩ ±0.2%, and RB = 2 kΩ ±1%.

A2 or T2 CMRR | Electronically balanced 3 dB NF | Electronically balanced 1 dB NF | Electronically balanced 0.1 dB NF | Transformer balanced 3 dB NF | Transformer balanced 1 dB NF | Transformer balanced 0.1 dB NF |

40 dB | -57.6 dBu ( 1.0 mV ) | -63.4 dBu ( 520 µV ) | -73.9 dBu ( 160 µV ) | -57.6 dBu ( 1.0 mV ) | 63.5 dBu ( 520 µV ) | -73.9 dBu ( 160 µV ) |

60 dB | -49.2 dBu ( 2.7 mV ) | -55.0 dBu ( 1.4 mV ) | -65.5 dBu ( 410 µV ) | -48.5 dBu ( 2.9 mV ) | 54.3 dBu ( 1.5 mV ) | -64.8 dBu ( 450 µV ) |

80 dB | -47.6 dBu ( 3.2 mV ) | -53.5 dBu ( 1.6 mV ) | -64.0 dBu ( 490 µV ) | -46.7 dBu ( 3.6 mV ) | 52.5 dBu ( 1.8 mV ) | -63.0 dBu ( 550 µV ) |

100 dB | -47.5 dBu ( 3.3 mV ) | -53.3 dBu ( 1.7 V ) | -63.8 dBu ( 500 µV ) | -46.5 dBu ( 3.7 mV ) | 52.3 dBu ( 1.9 m;V ) | -62.8 dBu ( 560 µV ) |

You can download a spreadsheet with the calculations here.

This is not really a part of phantom supply design, but as you may need to put DC blocking capacitors if you design an external supply, it has some relevance.

Fig.4: Phantom input circuits.

(a) is a microphone amplifier with built-in phantom supply. The over-voltage protection is part of A2.

(b) is DC blocking for an external phantom supply.

C3, C4 block the DC from the phantom supply.

R1, R2 limit the current through D1..D4 in case the phantom supply is shorted to GND. A typical value is around 10 Ω.

D1..D4 protect the amplifier against over-voltage. A value of 5.1 V..10 V works in most cases.

C5, C6 are not required if the input is designed for a dynamic microphone, but several low-cost mixers on the market have a non-standard phantom voltage in their inputs.

It is insufficient to power a 48 V microphone, but sufficient to turn on the protection diodes.

R3, R4 prevent the leakage current in C3, C4 ( C5, C6 ) from turning the protection diodes on. Their value should be as high as possible.

The input capacitors are normally electrolytics and should not be used to define the lower cut-off frequency.

The table below list some values.

Some microphone amplifier standards require that the input impedance is within specific limits over a given frequency range.

N10 ( see [1] ) require the input impedance to be within 20% of the 1 kHz value from 31.5 Hz to 16 kHz.

Using too small capacitors ( or using them for HP filtering ) will not meet the standard.

Input capacitor ( C1 = C2 ) | Input resistance ( RB1+RB2 ) | Lower -3 dB frequency | Lower -1 dB frequency | Input impedance rise at 20 Hz | Input impedance rise at 31.5 Hz |

1 µF | 2 kΩ | 160 Hz | 310 Hz | 690% | 410% |

10 µF | 2 kΩ | 16 Hz | 31 Hz | 28% | 12% |

100 µF | 2 kΩ | 1.6 Hz | 3.1 Hz | 0.3% | 0.1% |

1 µF | 10 kΩ | 32 Hz | 62 Hz | 88% | 42% |

10 µF | 10 kΩ | 3.2 Hz | 6.2 Hz | 1.3% | 0.5% |

100 µF | 10 kΩ | 0.32 Hz | 0.62 Hz | 0.01% | 0.01% |

Another serious issue with input capacitors is that they ruin the CMRR of the amplifier.

Their value is normally so high that electrolytics are the only realistic choice. You can of course match them, but their long-term drift makes that a waste of time.

Input capacitor ( C1 = C2 ) | Input resistance ( RB1+RB2 ) | CMRR at 50 Hz with 20% tolerance capacitors | CMRR at 50 Hz with 5% tolerance capacitors | CMRR at 100 Hz with 20% tolerance capacitors | CMRR at 150 Hz with 20% tolerance capacitors | CMRR at 1 kHz with 20% tolerance capacitors |

1 µF | 2 kΩ | 14 dB | 26 dB | 15 dB | 17 dB | 30 dB |

10 µF | 2 kΩ | 24 dB | 36 dB | 30 dB | 33 dB | 50 dB |

100 µF | 2 kΩ | 44 dB | 56 dB | 50 dB | 53 dB | 70 dB |

1 µF | 10 kΩ | 19 dB | 31 dB | 24 dB | 27 dB | 44 dB |

10 µF | 10 kΩ | 38 dB | 50 dB | 44 dB | 47 dB | 64 dB |

100 µF | 10 kΩ | 58 dB | 70 dB | 64 dB | 67 dB | 84 dB |

1 µF | 1 MΩ | 58 dB | 70 dB | 64 dB | 67 dB | 84 dB |

100 µF | 1 MΩ | 98 dB | 110 dB | 104 dB | 107 dB | 124 dB |

The calculation above for 1 MΩ is done for a differential input impedance of 1 MΩ.

You do not need to increase the differential input impedance to achieve better CMRR, only the common-mode impedance.

You can download a spreadsheet with the calculations here.

In some cases it is desirable to supply phantom powered equipment from batteries.

The following is an estimate of battery life with different loads.

It does not take into account that batteries are very different and their capacity varies a lot with discharge current ( up to a factor 3 for the same battery type ).

Phantom supply | 100% converter efficiency | 75% converter efficiency | 50% converter efficiency | |||||||

Current | Power | Power @ 9 V | Current @ 9 V | Battery life | Power @ 9 V | Current @ 9 V | Battery life | Power @ 9 V | Current @ 9 V | Battery life |

1 mA | 48 mW | 48 mW | 5.3 mA | 94 h | 64 mW | 7.1 mA | 70 h | 96 mW | 11 mA | 47 h |

2.5 mA | 120 mW | 120 mW | 13 mA | 38 h | 160 mW | 18 mA | 28 h | 240 mW | 27 mA | 19 h |

5 mA | 240 mW | 240 mW | 27 mA | 19 h | 320 mW | 36 mA | 14 h | 480 mW | 53 mA | 9.4 h |

10 mA | 480 mW | 480 mW | 53 mA | 9.4 h | 640 mW | 71 mA | 7 h | 960 mW | 110 mA | 4.7 h |

14 mA | 680 mW | 680 mW | 75 mA | 6.6 h | 900 mW | 100 mA | 5 h | 1.3 W | 150 mA | 3.3 h |

It is also possible to use 5 9 V batteries in series for a 48 V phantom supply.

It should give an overall better efficiency than a step-up converter, but I do not know how much of the battery capacity you can use with a lower limiting voltage of 8.8 V.

I have seen suggestions that a battery is too noisy for this, so I measured a new alkaline battery at different currents from 1 mA to 10 mA:

22 Hz-22 kHz noise: <-116.1 dBu ( the same as the analyzer measures with a shorted input ).

You can download a spreadsheet with the calculations here.

Connector type | Chassis | Channel 1 signal + | Channel 1 signal - | Channel 2 signal + | Channel 2 signal - |

3-pin XLR | 1 | 2 | 3 | ||

5-pin XLR | 1 | 2 | 3 | 4 | 5 |

3-pin DIN | 2 | 1 | 3 | ||

5-pin DIN | 1 | 4 | 2 | 5 | 3 |

¼" TRS jack | Sleeve | Tip | Ring |

¼" TRS jacks are not normally recommended for phantom powered circuits as they connect the socket's ring and sleeve to the plug's tip and ring during insertion and removal.

This applies 48 V through a 6.8 kΩ resistor across the microphone and discharge the cable and amplifier input capacitance through the microphone.

[1] | Danmarks Radio, Norsk Rikskringkasting, Rikisutvarpid, Sveriges Radio, Yleisradio: N10 Electrical Specifications for Sound Control Systems and Units, Fourth Edition. |

[2] | Jörg Wuttke: The feeble phantom. |

[3] | Chris Woolf: Powering Microphones. |

[4] | Glen Ballou: Handbook for Sound Engineers, Fourth Edition. |

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