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Some notes on phantom power.

Contents


Introduction.

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.

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.

Schematic of a typical phantom powered system.
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:

Table 1: Standard phantom voltages. The 3 left-most columns are from the standard, the rest are calculated values.
NameOpen 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 )
P12L12 V ±1 V3.3 kΩ4 mA7.2 mA22 mW87 mW6.0 V3.6 mA
P1212 V ±1 V680 Ω15 mA35 mA100 mW420 mW6.9 V15 mA
P2424 V ±4 V1.2 kΩ10 mA40 mA180 mW960 mW18 V10 mA
P4848 V ±4 V6.8 kΩ10 mA14 mA170 mW680 mW24 V7.1 mA
SP4848 V ±4 V2.2 kΩ22 mA44 mA520 mW2.1 W24 V22 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.


Phantom supply noise.

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.

AC model for typical microphones.
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.

AC model for typical microphone amplifiers.
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 ).

Table 2: Maximum allowed VP noise for different values of CMRR with a dynamic microphone.
A2 or T2 CMRRElectronically
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%.

Table 3: Maximum allowed VP noise for different values of CMRR with a capacitor microphone.
A2 or T2 CMRRElectronically
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.


Amplifier input capacitors.

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.

AC model for typical microphones.
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.

Table 4: Lower cut-off frequency and input impedance change with different capacitors.
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 µF2 kΩ160 Hz310 Hz690%410%
10 µF2 kΩ16 Hz31 Hz28%12%
100 µF2 kΩ1.6 Hz3.1 Hz0.3%0.1%
1 µF10 kΩ32 Hz62 Hz88%42%
10 µF10 kΩ3.2 Hz6.2 Hz1.3%0.5%
100 µF10 kΩ0.32 Hz0.62 Hz0.01%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.

Table 5: Worst case CMRR calculation for different capacitor values.
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 µF2 kΩ14 dB26 dB15 dB17 dB30 dB
10 µF2 kΩ24 dB36 dB30 dB33 dB50 dB
100 µF2 kΩ44 dB56 dB50 dB53 dB70 dB
1 µF10 kΩ19 dB31 dB24 dB27 dB44 dB
10 µF10 kΩ38 dB50 dB44 dB47 dB64 dB
100 µF10 kΩ58 dB70 dB64 dB67 dB84 dB
1 µF1 MΩ58 dB70 dB64 dB67 dB84 dB
100 µF1 MΩ98 dB110 dB104 dB107 dB124 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.


Battery powered phantom supplies.

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 ).

Table 6: Battery life estimate for a 9 V / 500 mAh battery driving a 48 V phantom supply ( the last row is for a shorted supply ).
Phantom supply100% converter efficiency75% converter efficiency50% converter efficiency
CurrentPowerPower @ 9 VCurrent @ 9 VBattery lifePower @ 9 VCurrent @ 9 VBattery lifePower @ 9 VCurrent @ 9 VBattery life
1 mA48 mW48 mW5.3 mA94 h64 mW7.1 mA70 h96 mW11 mA47 h
2.5 mA120 mW120 mW13 mA38 h160 mW18 mA28 h240 mW27 mA19 h
5 mA240 mW240 mW27 mA19 h320 mW36 mA14 h480 mW53 mA9.4 h
10 mA480 mW480 mW53 mA9.4 h640 mW71 mA7 h960 mW110 mA4.7 h
14 mA680 mW680 mW75 mA6.6 h900 mW100 mA5 h1.3 W150 mA3.3 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.


Microphone connector wiring.

Table 7: Connector wiring.
Connector typeChassisChannel 1 signal +Channel 1 signal -Channel 2 signal +Channel 2 signal -
3-pin XLR123
5-pin XLR12345
3-pin DIN213
5-pin DIN14253
¼" TRS jackSleeveTipRing

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.


References.

[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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