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Phantom power tester.



PT01 is a small phantom power tester housed in a XLR plug.
Indication is with a test tone and a LED.
The tester works for P12, P24 and P48 systems.

Phantom voltages.

The following phantom voltages are defined in IEC61938:

Table 1: Standard phantom voltages for microphones.
NameOpen circuit voltageSeries resistorMaksimum current
P1212 V ±1 V680 Ω15 mA
P2424 V ±4 V1.2 kΩ10 mA
P4848 V ±4 V6.81 kΩ10 mA


PT01 schematic.
Fig.1: PT01 schematic.

The circuit around Q1 is a "standard" 1-transistor phase-shift oscillator running with an average collector current of 250 µA.
The only nice thing to be said about this circuit is that it can be built with 1 transistor ( saving space ).
Changing any component, the supply voltage or the temperature will affect the frequency and output voltage.
The circuit oscillates at the frequency where the phase shift through the 3 R-C filters and the filter formed by C4 is 180 °.
The output from the oscillator is normally taken from Q1's collector, but this is a high-impedance point, so a buffer is needed.
C4 is normally a large capacitor that decouples Q1's emitter for maximum gain.
The value of C4 can be reduced to raise the signal voltage on the emitter giving a "low-impedance" output.
A value where C4, together with R9 and Q1's emitter resistance ( 100 Ω at 250 µA ), has a -3 dB point of 4..5 times the oscillator frequency works well.
Loading this output with an AC coupled load will increase the gain in Q1 so the oscillator will not stop when it is loaded.
R10..R12 is an attenuator that reduce the output level to around 10 mV.
C5 and C6 are AC coupling.
R4, R5, D1B, D2 and D4 is a simple voltage regulator with around 4.5 V output.
R4 and R5 are chosen to draw the maximum 10 mA from a 24 V supply ( less from 12 V and 48 V ).
D2 is a standard red LED mounted on the PCB and D4 is a 5 mm green LED mounted in the connector's cable entry.
Both LEDs must have a continuous forward current rating of 15 mA or more.
If you want more light, use a high efficiency LED for D4 ( or use a red LED for D4 and a green one for D2 ).
D1A prevents Q1 from being reverse biased when the tester is plugged into a powered cable ( discharging its capacitance ).
D3A protects the circuit from reverse voltage in case the cable is mis-wired ( pins 1-2 or pins 1-3 swapped ).
My initial intention was to have a frequency around 400 Hz, but my breadboarded prototype-circuit was at 439 Hz ( far too close to a tuning reference - which this circuit is not ), so I reduced the frequency to around 250 Hz.


D4 is a 5 mm LED that is soldered so it extends from one end of the PCB.
D1, D3 and Q1 are SOT-23-3 cases. The footprints are larger than normal SMD footprints for easy hand-soldering.
All other components are 1206 SMDs ( 0805 can be fitted ).
Resistors are 1% thick- or thin-film types. 5% types can be used except for R4, R5 which must be matched to 1% or better ( 0.1% ideally ).
C5 and C6 should be 100 V X7R or X7S ceramic types, but 5x11 mm, 63 V electrolytics can be fitted as well ( plus towards R12 ).
If you use electrolytics, diodes ( 1N4148 ) should be added from the XLR pin 1 to 2 and pin 1 to 3 with the anodes on pin 1.
Other capacitors are NP0 or X7R ceramic types with a voltage rating of 10 V or more and a tolerance of ±10% or better.
A complete schematic and an example parts list are in the files download.


PT01AAA photo. PT01AAA photo.
Fig.2: Photos of finished tester.

Download PT01A design files.

I have boards available for this project. See the PCBs page.

Known Issues / updates.

No known issues.

Poul Petersen, C/Faya 14, 35120 Arguineguín, Las Palmas, Spain.
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