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For reasons that I find somewhat puzzling, there are very few decent 48V phantom power supply schematics available on the Net. Fortunately, this is not a problem, as a voltage doubler supply will give more than enough voltage, and is easy to build. Of the designs that are available on the Net, there is the supply shown in the P30 mixer (see Figure 3), which is similar to the design shown here, but is not as refined. Another version of phantom supply (reasonably common) uses an oscillator and voltage multiplier to provide the 48V (or thereabouts) supply, but these are not suitable (IMO) due to very poor regulation and an inherent inability to provide enough current.
There are also quite a few suggestions that you can use as little as 18V for phantom powering (using a pair of 9V batteries), but unless the microphone or DI box (for example) is specified to operate at such a low voltage, then I would not recommend it - headroom will be reduced dramatically, and distortion will be a problem at even relatively low levels. It is probable that few people will ever need the maximum suggested output current, since 200mA is capable of supplying up to 20 phantom powered microphones at once. The circuit is shown in Figure 1, and as described above, uses a voltage doubler rectifier.
The regulator was a very common topology prior to the introduction of 3-terminal regulator ICs, and is used here so that high voltage regulators are not needed. As can be seen, there are no adjustments, and this means that the 48V may be a little higher (or lower) than rated. Load regulation is far better than you might expect, with typically 100mV variation between full load (100mA) and no load. To use a single 50-60V winding, simply replace D1 and D2 with 100k resistors, link the AC1 and AC2 terminals, and apply the output from an external bridge rectifier as shown.
The next problem is how to actually send the phantom power to the microphone and not the mixer input circuits. Although shown using bipolar electrolytic caps, some constructors will no doubt want to use something 'better', but polyester or similar caps at those values will be very large!
It is worth noting that many mixers use polarised electrolytics at the inputs of phantom power circuits. Zener diodes must be used as shown to limit the maximum voltage applied to the mic input circuits. Some mixers boast a "silent" phantom switch to alleviate the typical loud BANG through the mixer when phantom power is turned on or off. If you don't wish to buy the PCBs, you may use Veroboard, but unless great care is taken with the earthing arrangements, noise will almost certainly be worse than quoted. The power transformer does not need to be anything too fancy, but I suggest a separate box for it to minimise hum and noise. It is extremely important that the transformer is not used to power other supplies or equipment. Q1 on the power supply (Figure 1) must be fitted with a heatsink, and worst case power dissipation will be around 5W. The PCB is laid out such that the power transistor can be attached directly to the chassis if desired (using insulating washers and heatsink compound, of course), and this alleviates the need for a separate heatsink.
A Wheatstone Bridge is easy to set up, and while it is not especially accurate in absolute terms, it can be made extremely sensitive to variations between components.
Because the bridge circuit does not care if you use AC or DC, an amplifier can be used to detect the null, and it can be made almost unbelievably sensitive. You will have a number of 6.8k resistors to hand, all of the same nominal value but having normal (1%) tolerance.
The signal input (applied unsurprisingly to Sig1 and Sig2) must be floating if you use an amp for monitoring, since one side of the amp input will be earthed as shown. The Wheatstone Bridge is such a useful device that you can eventually expect to see a complete article describing its use for comparing resistors, capacitors and inductors - actually, you can match any passive components and even complete networks by this means. Typical standard values of some components involved in the first designs?Just guessing, interesting question indeedTony. 48V being considered the highest "safe" low-voltage voltage by legislation in many countries..
Circuit should look like this roughlyCheck that you have about 60vdc at the main storage capacitor. Depends on how much current the head-amp draws, but usually there?s only ~34 volts at the most that gets to the mic circuit anyway. Those that I have seen are either very crude, or require the use of a special transformer that (naturally) is all but unobtainable (or both).
The design featured here uses just that, and allows the use of a readily available transformer and other low-cost parts, to give a supply with extremely good performance.
The maximum current that can be drawn from a standard phantom circuit (using 6.81k resistors) is 14mA into a short circuit. Simulation gives a figure of 10uV peak to peak at 200mA, but this is likely to be rather optimistic.

These are much harder to get than the standard versions, and still require additional circuitry because 48V versions are not made. If output voltage increases (because the load current is reduced for example), Q3 is turned on harder, removing base drive from Q2 (and hence Q1), reducing the output voltage to the preset value. This is not a problem however, and all phantom feed microphones will handle the variation without any problems at all.
The normal range is between 25 and 30V AC, but at least one customer found they had a 50V winding available - ideal for a phantom power supply. Positive goes to the cathode connection of the D1 position (positive terminal of C1) and negative to the anode of the D2 connection (negative terminal of C2).
The latter will not be impressed with 48V DC applied, and will most likely voice their displeasure by failing instantly.
Fairly obviously, the closer the match the better, and the multimeter used does not have to be absolutely accurate, since you are measuring for a difference rather than an absolute resistance value. Only one channel is shown - subsequent channels are identical, up to a typical maximum of 10 (20 at a pinch) for a single supply module.
While this is quite ok while phantom power is applied, the caps will be unbiased when phantom power is not being used.
The worst possible scenario is when a mic lead is connected to a mic while phantom power is on.
All electrolytic caps should be rated at 50V or higher (NOTE: C5 in Figure 1 must be rated at 63V!). The centre-tap must not be connected to anything, and needs to be insulated to prevent contact.
Even so, there is no such thing as a heatsink that is too big, so I suggest that you use the largest one that you can. For only one or two phantom powered mics, a heatsink is not essential, but a small one is cheap insurance.
A Wheatstone Bridge used to be the standard method of accurately measuring resistance, inductance and capacitance, but new digital instruments have taken over.
Using a power supply, 9V battery (or even better, a 12V AC voltage from a transformer), the circuit shown in Figure 3 will resolve a difference of 1 ohm easily with a suitable test setup.
In use, wire up the circuit as shown, and use a meter or amplifier and headphones (or speaker) to monitor the difference signal. Wheatstone bridges are also used for precision temperature measurement, strain gauges (used to test mechanical movement in structures) and have many other uses. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro- mechanical, is strictly prohibited under International Copyright laws.
Perfect !Either the transformer could not supply enough current or something else is wrong. Any electronics above 50 vdc required a high voltage cirtification  from the fcc or some other entitie not sure exactly. In contrast, a 15-0-15V transformer can be obtained almost anywhere, but alas, does not have enough voltage after rectification and filtering. As shown, there is no short circuit protection, but a phantom supply is unlikely to be shorted anyway, so this is not a limitation. Since all phantom powered systems need some operating voltage (a typical value being around 10V), the maximum practical current drawn by each powered circuit is around 11mA. The lowest recommended phantom supply voltage is 30V, and I consider even that to be too low for most things.
Although the circuit looks complex, it is very easy to build (especially if the PCB is used). Line (input) regulation is also quite good, with less than 200mV output change with +20% and -20% input voltage, with a load of 100mA. Unfortunately, if that is applied to the input of the circuit as shown the supply will fail, because the total voltage will be around 140V.
The external bridge can be made easily on a small piece of Veroboard, or use a 1A bridge assembly if you prefer. If your multimeter refuses to measure to the number of digits needed, see the appendix for an alternative method you can use. Naturally, if the lowest frequency you need is higher, then lower capacitance is acceptable. This is usually alright, provided the instantaneous voltage across the caps never exceeds 1V. The cable is effectively a capacitor, and the sudden discharge of the cable and coupling caps can create a high current through the zeners, which must be capable of withstanding the surge without failure.
Where there is just no space at all, an external box can house the regulator and distribution board(s).

A standard 50V ceramic is recommended for C7, which is used to ensure that the regulator does not oscillate. The supply circuit uses the full 30V AC in a floating configuration, and connection to another supply or rectifier will cause a short on the winding. In this case, R1 and R2 in Figure 1 may be increased in value, and this will provide even better filtering.
This is a shame (I think), because the old methods actually taught you something as you used them - not the case with any digital instrument. Be warned - the amp input signal will be 6V RMS (for a 12V input) with the DUT (Device Under Test) removed, unless D1 and D2 are installed as shown. Press the TEST button only when the DUT is securely attached, or the output will be very loud indeed. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference while constructing the project. The resistor tolerances will mean that the voltage will be not exact even when using 1% types. Make absolutely sure that you get the polarity right - the filter caps and regulator will be destroyed if you get it wrong! Each pair of resistors should be matched to within 10 ohms (or less if possible) of each other for best results. Likewise, if the mic preamp input impedance is higher than 1.2k, less capacitance can also be used.
Fortunately, 1W zeners are rugged enough to take it, and this is almost an industry standard circuit. Not shown (or needed) are film caps in parallel with the electrolytics - if you wanted to, these may be added, but with the filtering shown high frequency noise should be non-existent. If the worst comes to the worst, you can use a pair of single winding 15V transformers, with the windings in series to give 30V.
One by one, connect the remaining resistors into the circuit, and aim for an output signal of less than 4mV - do not re-adjust VR1.
Problem is, I'm using dual 22V toroid transformer and the most it can do is 47VDC unregulated. That is why all of our kits have 25 turn trim pots as it is the only way to trim the voltages exactly to the right voltages. The voltage multiplier supply will be struggling to supply even this meagre current, and will have high battery drain as well. If you allow for a maximum of 10 phantom powered mics each drawing a typical current of around 10mA, that's still only 100mA so it's hardly a limitation.
I recommend a conventional 'EI' transformer if possible, as these have less capacitance between primary and secondary, and will allow less HF mains noise through. Hopefully, you will find a number of resistors that are close to the reference you chose - if not, simply choose a different resistor to use as the reference, and re-balance the bridge. This will keep the maximum level down to something more sensible if the Test button is pressed when nothing is attached to the DUT terminals. 10V zeners are specified because it is virtually impossible for any microphone to exceed that level, and the mic preamp will clip well before you achieve the 7V RMS input voltage limit imposed by the zeners.
Repeat these tests until you find as many resistor pairs as you need - each pair will now be very closely matched if you did the test and selection properly. If you choose to use polarised electrolytic caps for C1 and C2, the positive end must face the mic inputs (connected to the 6.8k resistors).
In addition, 10V zeners have a higher current capacity than higher voltages for an additional safety margin.
For the next 2,000 years, anybody making wheeled vehicles ended up making them the same size, to fit in the Roman ruts. The 10 ohm series resistors will have little or no influence on input level or noise, and help to limit the peak zener current. Once a few railroads got started (on Roman gauge), all other RRs had to use the same gauge, so they could inter-operate well. It is also a standard voltage for stacks of lead-acid batteries, the standard way to get clean big DC before good electrolytics.
Without the cap, a common DC meter will read the average voltage of a varying DC wave, which in this case should be 40V (though 47V no-load is reasonable). WITH the cap you get a steady DC voltage nearly equal to the peak of the AC wave, which computes to around 62V (as Joe says).

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