Phantom Power Supply
A simple-to-build and inexpensive design capable of powering up to 200 microphones!
A simple to build, high quality 24V phantom power supply for use with capacitor microphones.
With the introduction of budget capacitor microphones such as the excellent CM65 range by Calrec, it is likely that many of these will find applications in home studios and as yet, many otherwise exemplary budget mixers provide no onboard phantom powering facility.
As a proprietary phantom supply may be too expensive to consider if you only need to run one or two mics, this flexible design from HSR enables you to build a suitable alternative for around £20 that will provide sufficient phantom power for up to two hundred microphones simultaneously - so you probably won't need to build more than one.
The whole idea of a phantom power source is that it can be fed along the mic signal leads to avoid unnecessary wiring and this is accomplished by feeding the same voltage to the hot (+ve) and cold (-ve) lines via balancing resistors. To prevent this voltage from reaching the mixer, isolating capacitors are added and, in this case, these are built into the power supply beween the mic input and mic output sockets.
In theory, as the power supply is fed equally to both the hot and cold lines, any power supply noise or hum will cancel out, but this is a dangerous assumption. Any imbalance in the system will allow noise into the mixer, and this can occur in at least two places.
Firstly, the tolerance in the balance resistors will cause some imbalance and secondly, the common mode rejection of the mixer input stage will certainly be less than perfect.
These shortcomings, coupled to the fact that a microphone output is electrically quite small, mean that care must be taken to make the power supply as free from hum and noise as is possible.
The raw DC supply is derived from a 12-0-12V toroidal transformer via a full wave bridge rectifier giving around 35 volts before regulation. This voltage is highly smoothed by means of C1 and C2 before being regulated by the Darlington pair comprising TR1 and TR2.
A stable voltage derived from a 24 volt Zener diode is smoothed by R3, C3 and C4 and is presented to the base of TR2, finally appearing at the emitter of TR1, less the base emitter drop of the two transistors. This means that for a 24V Zener the final output will be nearer 22.5V but as most Zeners have a ten percent tolerance, this may vary a little but to no consequence.
The output from the regulator is again smoothed by capacitors C5 and C6 to remove any residual ripple and noise that may have crept through the system before being connected to the 6K8* feed resistors, R7 and R8. * See note in the XLR Wiring Connections diagram - the proper values for R7 and R8 should be 1k2, not 6k8. The resistors should also be matched within 0.1% to avoid degrading the common-mode rejection ratio and compromising the interference rejection properties of the balanced interface.
The easiest way to build this unit is to use a diecast box such as those sold by Maplin. The exact size will depend on how many mics you wish to connect as there must be room for all the XLR sockets, one pair per mic.
A toroidal transformer is used to minimise induced hum and this is fixed by a single central bolt to the case, preferably as far away from the XLR sockets as possible. Fit a switch and fuse holder, again positioned away from the mic sockets and sleeve all exposed mains connections for safety.
Veroboard is the most economical way to build the circuitry and, as it is so simple, any convenient layout may be used. The Maplin boxes have moulded guides for PCBs so a carefully cut piece of Veroboard can be dropped straight in.
As the power consumption of the regulator is small, the 2N3055 doesn't require a heat sink (unless you intend to use all 200 mics!) and so the plastic encapsulated version may be used and wired directly into the board. Likewise, single-ended electrolytics are recommended in order to save space on the board.
The two resistors and two capacitors associated with each pair of sockets may be mounted directly to the sockets and one pair will be needed for each microphone used.
Constructionally, there is nothing difficult, as diecast boxes can be drilled very easily but as with all equipment containing mains voltages, take great care, our readers are too valuable to lose!
The zero volt rail of the circuitry is connected to the case via a 1K ½watt resistor and the case must be earthed. This measure provides a ground lift to minimise the probability of ground loops, a full explanation of which was given in last month's HSR. It is also a good move to use an LED run via a 2K2 resistor from the output of the rectifier to indicate 'power on' status.
Make sure that the bunker door is tightly sealed and that all the sandbags are in place. Shout "firing" and switch on!
If all is well and you haven't forgotten the fuse, the unit should show no signs of excessive heat, smoke or sparks (and the LED should come on). With a voltmeter, check that the voltage between pins 3 and 1, and then 2 and 1 on the mic input sockets read around 24 volts. When satisfied that all is well, try inserting a capacitor microphone.
It is not normally a problem if a balanced dynamic mic is connected to a phantom PSU as both ends of the coil will be at the same potential, but it is unwise to plug in a mic that is wired for unbalanced use as DC current will flow through the coil forcing it to one extreme or the other, thus causing poor sound and possibly damage.
Feature by Paul White
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