Phantom Powering the Realistic PZM
Our final modification to this inexpensive 'pressure zone microphone' makes the unit suitable for more professional applications.
At £19.95, the Realistic PZM microphone represents fantastic value for money, although not directly suited to professional use.
However, with the conversion to 'balanced line' operation outlined in our March issue, and the modifications published below for 'phantom powering', the Realistic device becomes an even more appealing purchase.
The Realistic 'PZM' (Pressure Zone Microphone) offers an outstanding performance for such a low-priced unit, and makes available the attributes of the pressure zone mic, at a fraction of the original cost. The microphone closely resembles the original Crown product, both in appearance and in its characteristic sound; the principal differences being a less robust construction and an unbalanced ¼" jack output, with battery rather than phantom powering.
The quality of the audio performance and the remarkably low price make it well worthwhile undertaking the simple modification detailed in the March issue of HSR, which converts the Realistic mic to balanced operation. The modified 'PZM' exhibits significantly improved immunity to interference, however, many users may dislike the idea of still having to rely on batteries to operate the mic, but this can be solved by a further modification which facilitates operation on phantom power.
The technique known as phantom powering uses the microphone cable to supply the DC voltage required by a condenser mic, both to power its built-in preamplifier and to apply a polarising voltage to the diaphragm assembly. The positive side of the DC supply is applied equally to both signal wires of the balanced line, either through a transformer centre-tap, or by means of identical series resistors, while the negative side of the supply is connected to the screen.
As both signal wires are at the same potential, microphone circuits that are powered in this way can be used safely with both condenser and dynamic mics. Although many condenser mics are tolerant of supply voltages down to 9V, some of the highest quality types will only operate on a 48V supply, to ensure maximum headroom and dynamic range. In order to cater for all types, most mixers with built-in phantom supplies operate at 48V.
The Realistic 'PZM' uses a permanently charged or 'electret' capsule which requires no polarising voltage, power only being needed for the operation of the mic's internal preamp. A single AA size 1-5V battery is normally used, although the user is offered the option of utilising a 12V supply, obtained from two small 6V batteries which fit into the same battery compartment. The larger supply gives a significant improvement in headroom and output, and should certainly be used for applications where wide dynamic range signals are likely to be encountered.
In order to run on phantom power, the Realistic 'PZM' must first have been converted to balanced operation, as described in the previous article (March HSR), so the procedure for the disassembly of the power unit should already be familiar. However, before the additional components can be inserted to drop the 48V phantom down to the required level, some alterations must first be made to the existing system.
The 'PZM' circuit must be changed over from its original positive-earth operation into a negative-earth system. This is necessary because the phantom supply is a positive voltage and it is easier and cheaper to alter the microphone than it is to invert the supply at its destination.
Having removed the single PCB from the power unit, it is necessary to gain access to the component side of the board. The metal cover is very firmly soldered down at both ends, but fortunately the sides can be prized open and bent upwards without too much difficulty. You may observe that the components do not correspond exactly to the schematic diagram in the Owner's Manual, but the differences do not affect the modification that needs to be made in this region.
The 10uF polarised capacitor, the largest of the components on the board, must be carefully de-soldered and re-fixed with its connections reversed; a suction-operated desoldering tool can be valuable here, as it will clean out the PCB holes and so enable the component to be firmly re-located on the board in its new orientation, thus making resoldering much easier.
Having checked that the solder joints are clean and not contacting any of the adjacent tracks, the cover can be bent back into shape, taking care not to deform its top surface to the point where it interferes with the action of the switch. Next, the two battery leads should be swopped over at the terminals; at present the red lead comes from the negative terminal and the black lead from the positive contact; when reversed, the colouring in fact seems rather more logical.
An alteration must then be made to the capsule itself, necessitating the removal of the capsule housing assembly from the boundary plate. Viewing the underside of the assembly, the capsule cover plate can be seen, and its single retaining screw removed to facilitate the extraction of both the capsule and its circuit board. The capsule can then be re-soldered from the board, and the metal cap covering its terminals gently levered off; the cap is only retained by sticky tape which can be re-used when it is replaced.
It will be seen that the black wire's terminal is connected also to the casing of the capsule; this connection must be broken, but as it is too small to be de-soldered easily, it is best to just cut through it with a scalpel or similar sharp instrument. Then, using a strand of fine wire, an equivalent connection should be made between the red wire's terminal and the rim of the casing, the solder joint being made flush with the edge of the capsule so that the cap can then be replaced.
The capsule may then be reconnected to the board, but with the positions of its two wires reversed so that it is the red wire that now connects to the same track as the screen of the output cable. Before restoring the capsule and circuit board to the microphone body, use a test meter to check for continuity between the capsule casing and the negative terminal of the battery compartment, and also ascertain that the black wire's previous connection to the casing has been properly broken.
The 'PZM' circuit has now been altered to negative-earth operation, and it is possible at this stage to check that the preceding steps have been completed successfully. Without reassembling the power unit, insert a 1-5V battery between the terminals, and check that the mic produces an output as normal; if it does not, remove the battery and retrace the previous stages, examining each alteration until the fault is found.
With the unit established as working correctly, it is possible to then add the extra phantom supply components. The PZM's transformer does not have a centre-tap facility, but the supply can be picked up from both signal wires using 22kohm series resistors. If the component leads are accurately cut to length and pre-formed to shape, it is possible to neatly solder the resistors to the same points on the underside of the PCB as the Red and Black signal output wires, but with the components able to stand upright, just clear of the end of the circuit board and, thus facilitating easy refitting into the power unit.
The longer connection needed to reach over to the black wire's soldering point must be insulated with cable sleeving as it lies across several other joints. The free ends of the resistors should then both be attached to an insulated wire which is connected to the positive battery terminal, located at the opposite end of the power unit. Don't bother to try to make this easier and neater by physically swopping the positions of the terminals, because the mountings are slightly different and they won't fit.
Finally, a 3-3kohm resistor should be connected across the supply terminals; if the component leads are accurately pre-shaped once again, the resistor can be made to lie neatly along the edge of the compartment, still leaving enough space for the insertion of a battery, should this be necessary.
The circuit modifications and additions have now been completed, but before reassembly the actual operating voltage should be checked. Connect a test meter across the battery compartment terminals, measuring DC up to 50 volts, then, with the switch on the power unit set to OFF, connect the phantom supply. You should be able to measure between +10 and +12 volts. If the figure is out of this range, disconnect the supply and check the wiring of the three added resistors; the fault must lie in this area if the mic was established as working correctly after the previous alterations. If the correct supply voltage is indicated, the power unit can be reassembled, and the microphone's audio output then checked with phantom powering in operation. You may notice that the 'PZM' now produces a higher output level, which enables lower input gain settings to be used, resulting in a significant reduction in the microphone's output noise. The added components will have no adverse effect on normal battery operation, but when phantom power is in use, batteries must be removed in order to avoid damaging them.
Phantom supplies of less than 48V are very occasionally encountered, but the modified 'PZM' can still be used satisfactorily with these, although it will receive a proportionally lower actual supply voltage, ie. 24V phantom would operate the mic on about 5-5V. If you intend to regularly use the 'PZM' with a lower voltage phantom source, it is worth considering a different value of R3, which would still enable the optimum supply level to be achieved.
Phantom Voltage R1 R2 R3 (value to achieve 10-12V supply)
This modification costs very little, but adds significantly to the performance and convenience of use of the mic. The Realistic 'PZM' only performs at its best with its higher voltage supply, but when you consider that a pair of the necessary six volt batteries can cost over a third of the price of the microphone itself, and that they degrade fairly rapidly in use, some owners may be understandably reluctant to use them; with phantom powering in use, consistent optimum performance can be relied upon.
This microphone represents remarkable Value for money and makes the unique qualities of the pressure zone principle mic available to all recordists. When modified for balanced operation and phantom powering, the Realistic 'PZM' must even be considered as a serious rival to its professional counterparts.
The Realistic PZM can be obtained through your high street Tandy shop, or alternatively from Turnkey, (Contact Details).
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Feature by Dave Lockwood
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