Realistic PZM Modification
How to turn this inexpensive PZM into a balanced line microphone
Despite its extraordinarily low price the Realistic 'PZM' is capable of giving excellent results under the right conditions. Although it is obviously a product intended for 'domestic' use as evidenced by the unbalanced output and rather poor screening, the quality of its audio performance makes it well worthwhile undertaking this modification after which the mic can be used with confidence in any situation.
Professional mics almost invariably have balanced outputs using three conductors, 'live', 'return' and a separate 'ground' or 'screen'. Any induced noise current caused by an external field that gets through the screen will be at the same level and flowing in the same direction in both the 'live' and 'return' lines. At the balanced mic input there is usually a transformer and the induced currents will flow in opposite directions through the primary winding and will cancel. Unbalanced mics use only two conductors with the screen also acting as the 'return'. Any interference strong enough to get through the screen will impose itself on the signal and be amplified at the input.
Although the Realistic 'PZM' is an unbalanced system, the power supply also houses an output transformer and the output lead provided, although terminated in a mono ¼" jack, is already a two conductor and screen cable, making it a simple operation to convert this stage to a balanced output.
Simply cut off the moulded jack plug and fit a standard three pin male XLR connector using the red wire as Pin 2 ('live'), the black wire as Pin 3 ('return') and the screen as Pin 1 (ground). If for any reason you need to reverse the phase of the mic then just swop the positions of the red and black wires.
The Realistic 'PZM' is now compatible with any balanced mic input. To use it with unbalanced equipment, it is necessary to use an XLR - jack adaptor which has a connection between Pin 3 ('return') and Pin 1 (ground), or better still use one of the XLR - jack adaptors which contains a built in mic transformer and so retains the benefits of balanced line operation.
The modifications so far, however, has only dealt with the output after the power supply but the length of cable between this unit and the mic itself is also susceptible to interference. A simple solution is to replace this section with a much shorter length of better screened cable. For maximum immunity to interference the power supply, containing the balancing transformer, should be wired as close as possible to the mic. Its plastic casing is easily dismantled by removing the four screws in the base. The underside of the small PCB, to which the cable connects, is then accessed by removing the single retaining screw adjacent to the output transformer.
You will need to choose a replacement wire that has excellent screening properties, is physically robust, and yet which is thin and flexible enough to pass through the existing cable grommets and be knotted inside for strain relief. Foil screened cable is excellent for this purpose but rather difficult to obtain in small quantities, a tightly braided screen is preferable to one that is spirally wrapped. Looking at the underside of the PCB the screen can easily be soldered to the point where the previous screen connection was made, while the 'live' conductor can either be wired directly to its PCB track or, if you don't have the necessary tools for such a tight soldering job, then the existing wire can be cut leaving an inch of the 'live' conductor still connected to the PCB. The new wire is then easily soldered to this free end and the join protected with cable sleeving (preferably heat-shrinkable).
The casing of the power supply is made of plastic and offers no shielding at all so before reassembly the screening can be improved with the use of some conductive paint or self-adhesive copper foil (a most useful material, often obtainable from electric guitar shops). Remember to earth the extra screening and to use an insulating layer where the PCB comes close to the casing.
Disassembly of the microphone body to re-wire the other end of the new cable is equally simple. Just two screws pass through the boundary plate into the capsule housing, with the plate removed the tiny head-amp PCB can be seen. Removal of this board for re-wiring is a very delicate operation but need not be attempted if you don't feel confident. Instead, simply again cut the existing wire leaving an inch or so still connected. Pull off all the outer sheathing and separate the screen from the inner conductor. Twist the screen into a solid wire and place about half an inch of cable sleeving over each conductor. Pass the new cable through the grommet, knotting it for strain relief, and then solder the 'live' conductor to the short white wire, and the screen to the free end of the old screen. Finally protect the solder joints with the sleeving, being careful to cover all the exposed wire on the 'live' conductor. Re-assembly of the mic is made simple by a raised point on the boundary plate which locates into the base of the capsule housing.
With the power supply now connected close to the mic some extra thought has to be given to placement when in use. The plastic casing could easily be crushed so the floor is probably best avoided, but taping to a wall or the underside of a piano lid is no problem as, even with an XLR attached to it, the power supply is still a very lightweight unit.
So, for just the cost of the XLR connector and a short piece of high quality screened cable, a few minutes work with a soldering iron will make the Realistic 'PZM' as fully compatible with professional equipment as its audio performance suggests it should be.
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Feature by Dave Lockwood
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