Electro Music Engineer
Powering Capacitor Microphones
Although it is generally recognised that capacitor microphones provide the best available quality for recording, and PA work, many musicians and sound engineers, particularly those of us working on tight budgets, have been dissuaded from purchasing them because of the need to buy separate power supply units, which add considerably to the initial costs. However, as this article shows, substantial savings can be made by building your own power unit.
In the early days, capacitor microphones used valves in the head amplifier. These were powered through special multi-core cables carrying HT and heater supplies, and the return signal. Early transistorised mics also used this system, with appropriate voltages. Although perfectly workable, this system has the major disadvantage of using special multicore cables and connectors, which limits the flexibility of the system, and made interchange with other types of microphone difficult. Most modern capacitor microphones use a system of 'phantom powering', using the two signal leads of a balanced microphone cable for the positive supply, with the screen forming the return. Figure 1 is a block diagram of a typical capacitor microphone, showing how supply and signal share the same cable. The use of phantom powering means that standard balanced mic cable and connectors can be used throughout the system, and any balanced microphone, whether dynamic or ribbon, can be plugged in and used on a channel carrying phantom power without damage. This is very useful in the musicians environment, as it means that, if all channels have phantom powering, any type of balanced microphone can be used on any channel. However, unbalanced mics cannot be used.
An explanation of the balanced line system appeared in the March 1981 edition of E&MM. The term 'phantom' refers to the latency of the DC voltage: It isn't seen by dynamic microphones.
Figure 2 shows the basic phantom powering system. The power source could be dry batteries — particularly if your mics will function on 9V. However, dry batteries are an expensive way of buying electricity, and are not really justified if you need mains power for other equipment. A well stabilised mains power supply is preferable for this system; obviously, this is what is done in most mixers having built-in phantom power facilities (or you could modify an existing mixer). A main criterion here is that residual ripple cannot be tolerated in the mic supply, and so a very high degree of smoothing will be required. A disadvantage of building an external unit using transformers, as opposed to modifying existing equipment, is the high cost of a good quality component; and unless high-quality transformers, preferably bifilar-wound types, are used, the quality advantage of capacitor microphones will be lost.
The series resistor Rx serves three functions: Firstly, it provides isolation between channels if a common supply is used for more than one microphone. Secondly, they allow the unit to power most common types of microphone. Most capacitor mics fall into two categories, those working on a nominal 48v supply, and those working on anything between 7.5v and 52v. Most mics draw about 0.5mA from the supply. However, one of the most popular types, the AKG C451 series, although functioning on any voltage between 7.5 and 52v, need a series resistor, depending on supply voltage (the values are quoted in AKG's literature), and draw 2-6mA at 48v and 10mA at lower voltages. As the specified resistors only drop 0.17v at 0.5mA they can be included without affecting the operation of other microphone types.
Finally, the resistors provide protection for the power unit in the event of a short circuit on the microphone lines. Cut-outs and fuses are not recommended because their operation would shut down all the microphones running from the supply — not a nice thing in the middle of a concert or live recording!
The circuit diagram (Figure 3) shows an inexpensive unit suitable for home construction. Microphone transformers are avoided by the use of capacitors in the signal lines. A metal cabinet is essential for screening purposes; and the power supply section is built into a separate screened compartment, including a piece of steel plate behind the mains transformer to reduce magnetic field radiation. The power supply incorporates a simple series stabiliser, using transistors in 'Darlington' configuration. Additional smoothing is provided by the use of high value capacitors, especially C14, the effect of which is magnified by the gain of the transistors. C15 ensures that the output impedance of the power unit is low at high frequencies, therefore preventing any possibility of crosstalk between channels. The same power supply circuit could be used with centre-tapped transformers, as in Figure 3.
The use of a toroidally wound mains transformer would be advantageous in reducing the risk of hum pick-up, if the additional cost can be justified. It is often advisable to run capacitor microphones for about half hour prior to use, to ensure that any moisture in the capsule is dried out, as damp causes noisy operation. A small pack of silica gel in the microphone case is also a worthwhile precaution, as is arranging storage in a warm place.
With an ohmmeter, check that the chassis is connected to mains earth. Also check that there is no connection between any XLR pin and chassis. Switch on and check that there is 48V ± 4V at the emitter of Tr2. (±4V is the usual tolerance on mics.) Check that +48V appears on pins 2 and 3 of each female XLR connector with pin 1 at 0V. Switch off and connect a resistor of 1k2, 1W across C15. (This simulates 'worst case' maximum load conditions). The voltage should remain within the range 48V ± 4V. Next, connect a link between pins 1, 2 and 3 of one XLR connector, and check that there is no significant change in output voltage (having first disconnected the test resistor). Finally, leave the unit running 'off load' for 24hrs, to ensure reliability. If a long term full load test is desired, the resistor should be 3W rating. A 1W unit will overheat if left connected for more than a couple of minutes.
Feature by Tony Newnham
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