Powering Microphones V
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Powering Microphones Powering Microphones By Chris Woolf Simple dynamic microphones do not need any multi-rail supplies. There are a few examples where powering - they generate electrical energy from a the power requirement has been reduced enough for relatively low impedance at a level that is adequate, if a 48V phantom supply to be adequate but they are a not generous, to feed a preamplifier at some rarity. distance down a cable. And lastly there is the newcomer - the digital microphone - with a requirement for not only Capacitor (popularly, though inaccurately, called powering of the analogue elements that it “condenser”) microphone transducers do not inevitably has but internal clocks, converters and generate energy but modify an electrical parameter, digital control circuits. These do not come and have an extremely high output impedance which cheaply in terms of power - 2W is not makes them impossible to connect directly to uncommon. anything – even a short length of cable. This impedance needs to be reduced to a much lower Internal batteries value that will allow long microphone cables to be For the lowest power designs internal batteries are driven efficiently and will help to exclude an option – and can be very useful when interference. This impedance conversion, or sophisticated powering systems are not available. matching, is carried out by an amplifier immediately With rare exceptions batteries are only used for behind the transducer – and this circuitry needs to electret designs where the current drain is just the be powered in some way. impedance converter. A single AA cell is often the choice since it can fit into a slim body barrel. The Non-electret capacitor microphones (sometimes limitations of such systems are the need to switch even more inaccurately called “true condensers”) the battery on and off (though some designs expect also need additional power for polarising the the user to simply remove it), per haps check its capsule. RF designs use, instead, a small oscillator capacity with some sort of LED indicator, and the and demodulator in order to avoid some of the fact that the meagre power available does not problems associated with high impedance DC allow large output voltage swings or very high cable circuits but do not escape the need for power. driving ability. The field is even further complicated by the introduction of a few dynamic (including ribbon) Plug-in Power microphones that have been allied to buffer For microphones that are designed primarily for amplifiers to give greater cable driving ability or a the domestic market and have an unbalanced out- more consistent source impedance, though some put connection the system known as “plug-in would argue that in doing so they risk sacrificing the power” provides a comparable supply extreme Sound Pressure Level performance that configuration – perhaps 10-20mW at <5V. In this most dynamics inherently possess. case the feed is from the device that the microphone connects to – usually a recorder of The total power requirement for a microphone is some form, though computers and mobile phones commonly of the order of 100-200mW though it use a similar arrangement. The problems of might be as low as 50µW for some electrets and battery switching and condition no longer apply nearer 500mW for some high output designs. but the restrictions on output and cable driving Microphones using valves (tubes, in US parlance) are are unchanged. something of a special case since their power- hungry circuitry normally requires purpose-made Figure 1 shows a typical minidisk or video-camera 3.5mm stereo jack input with powering circuitry. +2.5V L R 2k2 3.5mm jack 2k2 + L preamp + Microphone R preamp Input Figure 1 - Typical Plug-in Power configuration 1 Powering Microphones Valve (tube) microphones T-Power With the exception of the unusual phantom- For the vast majority of semiconductor-buffered powered devices valve (tube) microphones are professional microphones lower voltages and associated with a proprietary power supply unit, powers are considered adequate and two which connects to the microphone via a multiway powering configurations cover virtually all of cable. By definition thermionic amplifiers need a these – the rather rare T-power, and the almost heater supply and also a voltage of the order of ubiquitous phantom. 50-100V (sometimes greater) between the cathode and the anode (or plate). These supplies T-power (alias Tonader, Tonaderspeisung, A-B are fed on cores that are separate from the powering, or parallel powering) is nearly obsolete audio. but still finds some adherents in the film and TV industry. It is an incompatible system, in that Capsule polarisation may be from the tube supply microphone lines using T-power must not be rail or a separate one. On many designs a dual cap- used for other microphones for fear of damaging sule is used and by altering the polarising voltage them. on each diaphragm the directivity can be changed. If the control for this is remoted to the power unit At the time of writing the only T-power micro- then still more cores are needed. Hence tube de- phones marketed are the Sennheiser MKH416T signs commonly use cables and connectors with 7 and the Schoeps CMC4U preamplifier for the or more “ways”. This has the merit of simplicity Colette range of capsules but a considerable even if it is mechanically and electrically somewhat number of older Sennheiser designs are still in heavyweight. More sophisticated arrangements everyday use and other companies have are technically possible but examples are few and produced T- power variants in the past. far between. Many users of T-power claim that it is more High voltage microphones robust than phantom. This claim is a difficult one A few semiconductor microphones also use a pro- to prove since some of the alleged benefits are prietary power supply to generate clean rails as connected with the very low output impedance of high as 120V . Semiconductors have an innate ad- T- power microphones in common use – vantage of being able to run from voltages that are something that is not exclusively their far lower than those that can be used with prerogative. It is also possible to operate T-power thermionic devices. Microphone amplifiers that with poor cables that have an intermittent or non- operate on 1V can be designed and 12-48V is the existent screen connection. However it might be most usual range for supply voltages (though inter- better to be aware nal generation of higher polarising voltages may be of such defects rather than oblivious of them. used). Parallel powering is covered by section 7.5 of the However the design of a capsule that can work with IEC 61938 standards. Figure 2 shows the usual extreme SPLs and an amplifier that can de- liver circuit. It exists only in a 12V form, with the +12V very high levels of audio is eased if high voltage rails being applied to the “positive” phase leg (XLR are available. Thus a few highly-respected pin2, Tuchel pin1). However some microphones microphone designs have been introduced that have been made with reversed polarity for use trade these advantages for the inconvenience of with “positive” earth Nagra recorders. non- standard connection systems. >100µF +ve phase 2(1) + 180R XLR connector (Tuchel or DIN + connector numbering 12V shown in brackets) - 10k 180R 3(3) Microphone Cable Powering Input 1(2) Figure 2 - T-Power or parallel powering configuration 2 Powering Microphones The 12V may be floating with respect to ground, contributed to its limited appeal. The supply which allows for very simple connection to voltage is superimposed on the signal voltage and unbalanced systems. However the standard also therefore needs to be exceptionally well filtered. In allows for one leg to be grounded. This may be multiple microphone systems the mutual more convenient in terms of symmetry for the decoupling - the isolation that pre- vents the leaking audio signal. Figure 3 shows a variant of the of audio signals via the common supply rail - must normal circuit that permits unbalanced similarly be of a high order. connection with a grounded supply. Historically many T-power location microphones It should be noted that signal and powering polarity were powered from portable regulated PSUs are unavoidably linked so signal phase can only be driven by a pair of miniature 9V batteries. since changed by switching beyond the power section. mixers and recorders frequently did not have the necessary powering circuits. Many of these de- Besides the dangers of accidentally connecting vices also usefully provided line and LF attenuation microphone lines with T-power to dynamic, and - functions that must be provided outside the particularly ribbon, microphones there are a powering loop. number of other disadvantages that have >100µF +ve phase 2(1) + 360R XLR connector (Tuchel or DIN + connector numbering 12V shown in brackets) - 10k 3(3) Microphone Cable Powering Input 1(2) Figure 3 - Unbalanced T-Power or parallel powering configuration Phantom Power The phantom power system has become by far Thus the filtering and decoupling requirements for the most common microphone powering method power supplies is significantly less critical and the and is an elegant and compatible arrangement. It risk of crosstalk between microphones on the same takes its name from an ingenious technique used power rail is greatly reduced. by telegraph engineers who realised they could treat a standard balanced pair as a single The standard for phantom power is IEC 61938 conductor and, by using an earth return, create an clause 7.4 and though the 48V version is the most additional circuit without any extra wires. The common a 12V variant is available and has some phantom circuit so formed had no effect on the advantages where economy of power is at a balanced pair since an identical phantom signal premium, as in portable equipment.