Panthro10, some electronic circuits are purely "passive"--they're made entirely from components that don't require a power supply, such as wires, resistors, capacitors, inductors, diodes and/or transformers. Many useful things can be done with passive circuitry, but the law of conservation of energy says that the power of their output signals must always be less than the power of their input signals; they can't "make up" energy from out of nowhere.
"Active" circuits on the other hand can amplify a signal such that there is greater power in the output than in the signal input. Such circuits require a power supply, of course--the law of conservation of energy still applies--but active components can divert some of the energy from the power supply into their output signals.
Electrical power equals current times voltage. Most amplifier circuits boost the voltage of signals, multiplying it by some factor which is called their "voltage gain" (or in common usage, just "gain"). But sometimes an amplifier is needed because a signal coming from a high-impedance source has to drive a low(er)-impedance load without major losses. That's a rather different scenario; "current gain" is what's required there, while any voltage gain that the circuit may or may not have is more or less incidental under the circumstances. That latter type of circuit is sometimes called a "buffer amplifier" or an "impedance converter," and it matters here because the usual type of condenser microphone needs such a circuit in a big way.
The capsule of a condenser microphone, the ultimate source of its signal energy, is a condenser--a capacitor--which by nature has very high impedance at audio frequencies. A capsule with 40 pF capacitance would have a source impedance of around 40 MΩ (40 million Ohms) at 100 Hz, which is an extremely high impedance in audio terms--practically an open circuit. If you want to preserve a signal that's defined by its voltage, you need to feed it to something that has a distinctly higher load impedance than the source impedance of that signal. But the capsule's output has to drive the input of a preamp, mixer or recorder which is usually in the 1,000 - 2,000 Ohm range, which is much lower than the source impedance in this case.
So some kind of impedance conversion is required--otherwise the signal would nearly all be lost in the connection, and due to the capacitive character of the source impedance, the frequency response of the hookup would be far from flat as well. A condenser microphone's amplifier circuit provides this impedance conversion, generally bringing the output impedance of the complete microphone into the range below 200 Ohms and providing the output impedance with a mainly resistive character (i.e. it's about the same number of Ohms across the audio band, as compared with a condenser which has higher impedance at lower frequencies and lower impedance at higher frequencies).
OK. Now let's say you want to mount the capsule of a condenser microphone at some distance from its amplifier so that the capsule can be placed unobtrusively in a scene that's being filmed for a movie or TV, or so that your microphone arrangement at a concert doesn't block too many people's sight lines, or so that you can suspend just the capsule on the end of a 12' boom whose leverage causes even a few ounces of extra weight to be multiplied by a large factor. Then the problem is, the wiring between the capsule and the input of the amplifier becomes a vulnerable point for interference, distortion and signal losses due to what's called "stray capacitance"--the sometimes baffling tendency for electrical charges to go where they go instead of where we would like them to go.
In the 1960s and 70s, a few companies (Neumann and AKG among them) offered rigid, short- to medium-length, all-metal extension goosenecks for their condenser microphone capsules. That was the best available compromise, which however didn't help most of the applications just described. Then in 1973 Schoeps introduced their "Colette" (CMC) microphone series with a patented design which allowed them to connect active accessories between the capsule and amplifier of a microphone. The amplifiers were arranged to provide powering for these active accessories, which contained the equivalent of the first stage of the amplifier's own circuitry. This approach allowed a capsule to be separated from its amplifier by a 20-meter (~65') active cable with no loss in audio quality, and the relative immunity to interference was excellent for that time period. Schoeps soon had rigid "active extension tubes" and flexible "active goosenecks" available as well.
Neumann responded within a few years with a great little cardioid microphone called the KMF 4, which featured a permanently installed extension cable between the capsule and amplifier. Then they introduced the modular KM 100 series, which has since grown to include seven different capsules, each of which has the "first-stage" (impedance converter) circuitry built into a little cylindrical barrel that is permanently attached behind the capsule proper. And they also introduced a wide selection of extension cables, goosenecks and rigid mounting tubes for that series.
So in the Schoeps approach the accessories are active while the capsules are passive, while in the Neumann approach the capsules are active while the accessories are passive.
I hope that's not too much more than you wanted to know ...
--best regards
Topic: Actives? (Read 3919 times)