(sigh) Many, perhaps even most, published specifications for microphone "impedance" are given in a rather sloppy way. Very often you'll see something like: "Impedance: 1000 Ohms", for example. A figure that high almost certainly isn't the microphone's own impedance (i.e. its output impedance); instead, it's the minimum load impedance that it needs to "see" (work into) in order to operate correctly.
On the other hand, you sometimes see something such as "Impedance: 150 Ohms"--and when it's that low, it _does_ mean the microphone's output impedance. Then you're supposed to realize that the input impedance of the preamp, mixer or recorder inputs needs to be at least 10 times that amount, whether they tell you that or not. And for professional microphones, the minimum load impedance is 1 kOhm even if the microphone has a very low output impedance such as 50 Ohms. That's because microphone manufacturers design their circuits with that minimum impedance in mind. The output stage of a microphone isn't meant to be a power amplifier.
However, the standard way of specifying output impedance is at a single frequency--generally 1 kHz. With condenser microphones the output impedance will usually be pretty constant throughout at least the midrange; in some models (especially ones without output transformers) the output impedance will be essentially flat across the whole audio range. But with dynamic microphones that is often far less so. Moving coil microphones often have a generally rising output impedance across the frequency spectrum. Ribbon mikes generally have their main resonance at some low frequency, and that resonance is accompanied by a rise in output impedance, sometimes drastically so. A ribbon mike with a 200 Ohm output impedance at 1 kHz may have a 2000 Ohm output impedance at (say) 70 Hz. If you pair such microphones with a preamp, mixer or recorder that has a 2000 Ohm input impedance (and some go below 1 kHz, at least at some frequencies), Ohm's Law says you'll get losses at low frequencies of several dB, which are easily audible.
This type of active cable would prevent such frequency-dependent losses. Since it has no (0 dB) voltage gain, it avoids putting much of a limit on the maximum SPL of the microphone. That's the only questionable aspect of the cable to my mind--maybe you could trade some of that headroom for increased immunity to noise and interference at the preamp input, depending. But if I were still working with ribbon microphones (which I flirted with for a few years), I might still want to try this type of cable instead of, say, a "CloudBuster" type of in-line preamp with 20 dB voltage gain (Sanken and other high-quality manufacturers offer such things).
And yes, there's something to the fact that a dynamic microphone's damping can be affected by the load that it's working into, thus influencing sound quality in other ways to some extent. (The Web site for this product calls "damping" something else, I forget what by now; they could generally use a better editor and/or translator for their site.)
Topic: Umbrella Company Active Mic Cable (Read 73887 times)