[ProAudio] Microphones question
dlj at josephson.com
Fri Jun 11 22:28:45 PDT 2021
There is a wide variety of microphone electrical topologies, each with distinctly different impedance characteristics, even within the “balanced” microphone world. There is an effort underway in AESSC led by Anthony Kuzub of CBC to get a little more clarity but I think you know about that.
You asked about the “real” i.e. commonly found, impedance levels. The most common topology these days is the so-called “impedance balanced” circuit where the microphone feeds audio to either pin 2 or pin 3, depending on whether a polarity inversion is required. It is almost always through an electrolytic cap and a series resistor of 20 to 50 ohms. The other audio output pin has the same R and C, but to signal return. Phantom power is usually drawn from both signal leads. With ideal cables, the CMRR of the preamp produces the same interference rejection as if the signal were symmetrical. As it is only the impedance is symmetrical. Also relatively common is a pair of pnp emitter followers directly connected to pins 2 and 3, with the collectors returned to ground through a network held above ground by a few volts, to provide a bit of voltage for the input stage of the mic. These also typically have a few ohms of resistance, either resistors or RFI-suppression chokes in series with each output leg, typical impedance is on the order of 40 to 100 ohms. Transformer output mics are less common as electret condenser mics have become the defacto standard; they are cheaper to make than dynamic elements.
We have been over this ground dozens of times with the microphone manufacturers. The best consensus we can reach is to recommend that the manufacturer specify the minimum load impedance, which is typically around 1500 ohms.
> On Jun 11, 2021, at 7:49 PM, Dan Lavry via ProAudio <proaudio at bach.pgm.com> wrote:
> Hi Bill,
> I am not trying to change the standard. Certainly not before I have the understanding of what is going on. Of course in an ideal world, each mic would come with an impedance information. In a somewhat less ideal world, each mic would come with, at least, the best resistor value. But here we are stuck to the "typical" old dynamic mic.
> I know that most people don't understand technical details, and it will confuse customers. But I am not talking to customers here, or level playing field. I asked the opinion of mic experts. I want to know what real impedance levels are. That is really all I want to know. So far, I learned about the 150 Ohm relation to the old dynamic. There must be more information, I thought I would start here.
> Thanks for all the comments
> Dan Lavry
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