[ProAudio] Microphones question

Bob Katz bobkatz at digido.com
Mon Jun 14 07:45:33 PDT 2021

David: It might also be useful for manufacturers to specify an optimum 
load impedance. I heard (anecdotally) that some condensor microphones 
perform better with a 10k load instead of a 1500 or 2k load.


On 6/12/21 1:28 AM, David Josephson via ProAudio wrote:
> Dan,
> 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.
> David Josephson
>> On Jun 11, 2021, at 7:49 PM, Dan Lavry via ProAudio 
>> <proaudio at bach.pgm.com <mailto: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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DIGITAL DOMAIN MASTERING STUDIO Author: *Mastering Audio* Digital Domain 
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