Dan, many of the topics that go in circles here, and has for years,
concern the relationship between 3 parameters:
1/ Input noise (Measured EIN??)
2/ Microphone output voltage (measured mV/Pa??)
3/ Microphone noise (measured dB(A)??)
I wish that someone with a good enough mathematical skill would make
a 3D diagram describing these relationships, perhaps also a computer
program where you fill in 2 of the parameters and get the third.
A IRL example: If I want to buy a microphone with a self noise of 10
dB(A) and a output of 10mV/Pa - how good (EIN) must the recorder
perform, not to add noise?
I don't think such a formula would be used a lot in such IRL matters,
but I do think that a 3D diagram would have a great value to help
understanding.
(I am well aware of that such a diagram would disregard impedance and
a few other factors, I still think it would have a great teaching value.)
Or is there already such a diagram somewhere? Link?
I have never seen any.
Klas.
At 07:07 2012-12-17, you wrote:
> > Still, I think the "unclarity" resembles the debate decades ago, when
> > there was a lot of talk about impedance matching, preamps and
> > transformers. Some combinations between dynamic mic=B4s, input
> > transformers and transistors worked, others did not, and noone
> > actually knew why, even though "everybody" said they did.
>
>
>The frequency response and output level of a mic will change
>depending on what kind of load it's driving. An understanding of the
>history of audio connections might help to explain current practice.
>
>The original audio craft was telephony. The talker's mic had to
>produce enough power to drive the receiver at the other end, with no
>amplification involved. Experience taught that impedance matching
>carried the maximum power from one point to another. Pro audio
>adopted the practices of telephony. Thus line outputs had 600 ohm
>impedances, and inputs 600 ohms too, "matching" and "terminating."
>
>Very long audio lines, meaning miles, still use terminating
>impedances to prevent the signal from being reflected back. Very
>high frequency lines, like video and digital audio, use terminating
>impedances even on short cables, for the same reason.
>
>Where there was a "bus" that had to drive several loads, the concept
>of "bridging" developed, tapping the voltage off a line without
>loading it. Whereas a terminating input was 600 ohms, a bridging
>input was perhaps 15 Kohms.
>
>A generation later, it was acknowledged that maximum voltage
>transfer was a better goal than maximum power transfer, and all
>connections gradually became bridging. In this practice, it's best
>to have the lowest possible impedance source and a high impedance load.
>
>A practical professional microphone circuit has a source impedance
>of from 50 to 200 ohms, and a load (preamp input impedance) of 2000
>ohms or more. The factor of ten between source and load insures that
>the voltage of the source is minimally reduced by the load, i.e.
>less than 1 dB of loading effect. I dare say all mics sound their
>best when bridged.
>
>PIP mics are a higher impedance source than the balanced
>professional mics. They are typically 2 to 3 K ohms. That means
>ideally input impedances of 20 or 30 K ohms. But there are practical
>limitations to how high the input impedance of a preamp can be
>without increasing noise, and I imagine that they are generally
>lower than that. Perhaps someone has measured the actual input
>impedances of some PIP mic preamp inputs.
>
>-Dan
>
>------------------------------------
>
>"While a picture is worth a thousand words, a
>sound is worth a thousand pictures." R. Murray Schafer via Bernie Krause.
>
>Yahoo! Groups Links
>
>
>
Telinga Microphones, Botarbo,
S-748 96 Tobo, Sweden.
Phone & fax int + 295 310 01
email:
website: www.telinga.com
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