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Re: match that impedance

Subject: Re: match that impedance
From: Dan Dugan <>
Date: Fri, 17 Jan 2003 12:50:38 -0800
Oryoki, you wrote,

>My understanding of impedance matching (and the
>physics of electricity in general) is pretty
>poor.  Here's what I think is happening.
>
>By reducing the difference in impedance between
>mic and recorder, the amount of power (energy
>per unit time) received by the recorder is
>increased.  This gives the recorder pre-amp
>a stronger signal, and results in a louder
>recording without requiring the pre-amp to add
>gain.  Since consumer minidisc pre-amps are
>pretty poor, getting a stronger signal is
>a good thing -- up to a point.
>
>The reason a low impedance mic is usually attached
>to a medium impedance recorder input is to
>reduce the likelihood that the mic will over-drive
>the recorder pre-amp, thus distorting the recording.

No, it is normal to connect a mike to an input having 5 to 10 times 
the source impedance of the mike. A 200-ohm mike is typically 
connected to an input of 1000 to 5000 ohms impedance, for example. 
This gets the most level out of the mike, and prevents distortion in 
the mike electronics from driving too low-Z a load. You will hardly 
ever see the manufacturer of a mike preamp state the actual input 
impedance of the preamp, because they know that would confuse people. 
The Sound Devices MP-2, for example, has a 2K input impedance, 
suitable for all mikes from 50 to 600 ohms source impedance.

>So by reducing the difference in impedance, you're
>increasing the chance that the mic signal will be
>too powerful.

Yes, a transformer steps up the mike's output voltage. But it also 
limits the frequency response both top and bottom, and it can create 
distortion at high levels, so it isn't entirely a free lunch. 
Transformer quality counts.

Transformers (unless built into the preamp) also block phantom power 
and plug-in power, so external transformers can only be used with 
dynamic or self-powered mikes.

>In nature recording, the issue most commonly
>encountered is too weak a signal, not too strong.
>That's why we're always trying to get the mic
>closer to the subject.  So the risk of impedance
>matching can be worth the reward.  Just keep
>an eye on the meters on your recorder!

Right on.

>And if you find yourself in a situation where
>the mic signal is too hot (you're unexpectedly
>surrounded by howler monkeys and bellbirds, for
>example), it's a simple matter to unplug the
>impedance matching circuit from the mic input
>and return to recording. 
>
>As I said in my original post, people should
>give the impedance matching cable a try with
>their consumer grade minidisc and see if it helps.
>It works for me.

Agreed. Of course it's better to have the transformer built into the 
preamp, and that's one of the reasons better preamps are more 
expensive--they have expensive input transformers in them.

-Dan Dugan
Who isn't an engineer, but plays one at work.


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