[Boatanchors] Mic. Hi, Low Z

[Dave Brown]

George
Its probably safe to assume that even if the mic is high-Z,  a scope 
or VTVM input impedance will be high enough not to load it 
significantly.
So, feed the mic output into a scope(or VTVM) with a steady level from 
a tone source (WWV ex rx speaker etc) into the mic.
Put a pot or variable R across the scope(or VTVM) input and reduce its 
resistance till you see about half the voltage you started with on the 
scope. Start with a 1 or 2 meg pot. This should be OK for hi-z mics 
but you will need to use a 10k or lower if the mic is lo-Z.

At the 'half voltage' point, the scope/VTVM input resistance, in 
parallel with the added variable, is the mic output impedance---at the 
test frequency. It should be pretty much the same over the usual voice 
range if you pick a mid band tone to start with- say 1200 Hz etc.
For lo-Z mics, the scope/VTVM input resistance can be ignored-- it 
will be so high compared to the added shunt R that you can just 
measure the added shunt R. Hi-z mics may require the correction as 
noted above (ie  figure the parallel combo of the scope/VTVM input R 
and the added R)
 73
 Dave,
ZL3FJ

----- Original Message ----- 
From: "George KB2Z" <Thermionic_Emission at earthlink.net>
To: "boatanchors-mailman.qth.net" <>
Sent: Thursday, October 28, 2004 11:48 PM
Subject: [Boatanchors] Mic. Hi, Low Z


> Is there any way to find if an unknown mic. is high or low 
> impedance? I
> have a few unidentifiable mics that I would like to put into service 
> but
> the impedances are unknown. The actual values are unimportant as 
> long as
> they could be labeled as high or low z.
> Thanks in advance, George KB2Z
>


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