Microphone Impedence

[Bill Turner]

On Sun, 25 Jan 1998 09:55:22 -0800, Adam McLaughlin <kd6poc at JPS.NET>
wrote:

>After reading our recent discussion on microphone impedences, I have a
>question regarding gain and Hi-Z mikes.
>
>I have an SBE-34 that runs full output power when tuning it up, but when
>you speak into the microphone and talk normally the power output is way
>down from max, even when the mike gain is all of the way up.
>
>The manual for the SBE says that it wants a medium to high impedence mike.
>The manual refers to a shure model 90B microphone as an example. I am using
>an old Yaesu mike that is marked as having 500 ohms impedence.
>
>Is this high, medium or low? Do you think that changing the microphone
>would give me all of the talk power that I should have?
>
>As a dedicated CW guy, SSB is new to me and I am unfamiliar with the
>operating conditions of SSB radios.
>
>Thanks for the help,
>
>Adam
>Adam McLaughlin KD6POC
_______________________________________________________________
500 ohms is considered low impedance, and that's what is causing your
low volume problem.  There are two ways of solving it:

1.  Install a low-to-high impedance transformer.
2.  Make or buy a preamplifier.

Either way will work fine, but if you go the preamplifier route, be
wary of RFI.  Preamps are notoriously sensitive to picking up some of
your transmitted RF and causing either feedback or distortion or both.
Shield the preamp well and use bypass capacitors on the input and
output, and if you really want to be sure, install ferrite chokes on
the wires going in and out.

73, Bill W7TI

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