[heathkit] need a cheap hi-Z microphone

[Garey Barrell <k4oah@mindspring.com>]

John -

The high impedance microphone is a high voltage "generator" that expects 
to see a high impedance LOAD.

The low impedance microphone. is a low voltage generator that expects to 
see a low impedance LOAD.

High impedance mics are typically piezo-electric crystals.   D-104 is 
Rochelle salts crystal, which is susceptible to heat and humidity.  
Later technology was ceramic, much sturdier, but some claim less 
"accurate".  The Shure 444 is one of these.  Same technology used in 
phono cartridges of the time, mechanical force applied to a crystal or 
ceramic element generates electric current.  Output is measured in Volts 
into high impedance loads, and frequency response is affected 
considerably by load impedance. 

Low impedance mics are typically a "speaker in reverse", i.e., a coil 
moving through a magnetic field to generate a small voltage in the 
millivolt range.  Much more rugged, both mechanically and 
environmentally.  Sometimes used with a step-up transformer to generate 
higher voltages.  The Astatic 10D microphone is one of these.

73, Garey - K4OAH
Glen Allen, VA

Drake 2-B, 4-B & C-Line Service Supplement CDs
<www.k4oah.com>



JJJUREK wrote:
> Hi Randy,
>  
> You wrote:  "/Can you let me know the impedance you are looking for so 
> that I am not miss leading you here./"
> I wish you hadn't brought this up; i'm starting to feel ignorant.
>  
> I consulted the SB-401 manual, and there is no information in the 
> specs on actual microphone impedance.  All I found was the following:
> "It is recommended that a high impedance microphone that is equipped 
> with a push-to-talk switch be used with this transmitter, so advantage 
> may be taken of both the PTT and VOX operation."
>  
> Looking at the circuit and trying to remember my knowledge of vacuum 
> tube amplifiers, it still does not seem obvious to me why a /High/ 
> impedance is required or recommended.  Yes, the input of the speech 
> amplifier is high impedance to audio frequencies (around 1 megohm) and 
> impedance matching issues come to mind, but this is not a max power 
> transfer situation.  I would think we just want the highest 
> signal /voltage/ possible supplied to the grid, regardless of current, 
> which I presume is very low.  Wouldn't you think it would call for a 
> /low /microphone impedance for that?
>  
> Also, I don't see how the impedance of the microphone would affect any 
> dc bias-ing because it is capacitively decoupled.  The vox circuitry 
> is down-stream from the microphone,  and has no interest in microphone 
> impedance, I would think.
>  
> A few weeks ago I went to a local amateur radio supply store, and the 
> guy said he didn't have a single microphone in the house he could sell 
> me, as they were all /low /impedance, and recommended I look online 
> for a used Shure 444 or Astatic something.  I suspect that if you used 
> your D-104 on an SB-103 which I presume is tube, then it will work for 
> me on the SB-401.
>  
> Yes I have paypal.  Give me the postage costs etc. and I'll drop the 
> dough.  I presume this is 35.00 US $?
>  
> Regards,
> John  KC9MJO
>  
> P.S.
> Can anyone out there technically nail down this high-impedance 
> mic issue, just to satisfy my need to know?
>  
>