[heathkit] More microphone theory belaboring

["JJJUREK" <jjjurek22@att.net>]

Garey,

Yeah, I see what you mean.  It's just like the difference between crystal 
phono and magnetic phono pickups.  The magnetic inputs on an audio amplifier 
always have much higher gain to accommodate the low output magnetic 
cartridges.  So I guess what you are saying is that a magnetic (inherently 
low impedance? like an 8 ohm speaker)  microphone won't have enough output 
voltage to drive my transmitter speech amplifier adequately.  Not quite sure 
though why Heathkit just talks about high impedance.  Why don't they talk 
about recommending a high-voltage output microphone?  Is "high impedance" a 
code phrase for high voltage output, or crystal device or such?

Because if I were to build say an op-amp preamp for a little speaker and 
used that as a microphone, assuming it had enough voltage swing output, it 
should work, even though the op-amp output would have a low output 
impedance.  Corrrect me if I'm wrong but a low impedance driver into high 
impedance load is desireable if all you are looking for is high voltage on 
the amplifying device's control element (rather than maximum power 
transfer.)
Forgive me for belaboring this but I guess I 'm not clear what you mean by 
the term "expects."

John
KC9MJO







----- Original Message ----- 
From: "Garey Barrell" <k4oah@mindspring.com>
To: "heathkit List Member" <jjjurek22@att.net>
Sent: Friday, December 07, 2007 9:09 AM
Subject: [heathkit] need a cheap hi-Z microphone


> 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?