[TenTec] 566 v3.029 and 707 Regal mic

Richards jruing at ameritech.net
Fri Nov 30 16:18:57 EST 2012


On 11/29/2012 12:46 PM, Rsoifer at aol.com wrote:
	> FYI, the output impedance of the 709A is
	600 ohms, while that of the 707 is > 50k ohms.
	The O2 specs call for >10k.
> __________________________________________

Hmmm...  Not sure, but...

I don't think these impedance variations are material - at least not 
enough to cause problems or to really explain what we are seeing in 
differences in performance in this case.  Generally, modern audio gear 
does not seem to care about impedance. RF does, but audio gear not so 

Others, with serious degrees like Jim B  K9YC,  can better explain why, 
but I have read, and have long believed, that wide variations in 
impedance in this case is not material, except in certain cases, such as 
when you want to drive a PA with a high impedance microphone, or a 
guitar amp with a high impedance pickup, and you wanna use a 100 foot 
cable in between.   (Then  you might want a DI box or other 

Try this:

especially the bit on microphone impedance, and how the audio chain 
differs from an RF chain.

Or try this:

which says,
	"Most audio inputs are 'bridging' inputs, where
	the load impedance is much higher than the
	signal source impedance. In that way, when
	the signal is fed to an extra piece of kit in
	parallel the signal level does not fall appreciably.
	Thus at line level, most modern outputs are
	< 100ohm impedance and inputs are
	usually > 10kohm.

	When Should Source and Load Impedance Be Equal?

	When a cable length is more than about a tenth
	of the wavelength of the highest frequency
	present, then the frequency response may
	become uneven. For audio, the highest frequency
	is about 22kHz for CD, making the wavelength
	a little over 8km assuming a cable velocity factor
	of 0.6. 800m is much longer than most audio
	cables in practice, so this is not usually a worry."

Or what it says here:

particularly re: mic input matching.

Or here:


-- Thus, I am thinking variation in microphone impedance may not be 
material, and at least not the reason we are seeing differences in the 
case at bar.

Just MY take...

At least my intentions are good... and my road to He** is very well 
paved with my good intentions !     ;-)

-----------------  K8JHR  ----------------------------


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