In a message dated 12/1/2012 4:53:56 PM GMT Standard Time,
jruing@ameritech.net writes:
> Subj:Re: [TenTec] 566 v3.029 and 707 Regal mic
> Date:12/1/2012 4:53:56 PM GMT Standard Time
> From:jruing@ameritech.net
> Reply-to:tentec@contesting.com
> To:tentec@contesting.com
> Sent from the Internet
>
>
>
> ____________________________________________
>
> On 11/29/2012 12:46 PM, Rsoifer@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
> much.
>
> 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
> transformer...)
>
> Try this:
> http://sound.westhost.com/impedanc.htm
>
> especially the bit on microphone impedance, and how the audio chain
> differs from an RF chain.
>
> Or try this:
> http://suite101.com/article/audio-impedance-matching-a187871
>
> 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:
> http://hyperphysics.phy-astr.gsu.edu/hbase/Audio/imped.html
>
> particularly re: mic input matching.
>
>
> Or here:
>
> http://www.soundonsound.com/sos/Jan03/articles/impedanceworkshop.asp
>
> -- 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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>
Tnx for that. Bob, K4TAX, explained it off line. BTW, T-T has since
changed the source impedance of the 707 mic to 250 ohms. Maybe it should be
called the 707v2?
73 Ray W2RS
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