[AMRadio] Microphone recomendation


Paul Christensen w9ac at arrl.net
Tue Sep 22 11:34:43 EDT 2009


Jim,

The LTSpice plot shows why the stock Astatic 2-stage pre-amp with its ~ 
470K-ohm input Z does little to extend low-end response.

The model also likely assumes a perfect C in series with the source.  After 
testing several D-104 cartridges of different vintage, I've come to the 
conclusion that distributed resistance appears across the crystal, thereby 
limiting any additional benefit of an extraordinarily high inout Z beyond 10 
meg-ohm.

Although not shown through the model diminishing returns hits pretty fast 
just above 5 megohm.  I've not seen any element produce a reasonably flat 
response below ~ 50 Hz.  Still, as long as the target is about 70 Hz to hit 
the lowest of vocal fundamentals, there's no reason to strive for anything 
better unless one relishes the sound of room rumble which in turn also 
unnecessarily raises RF duty cycle.

Paul, W9AC



----- Original Message ----- 
From: "Jim Tonne" <tonne at comcast.net>
To: "Discussion of AM Radio in the Amateur Service" 
<amradio at mailman.qth.net>
Sent: Tuesday, September 22, 2009 11:07 AM
Subject: Re: [AMRadio] Microphone recomendation


>
> I have been lurking here and reading the discussion
> which seems to me to have reduced itself down to a
> kind of "my daddy is better than your daddy" kind of
> thing.
>
> The person who said the D104 microphone could be
> thought of as a voltage generator in series with a
> 500 pF capacitor hit the nail on the head!  Exactly
> correct (although I thought it might be a  *slightly*
> higher capacitance value).
>
> I have spent a few minutes with LTspice and made
> a plot which clearly shows what happens when you
> change the value of the load on the D104 microphone.
> With a 10 megohm load the -3 dB point will be right
> at 30 Hz.
>
> Please, fellows, take a quick peek at this plot:
> http://tonnesoftware.com/appnotes/D104loadTest.gif
>
> The bottom line of this note is that going higher than
> 10 megohms for a load is nonsense.   Pure nonsense.
>
> - Jim W4ENE
>
>
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