[AMRadio] Mike Cable Capacitance

[Jim Bromley]

John Coleman, WA5BXO, posted:

(Regarding 50-ohm coax attenuating the high audio
 frequencies when used as a microphone cable).

> I feel there must be some other thing that is overlooked 
> about the connector or cable.  15 ft of cable would not 
> normally cause any noticeable effect on audio unless the 
> terminating Z was greater that a few mega ohms as it would
> be for some of the older equipment made for Xtal mike input 
> but I can't imagine a modern day line input being greater 
> than 100K ohms....

The impedance to be concerned with in this case is not
the terminating impedance, but rather the source impedance
of the microphone itself.  In the case of the D-104, although
the impedance of the microphone is specified at a nominal
10K-ohms, its actual source impedance is much higher.  It is 
a well-known fact that the element must be terminated in a 
resistive impedance of at least 10-Megohms to obtain adequate
low-frequency response - indicating a source impedance of
at least that amount.  Additionally, since there is no DC path 
through a crystal acoustic transducer, one would be suspicious
that whatever the actual source impedance is, it would  contain
a capacitive reactance as a series component.

The best solution to the problem is that advocated by several
other posters here - incorporate a preamplifier very close
to the microphone element (inches, not feet, away) having
an imput impedance in the tens of megohms and capable of
driving a 500-ohm termination at unity gain.

Jim Bromley, K7JEB
Glendale, AZ

James E. Bromley            Tel: 623-848-8711
5128 N. 69th Ave.           E-Mail: K7JEB at COX.NET
Glendale, Arizona  85303