Both of my D-104 mics contain this modification and it costs less than $2.00
to complete. It consists of one MPF-102 FET, two resistors and two
capacitors. The revised circuit is so simple and effective, that it can be
built on a 0.75-inch piece of square perfboard in a few minutes.
The theory behind the revised circuit is as follows: the D-104 crystal
element takes on the function of a high-impedance voltage generator, whose
output has an effective series capacitance of only a few hundred pF. The
low-end response of the microphone begins to suffer when it is loaded by
anything less than 10 meg-ohm, due to the low series capacitance inherent in
a crystal microphone cartridge. Astatic's solution to the problem is their
well-known two transistor preamp. Nevertheless, the input impedance to the
preamp represents approximately 100K-ohm, far too low to realize a low
frequency response from the crystal element.
A better solution simply involves connecting the element directly to the
gate of a FET transistor who's input impedance is approximately 11 megohm.
The resultant circuit is a voltage source follower with a slightly negative
gain just less than 1.0. According to Astatic, the preamp functions as an
impedance matching device, not to introduce gain. Nevertheless, gain
resulted in the design and a control was added, which became loved by so
many CB operators. The FET source follower presents a near unity gain
device, has an extremely high input impedance, and a low output impedance,
determined by the series resistor in the FET's drain lead. R.F. has never
been a problem on my mics, regardless of power and frequency.
----- Original Message -----
From: JEFF S JOHNSON <firstname.lastname@example.org>
Sent: Saturday, October 02, 1999 10:02 AM
Subject: Re: [TenTec] D-104 Modifications
> Gadzooks for 75 bucks it should dance the mombo and make breakfast! 8-)
> On Fri, 1 Oct 1999 13:23:01 -0700 (PDT) Carl Hyde <email@example.com>
> > As promised here is the URL for the company that
> > modified the Astatic D-104 to sound like a real ham
> > radio microphone. The site is interesting just for a
> > good explaination of mic response.
> > http://www.angelfire.com/ma/electroservepro/d104.html
> > =====
> > Carl S.HYDE
> > W2CSH
> > Eagles May Soar, But Weasles Never Get Sucked Into Jet Engines!
> > Politics: From the Greek word Poli, meaning many and
> > the word tics, meaning blood sucking parasites.
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