The secret to making the D-104's high-impedance crystal mic element sound
great rests in adequate buffering between the crystal element and the
relatively low input impedance of the transmitter.
Think of the crystal element as a network consisting of a voltage generator
in *series* with a small value capacitor. The more this simple two element
circuit is loaded at the transmitter, the more low-end roll off occurs.
Typical values of the source capacitance for the crystal microphone element
are from 800-1200 pF. Now, calculate the -3dB point using the reciprocal of
2*Pi*RC. Assuming a source capacitance of 1200pF and a high input impedance
(e.g., 1 MegOhm) of the transmitter, the -3dB turnover point is 132 Hz!
With 100K of loading it worsens to 1.3 kHz! No wonder why a straight D-104
sounds so "tinny" on modern transceivers.
At one time, Astatic offered the D-104 without a two-transistor preamp for
use with grid-driven vacuum tube speech amps. Because of the very high
input impedance of the tube's grid, the D-104 can sound wonderful on a tube
transmitter. Later, Astatic employed the two-transistor "preamp" as a
buffer for the crystal element, not to be meant as a true preamp.
Nevertheless, Astatic's buffer still presents a load far too low for the
crystal element. A much better and simplistic method involves the use of a
single MPF-102 FET transistor configured as a source follower. The input
impedance is 11 MegOhm, the voltage gain is slightly less than 1.0, and the
output impedance is is determined by a single resistor from the FET's Drain
to ground. I typically use 330 ohms for use with a high or low input
transceiver. Now, my D-104s sound every bit as good as any dynamic mic.
The cost of the circuit mod is less than $3.00 and it can be installed in
place of Astatic's 2 transistor preamp. Also, it will sound every bit as
good as that $125.00 mod that is available on the Internet. Hmmm, I wonder
what circuit they're using in theirs!
-Paul, W9AC
-----Original Message-----
From: John Vickers <wa4tt@nlamerica.com>
To: Tentec Users Reflector <tentec@contesting.com>
To: <tentec@contesting.com>
Date: Monday, March 20, 2000 1:29 PM
Subject: Re: [TenTec] d-104 and omni V
>
>Carl S. Hyde W2CSH Wrote:
>
>
>>
>>The D-104 mics are made for CB radios. They have a low
>>frequency roll-off from about 1 KHz on down. They also
>>boost higher frequencies. In other words they suck!!!
>>But if you insist on using one there is a company that
>>for $125.00 will make a D-104 sound like a real ham
>>radio microphone.
>
>
>Hi Carl es all--- The D104 was in use BEFORE the 11 Meter CB band was even
>established. The CBers starterd using theD104 because it made their
>stations look more like ham rigs. Then the Mfg started targeting that
market
>with its Golden Eagles, silver Eagles, Black Crows, Dead Ducks, Etc, Etc.
>
>In the hands of someone with a basic knowledge of radio the 104 can be made
>to work just fine on the Ten Tec rigs. Some of the guys with higher
pitched
>voices need to loosen their shorts or try another brand of mic. BUT PLEASE
>don't pay $125 to some outfit to do to your mic -- what any true ham should
>be able to do for himself.
>73 John WA4TT
>
>--
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>
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