Couldn't agree more with Paul. I use a very similar circuit in my DR-10 ( a
"studio" high impedance mic made by Astatic that I bought new back in about '51)
with my Omni VI (+3) and I have placed the circuit in D-104s for other hams. As
Paul says, they work as well ( or frequently better) than most "ham market"
dynamic mics.
73,
Troy, W6HV
Paul Christensen wrote:
> 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
> >
> >--
> >FAQ on WWW: http://www.contesting.com/tentecfaq.htm
> >Submissions: tentec@contesting.com
> >Administrative requests: tentec-REQUEST@contesting.com
> >Problems: owner-tentec@contesting.com
> >Search: http://www.contesting.com/km9p/search.htm
--
FAQ on WWW: http://www.contesting.com/tentecfaq.htm
Submissions: tentec@contesting.com
Administrative requests: tentec-REQUEST@contesting.com
Problems: owner-tentec@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|