[TenTec] Modified D-104 versus Heil on the Omni-VI.

[Jerry Volpe <kg6tt@tomorrowsweb.com>]

Yesterday a ham friend of mine reminded me that I had some time in the 
past mentioned seeing an article on modifying a D-104 for use on 
lower-impendance mic input rigs... and that he wondered if I remembered 
about the article. Well, fact is I had and that a few weeks ago I 
actually stripped out the stock preamp in my circa 1980s D-104 and 
installed my home-brewed version of the MPF-102 based amp detailed in:

http://members.cox.net/n4jk/d104.htm

My only modification to the circuit was the addition of a resistive 
attenuator in the output as it was initially a bit too strong for my 
Omni-VI Plus. In short I was more than pleased with the results with the 
modified D-104 on my Omni-VI Plus. I had made recordings before and 
after and there simply wasn't any similarities whatsoever. The stock 
D-104 was tinny, thin and even somewhat distorted. The modified D-104 
was full without being muddy and had the necessary peak around 2kHz to 
add voice clarity.

Anyway, back to yesterday. I told my friend that I could do one better. 
I asked him to meet me on 40 meters (he lives about 10 miles away in San 
Francisco) and that I would demo the modified mic. Before I got to the 
modified D-104 I began the on-the-air test with my newly installed 
boom-mounted Heil Dual Element HM-10 (I had just used the Heil 
microphone on the previous weekend for the SS SSB contest). My friend 
immediately commented that my audio was really nice. I changed from the 
HC5 to the HC4 element and he respoinded kindly. I changed microphones 
to my Ten-Tec 700 hand mic and he said that it was nice but not 
particulary distinguished. I then put on the modified D-104 and got a 
rave review. In fact an easedropping ham friend of mine came on 
frequency and echoed how great the modified D-104 sounded. Just to 
illustrate the difference I placed a stock D-104 on and they cringed.

eBay is full of adds for these fine microphones that nearly all of us 
totally ignore as 'CB fare'  from the bygone AM days. Fact is they are 
really a treasure to be rediscovered. The D-104 crystal cartridge needs 
a 2-3 megohm working impedance to give its fullest audio response but 
the versions with the built in amplifiers (several designs over the 
years) just loaded down the mic element at the input and mismatched to 
the newer transceivers on the output. A recipe for audio disaster. The 
FET buffer detailed in the web article above provides a more correct 
match to both ends and results in simply great audio with both my Omni-C 
and my Omni-VI Plus. Now I no longer have my Jupiter so I couldn't test 
it but the article's author created his mod for the Jupiter/Pegasus so I 
have to imagine it does well there too. The Orion? Well, I am saving my 
pennies but manybe someday.

So if you have one of these D-104 gems in your junk box and are not 
afraid of putting a few cheap parts on a tiny perf board why not give 
this a try.

My only complaint is that the modified D-104 is still a handfull and my 
operating desk is centered around the pull-0ut keyboard tray so I have 
to lean into the mic or hold it (Bah).... that is why I spent the money 
on the Heil (but I got the boom from Radio Shack for $15 so that wasn't 
too bad.). Now I have the quandry... The Heil is easy to use but the 
modified D-104 just sounds better. Oh what to do? Anyone know how to 
boom mount a D-104?

73,

Jerald, KG6TT
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