[Elecraft] Astatic D104 mics

[Joe Subich, W4TV]

The unamplified D104 has been a subject of discussion here on several
occasions.  The easiest, least cost way to use one with the K3 (and
Icom transceivers) is described in this three year old posting by W9AC:

> Last night, I built up the following D104 buffer circuit as discussed by
> Joe, W4TV.
>
> http://72.52.250.47/images/D104.jpg
 > http://72.52.250.47/images/D104.pdf
>
> Joe's idea works very well.  The difference between this circuit and other
> D104 buffer circuits as seen on the Internet is that it is powered directly
> from the K3's mic lead when the bias menu is activated.  One benefit is that
> the circuit can be ported to a PC sound card.  Another benefit for most
> builders:  Only one part is required -- either a J201 or MPF-102 nFET
> transistor.  When using the PC input configuration shown, I also had good
> results with a Samsung NC10 netbook.
>
> I tried the MPF-102 and J201, and each works well, although the J201 FET can
> be operated with an Ids current down to 0.5 mA.  R89 in the K3 limits the
> Ids current.  The minimum Ids spec for the MPF-102 is 2.0 mA but seems to
> perform the same, albeit with slightly less gain as expected.   The D104
> cartridge level is relatively high anyway and I found that in either
> configuration, the K3's mic menu could be set for the "low gain" setting.
> Also note the inclusion of the optional Rg and Rs resistors.  My
> breadboarded pre-amp is stable without those components and the gate biases
> just fine.  I'm also not too concerned about static here in Florida.  At
> 15-cents ea., the FET is easy and cheap enough to change.
>
> Finally, if RFI becomes an issue, it may be helpful to experiment with a
> bifilar-wound, common-mode choke at the shielded cable entry, close to the
> FET.  I would try that before attempting differential-mode abatement
> techniques (e.g, bypass caps to ground).  Similarly, the mic cable at the K3
> connector end can be wound through a #31 core, if necessary.
>
> Paul, W9AC

The circuit requires a single FET (it used to be available at Radio
Shack) and two optional resistors.  The "D104 with FET" becomes an
electret equivalent microphone.  If low frequency response becomes
a problem Rg may be reduced to as little as 470K to roll off low
frequency response.

73,

    ... Joe, W4TV


On 10/8/2013 12:01 AM, Tim Groat wrote:
> The basic D-104 (without amplifier) is a crystal microphone, designed
> for very high impedance (>100K ohm), and delivering fairly high output
> voltage. It worked well with the vacuum tube amplifiers used when the
> D-104 was new. The K3 (and other modern rigs) have much lower input
> impedance and will load down the crystal mic element considerably.
>
> There were several amplified versions, the "power mikes" of the CB
> craze. These have low output impedance (a few K ohms, varies with the
> output level pot setting) combined with high output voltage. Their
> output level can over-drive radios designed for dynamic microphones
> unless the gain setting is kept very low, or an attenuator is placed in
> the output.
>
> Links:
> Adapting D-104 to modern radios (using an op-amp buffer)
> <http://www.w6ier.org/images/9908034%5B1%5DAdapting%20the%20Astatic%20D104%20Mic.pdf>
>
>
> Schematics for amplified Astatic mics
> <http://www.barovelli.com/projects/radio/radioastatic.htm>
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