Re: K3 - D104 Buffer Schematic
Posted by
P.B. Christensen on
Jul 22, 2010; 10:05pm
URL: http://elecraft.85.s1.nabble.com/D-104-preamp-tp5319528p5327500.html
Adobe .PDF of the same schematic:
http://72.52.250.47/images/D104.pdfPaul, W9AC
----- Original Message -----
From: "Paul Christensen" <
[hidden email]>
To: <
[hidden email]>
Sent: Thursday, July 22, 2010 5:29 PM
Subject: K3 - D104 Buffer Schematic
> Last night, I built up the following D104 buffer circuit as discussed by
> Joe, W4TV.
>
> 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
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