Last night, I built up the following D104 buffer circuit as discussed by Joe, W4TV.
http://72.52.250.47/images/D104.jpg 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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
