Paul, > Last night, I built up the following D104 buffer circuit as discussed > by Joe, W4TV.
Thanks for the confirmation. I have found one other reference to this approach ... it is mentioned at the end of a QST "Hints and Kinks" article by K8RRH in June 2002. Although most of the article concerns the K0SF opamp preamp from the August 1999 QST, there is a passing reference to common source FET designs for both "bias on the mic" and conventional externally powered configurations. The K8RRH article shows 2N5484 JFETs which have performance characteristics similar to the MPF-102 (available at Radio Shack). Since my original goal was to create an "electret replacement" for use with both the K3 and computer sound cards, there is a similar approach for those using dynamic mics like the Heil HC4/HC5/HC6 elements. See: http://www.ifwtech.co.uk/g3sek/in-prac/best-of.htm#icom by GM3SEK. The GM3SEK circuit is also "mic powered" ... but its input impedance is too low for use with a crystal element. An externally powered (e.g., Elecraft pin 6) version of the circuit can be found here: http://www.tkk.fi/Misc/Electronics/circuits/sbmicamp.html. The NPN circuit can be built using the 2N3904/MPS3904 also available at Radio Shack. Converting microphones for "electret replacement" operation has two advantages ... the K3 can be operated with "low gain" settings for lower sensitivity to external hum/noise and microphones/headsets can be interchanged without concern over changes to input sensitivity or bias. 73, ... Joe, W4TV On 7/22/2010 5:29 PM, Paul Christensen wrote: > 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:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
