> Does anyone have a design for a FET preamplifier for a > D-104 that will work off the K3 mic bias (rather than the > separate +8V line) and the typical 3.3V mic bias in the > current crop of laptops?
Joe, I would try the circuit shown in the link below. http://72.52.250.47/images/ECM.png Referring to the diagram, note that in simplified form the resistor and capacitor as shown are integrated inside the K3 (and laptop PC). Bias (shown as V+) is de/selected from the K3 menu. The cap is inside the K3 and is used to couple and de-clamp the AC portion to the K3's mic circuit. Again, the R and C components shown in the above diagram are a simplified representation of what's in the K3 -- but it's close enough. The common node between the cap and resistor is the K3 mic pin. The K3 then supplies bias to your external nFET (e.g., MPF-102) and D104 element as shown in the dotted "capsule" area. When bias is active on the K3 mic lead, audio is clamped and elevated to the DC bias potential.of ~ 3VDC to power the nFET. The circuit shown is not a source follower (common drain) that generates less than unity gain, but it is a common source pre-amplifier with a similar attribute of very high input Z. Additional experimentation is needed to optimize the level between the nFET and K3, perhaps by varying the Rs source resistance to ground. The diagram shows no Rs at all. Rd (drain resistance) is the resistor going to V+ in the diagram and is fixed inside the K3. However, the V(gain) expression for a common source FET amp shows that Rs (source resistance) is a denominator value. So, by adding Rs, the circuit gain can be reduced. Although the minimum value of Rd is fixed in the K3, it's possible to add additional Rd at the FET drain. That resistance is also in series with the coupling cap in the K3. By adding more Rd, a voltage divider is created that could impact the operation of the nFET with it's already low +3V at the drain. For that reason, I would start affecting gain by adding Rs instead. 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
