D-104 preamp ...
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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? I'd like to avoid providing separate preamp power when the mic is connected to a computer soundcard ... 73, ... Joe, W4TV ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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> 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:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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In reply to this post by Joe Subich, W4TV-4
I have used the following circuit on a D104 to my K2:
http://members.cox.net/n4jk/d104.htm Works great and you wire it for power through the mic connector. Mark W5EZY |
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> I have used the following circuit on a D104 to my K2:
> > http://members.cox.net/n4jk/d104.htm > > Works great and you wire it for power through the mic connector. That circuit won't work with the K3's bias on mic pin (Joe's intended use). To work that way with mic bias, power and audio must be joined on one conductor in the same way that an electret condenser element is powered. However, the N4JK circuit will work when using the K3's +8V pin to power the nFET. Also, in its shown configuration as a common-source (source follower) amplifier, I would eliminate the 100pF, 0.01uF input caps, and if absolutely necessary to kill static charge, increase the 2.7-meg shunt R to 10-megs. The common source nFET circuit is self-biased and no "gate leak" is necessary. The rest of the circuit looks fine. Paul, W9AC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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Joe,
If you aren't married to the original element you could change it out to an electret element wired directly to the K3 input with the bias turned on. I did that than trying to find a replacement crystal element. The electret element is available from many sources (including your local Radio Shack) for a couple of dollars. Make sure to get the 2 wire version. Otherwise you could create a "phantom" voltage source in the D104 using a blocking capacitor on the FET output and a diode to a pad-out resistor and filter capacitor. I get good reports on audio quality on my electret but suspect it is the K3 not the microphone! George AI4VZ --------------- That circuit won't work with the K3's bias on mic pin (Joe's intended use). ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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George, > If you aren't married to the original element you could change it out > to an electret element wired directly to the K3 input with the bias > turned on. I did that than trying to find a replacement crystal > element. If I were going to change the element I would change it to a Heil HC5 or the new HC6 - I am no fan of electrets or bias on the mic line. However, the whole purpose of the FET buffer is to use the outstanding, naturally pre-emphasized (rising) response of the Astatic crystal element on both the K3 and computer soundcard without the need for an external "battery box." I suspect the solution lies in an MPF-102 as a common source amplifier with a 470 Ohm resistor in the source lead, 680K to 1M as a gate leak to set the low frequency roll of at around 150 Hz (my preference - since I see absolutely no benefit in reproducing unnecessary and power wasting low frequencies and shack noise) and connect the drain directly to the K3 mic input. The source resistor might need to be adjusted for use with a laptop that only provided 3.3V of bias. Another solution is probably a common emitter NPN transistor with 330K - 1M collector to base, a .47uF - 1uF blocking capacitor to the crystal element with a shunt 1M resistor. However, that would take a lot more experimentation. I was hoping that someone had already solved the problem and could save me some cut and try ... 73, ... Joe, W4TV On 7/21/2010 2:49 PM, George & Jan wrote: > Joe, > If you aren't married to the original element you could change it out to an > electret element wired directly to the K3 input with the bias turned on. I > did that than trying to find a replacement crystal element. > The electret element is available from many sources (including your local > Radio Shack) for a couple of dollars. Make sure to get the 2 wire version. > Otherwise you could create a "phantom" voltage source in the D104 using a > blocking capacitor on the FET output and a diode to a pad-out resistor and > filter capacitor. > I get good reports on audio quality on my electret but suspect it is the K3 > not the microphone! > George > AI4VZ > --------------- > That circuit won't work with the K3's bias on mic pin (Joe's intended use). > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[hidden email] > > 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:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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In reply to this post by Mark - W5EZY
I said it once, and I'll say it again.
My 1956 crystal-element D-104 works great with my K3. I plugged it in and did an on-the-air check with K9YC. Jim reported outstanding audio. Before modifying your D-104 or building a preamp, try the bare-bones D-104. K9YC: If you don’t have a problem, don’t solve it. 73, Hank, W6SX Mammoth Lakes, California Elevation 8083 feet in John Muir's Range of Light ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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In reply to this post by Joe Subich, W4TV-4
> I suspect the solution lies in an MPF-102 as a common source amplifier
> with a 470 Ohm resistor in the source lead, 680K to 1M as a gate leak > to set the low frequency roll of at around 150 Hz (my preference - > since I see absolutely no benefit in reproducing unnecessary and power > wasting low frequencies and shack noise) and connect the drain directly > to the K3 mic input. The source resistor might need to be adjusted for > use with a laptop that only provided 3.3V of bias. The K3 and the laptop will effectively provide their own drain resistor in a common source configuration. For example, in the K3, bias appears to be turned on/off through Q8 and injected to the mic through R89 on the FP Board. R89 would become the FET's drain resistor (Rd) at 5K, so no new Rd is needed. From R89, audio is coupled through a 1 uF cap at C28 on the Main DSP Board. As a common source amp, a source resistor (Rs) is optional but helps set stage gain. The K3's 5K and 1 uF values are a really good choice since the FET's output Z is roughly equal to the 5K drain resistor. Thevenizing from C28, the cap sees 10K in parallel with 5K, leaving a load of about 3.3K. The -3dB turnover point should be about 48 Hz. When using the K3, it looks like the whole D104 interface can be done with just one external part -- a single MPF-102. That's even simpler than the already simple source follower circuit that's become popular. In your case, one extra gate shunt resistor can be used to optionally limit the low-end response and provide for gate static protection. I would bet the laptop's circuit is close enough if it also provides a switched pull-up resistor (to function as Rd) so that one homebrewed buffer could be built and inter-changed between the laptop and K3. Great concept, Joe! I'll try this the next time I work on an old D104. Paul, W9AC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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In reply to this post by Mark - W5EZY
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:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
Re: K3 - D104 Buffer Schematic
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Adobe .PDF of the same schematic:
http://72.52.250.47/images/D104.pdf Paul, 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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
Re: K3 - D104 Buffer Schematic
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In reply to this post by P.B. Christensen
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:[hidden email] > > 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:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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