D104 FET Transformer Circuit Boards

[w9ac]

I've recently completed a PCB design of a direct drop-in "FET transformer" as a replacement for the horrid 2-transistor "power mic" circuit used in the T-UG8/9 versions of the Astatic D-104 microphone.  A description of the boards can be found on my QRZ.com page:

http://www.qrz.com/db/W9AC/

A second board uses all SMD devices since most of the big semiconductor manufacturers have now "obsoleted" the J201 and MPF-102 in the through-hole TO-92 style case.  Even the SMD versions of these FETs will likely become obsolete soon - although for the moment, InterFET is making both case styles.  I've also posted some info about why these two parts should not be sourced from common on-line suppliers unless accountability in the supply chain can be verified.

I'll have more info concerning board availability after I receive the first set from the PCB manufacturer and will update my QRZ.com page at that time.

Paul, W9AC

[W3NE]

Very interesting projects on your QRZ page. And photos of your station are impressive as well. Thanks for sharing them. What are your plans for the D-104 boards?
Bob - NE

[w9ac]

Bob,

Not sure yet.  As of this morning, I've completed design of another SMD board with the same mounting footprint as the thru-hole version.  Since the two mounting tabs are only 0.4" wide, they can be cleanly removed with standard diagonal cutters if desired.  This might be helpful if placed into an older version of the D104 or other crystal mics.  That would eliminate the need to produce two SMD boards.

Paul, W9AC

[W1DAN]

Paul:

Incredibly beautiful work on all of your projects!

Very impressed.

73,
Dan

[WBear2GCR]

Yes, nice station, nice restores, nice link coupled tunas... too clean, way too clean!
I can't be that clean no matter how hard I try... no skills in that department!

Paul, I think *any* jfet will be more than adequate for source follower application here.
Nothing magic about the J102. The main question will be Idss, but even that isn't terribly critical. A higher Idss may require a larger source resistor, but that isn't all bad either.

The bogus asian parts seem to be mostly rebadged Fairchild parts. At least the ones I've been able to ID are. For this stuff, where noise figure is not important and gain isn't terribly important, they should work jes fine!

                  _-_-

[Steve - K4HX]

Very nice. Thanks for sharing.

I see you have the ADAT transceiver. I'd be interested to hear your impressions.

[N4LTA]

MPF102 has been obsolete for several years. Much better lower noise options now available at low cost.
J310 is what I usually use for general purpose. They are also in the end of life cycle.

I bought 200 several years ago for less than 2 cents each after they were declared obsolete.

Pat

[w9ac]

Paul, I think *any* jfet will be more than adequate for source follower application here.
Nothing magic about the J102. The main question will be Idss, but even that isn't terribly critical. A higher Idss may require a larger source resistor, but that isn't all bad either.

The low Idss of the J201 can be helpful when the D104 is powered through a 9V battery.  Even with heavy use, the battery will pretty much last its shelf life.  Of course, not a concern when the preamp is powered from a mic jack.

I did find what may be the last big stash of SOT-23 cased Fairchild J201s through AvNet, one of the big industrial component suppliers.  Cost is about 17-cents ea. in small  quantities.  I ordered 200 and the price dropped to just 11-cents ea.  If there's little fear of SMD work, this may be the last chance to get known-quality devices at a reasonable price before the "bottom feeders" take what's left of remaining stock, then sell them at $5 ea., or more. 

Linear Systems is making some high quality low-noise JFETs like the 2SK170 -- now called the LSK170.  Prices aren't cheap but not unreasonable either.  I guess we should be happy to have a source of high-quality parts in both TO-92 and SOT packages.

Attached is a virtual reality image of the SMD version of the FET Transformer board with mounting tabs.  Before ordering, I'm tempted to add a line of unplated "vias" along the tab lines so they can be easily snapped off for use with other crystal mic types or other audio and RF circuits where a source-follower is needed.  The board density is high with short trace distances so it should work well into the VHF region with the right FET and use of the poured ground plane.

Paul, W9AC

[W9BHI]

Paul,
When you get these made, I would like to purchase a few.
Thanks,
Don W9BHI