FET Impedence Transformer, 8 Ohm output into 600 Ohm input

I just stumbled across this old schematic from 1992, originally lifted from the Late Doug DeMaw's (W1FB) column in Monitoring Times. more I really built this back then (in fact, I just found it in my goodie box) to connect my auto reverse tape deck to my very first transmitter. The XMTR was built from James Cunningham's schem in the Low Power Broadcasting handbook now in our library. FIR, Friendly Information Radio, AM 790 :-) broadcasting a loop of tourist announcements.
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Might come in handy!


radio8z's picture

Interesting circuit. How well did it work? I am interested because a JFET is a square law device and distorts high level signals. Maybe in this configuration it is linear.

I built a similar circuit using BJT for DC and it worked great, though the biasing was tricky.


scwis's picture

From a user's perspective it worked great. I powered it with a fairly hefty bench top regulated DC power supply, not a wall wart.

This circuit made all the difference in the world in connecting my cassette player to my crude AM transmitter (three 2n2222's and a handful of passive components).

I had tried this audio chain (ha ha, one link!) without this unit and it was distorted and unintelligible. With the unit I could use the first third of the volume knob rotation to set a useful volume level for the input to the transmitter and tweak that with the modulation adjustment.

It was also very useful in connecting to that leaky cable FM transmitter experiment of which I should not speak :-)

I don't think it's audio broadband, but I'm guessig it has bandwidth somewhat similar to AM radio.

As far as the components, I saw the diagram and remember some of the text from the article but not the part where component choice was discussed.

If a library or any visitors or anybody else has access to the 91-92 issues of MT it was from the regular Doug DeMaw circuit column, published until shortly before his death. Unfortunately the earlier issue indexes aren't available on line.

A suggested modified/better/replacement might be handy :-)

Experimental broadcasting for a better tomorrow!