KD9SV VARIABLE PREAMP

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This 20+ db variable-gain preamplifier was described in an article by Gary Nichols, KD9SV, in HAM RADIO MAGAZINE for October, 1989. It has proven to be very popular with lowband enthusiasts as it can be configured for either 160 or 80 meters, has variable gain of from near 0 to over 25 db and uses readily-available parts. Noise figure is quite low and the design appears to have no vices. The only component no longer available is the 3SK88 FET; however, this can be replaced by a 40673 FET or an NTE 455 with no sacrifice in performance.

As a matter of interest, I did not include the protective diodes across the input of my preamps, relying instead on a front end protection circuit (described on another page). Inclusion of the back-to-back diodes may induce undesireable IMD response in the presence of strong broadcast signals. Having said this, if you are not inclined to use a front-end protector and your preamp is in the line at all times then the diodes are a necessity.

PARTS LIST for 160 METER VERSION

C1, C2-ARCO #429 COMPRESSION TRIMMER

CR1, CR2-IN914( or equivalent) silicon diodes

L1, L2-FT50-61 TOROID

L1-22 turns # 26 enamelled, taps at 11 and 2 turns

L2-22 turns wound on opposite side of toroid, tap at 11 turns

L3-2 turns wound over L2

Q1-3SK88, 40673 or ECG/NTE 455 (other dual-gate MOSFETS will work too)

R1-10K with ON/OFF switch

S2-DPDT toggle switch (use RG174 coax cable to wire to by-pass switch)

Resistors are 1/4 watt, capactors are disc ceramic

Circuit board: the foil outline is included for those who wish to roll their own. High-quality boards are available from FAR Circuits for $4.75 US

Connectors: I have standardized on the "F" series of TV connector for all my preamps and Beverage switching feeds. They are inexpensive, easy to use and lossless at lowband frequencies.

1-There is a 7.5K resistor across the input to broaden the bandwidth. It does not appear on the board layout or the parts placement. It is not really needed on 160M but is desireable on 80M if you wish to use the preamp over most of that band.

2-There is an error in the schematic regarding the 1 meg resistor attached to gate 1 of the FET. It is shown unterminated but should, in fact, be attached to the junction of the gain pot and the fixed 10K resistor.

3-Tuneup is simple: just peak both trimmers at 1850 and the preamp will provide 20db or more gain 75 khz either side of this frequency.

4-For 80 meters, use either a T50-3 or an FT50-63 core with the same number of turns on each coil.

5-The size of the board foil outline as shown in the link below will have to be adjusted. Actual size of the board is 64mm wide by 40mm high. This can be adjusted in a program such as Corel Draw8 or Photopaint.

• SCHEMATIC DIAGRAM
• BOARD LAYOUT
• PARTS PLACEMENT
• KD9SV WEB PAGE