4m pre-amp for the FT-847
I had bought a kit for a pre-amp for the one 847, but although there isn’t anything wrong with it, I was not over impressed with the performance, and it was very expensive (£50)
I decided that for the second one I would build a pre-amp.
The first thing I looked at was the need for the BC band filter in the 847. The instructions in the kit I bought mentioned bypassing it, there are no strong stations that ever worry me on the bands I use, so I felt I did not need to include it, and I could remove the one in the 847 to make place for the pre-amp.
The circuit I used is one published by G4IZH, fairly standard, but he has built, tried and optimised it, so it was a good place to start. I added relays, relay driver and designed the PCB, the circuit design is as G4IZH.
Then on discussions with a couple of other people, I was asked couldn’t I use “big parts” instead of SMDs, so that they could build one if it worked.
The board has to fit in a very limited space, there has to be a reasonable space between the two inductors, and it has two relays, so the end result is not as tidy as I would have liked, but it does use TtH components, and it does work well.
G4IZH 4m Preamp The place to start, print this out as all component numbers use this diagram.
NOTES Changes made since the first one was made. Need to read.
Note:- I used a slightly different coil, it had 5.25 turns instead of 6.25 on the “secondary”
The current draw on all the ones I have built has been 3.5mA
They had too much gain, so I changed the attenuator values
These values have a very good match to 50 ohms
| att | 6.1dB | 9.6dB | 12.5dB | 14dB | 15.9dB | 25.5dB |
| R5 | 39R | 68R | 100R | 120R | 150R | 470R |
| R6 & R7 | 150R | 100R | 82R | 75R | 68R | 56R |
These values have a near match to 50ohms, the actual figure is in red
| att | 7.5dB (48.8) | 8.1dB (51.8) | 10.1dB (52) | 17.8dB (52.4) | 20dB (55.4) |
| R5 | 47R | 56R | 75R | 200R | 270R |
| R6 & R7 | 120R | 120R | 100R | 68R | 68R |
I tried 10dB, then 20dB, I preferred the 15.9 dB, less likely to overload on strong signals. I think this must be due to a difference in the FET specs.
Relay Switching Circuit This is the added circuitry to switch the relays, which bypass the preamp when not on 4m
Build photos and notes A few photos taken during the build