I've built and applied many of these with good results. The two 2498's seem to be quieter that the MAR 5 MMIC circuits I had been using, although the MMIC's are much easier and faster to build (see Minicircuits Web site). I have since found that Ramsey has a preamp called the SA-7 which has a close configuration to the above one. That is where I found out about the 2570A. I can't tell you one is quieter or more broadbanded than the other since in my usage it really doesn't matter.
In my Preselector circuits, I use these amps at 18 dB (3 S-units), from 100 kHz to 30 MHz. They are fine way up to above 900 MHz.
Maybe I'm getting lazy, but for about $19.00, the SA-7 (with mods as required) seems to be a good deal. I get all the parts and a "real" PC board and no tendency to oscillate as was a bit of a problem when I "dead bugged" the first versions of this on perfboard.
For a gain control to serve unknown receiving antennas, hang a 5k pot off the input. For a 50 ohm invironment I use a 500 ohm pot or 50 ohm step attenuator, and transformer couple the input and output at 1:1 for isolation (minicircuits or coilcraft).
9-23-06
