Do a google search on things like 'active antenna' or 'active antenna AA-7' and youll find a few things. 'dxing.com' or organizations like National Radio Club might also be good places to look.
I have lots of schematics though none in electronic form that I can post. I ran across a nice article by Joe Carr that discussed filters, pre-amps, attenuators - collectively - and what you might expect in terms of performance improvements.
My own experience suggests that if you live in or near a city and your receiver is not a high performace model that you might benefit from some filtering. Strong signals that are not on the frequency of interest can still affect the receiver. I built and installed a broadcast band filter and it eliminated a large amount of junk/noise that I heard from about 2 mHz to almost 7 mHz. My receiver is a superhet/moderate performance.
Another alternative is a tuned loop. This can work well on stuff from 10 mHz on down. I built one for broadcast band (550-1600 mHz here in US) and even without a pre-amp the results were impressive.
A pre-amp may not improve things but only way to know for sure is to try it. If strong out-of-band signals are causing problems they might get worse with pre-amp. If you construct the pre-amp you can then add filtering and other features - so all won't be lost.
Good luck.