The main way of thinking is that you want to put the pre-amp after the band pass filter. The reason for this is that if it is before the b/p filter, it amps anything it sees, noise and unwanted stuff alike. If its behind the b/p filter, it only amps the signals that are left and need it.
You might need to add a several DB attenuator between the pre amp and the receiver to keep from overdriving the front end. Advanced Receiver Research makes very good low noise preamps as so do several other companies. Hope this helps, Mike Specialized Communications KB5FLX ----- Original Message ----- From: skipp025 To: [email protected] Sent: Wednesday, November 25, 2009 11:36 PM Subject: [Repeater-Builder] Re: pre-amp placement Hello John, > "W3ML" <w...@...> wrote: > Hi, not since high school... :-) > I have now read two different things about where to put > the pre-amp. Only two? > One says before the Bandpass and one after. > What I have now is the 6 can duplexer is hooked to the > bandpass and then on the other side the pre-amp is > connected and then a cable goes from preamp to radio. > The other article I read this past week says the preamp > should go between the cans and the bandpass. > Which is right? Or does it matter? Depends and it does matter... based on how busy your radio site is, where any other rf activity is relative to your frequency, the type of preamplifier, how it's constructed, your receiver front end, your duplexer type with number of cavities, your Tx Power level and a few other tidbits... got a headache yet? I'd venture to say... if your receiver front end is of decent Q (quality) and reasonably narrow band-width along with a decent duplexer... then the preamp might best go after the duplexer, between it and the band-pass cavity. If your receiver front end is average or fairly broad (a few MHz band-width) there might be a case to include the extra band pass cavity after the preamp before the receiver input. Some of this option depends on the duplexer and TX Power Level. The point of what I write above is about trying to obtain the best overload prevention performance and or damage control when the preamplifier is overloaded and generating unwanted signals. Many but not all the variables are are in the list. You could of course try both positions and measure the system performance. s.
