On Oct 17, 2007, at 12:01 PM, [EMAIL PROTECTED] wrote: > How can I tell if my MASTR II has a preamp built in? If > it doesn't what is the best preamp to use?
The other answer for how to see if it's in there, is correct. Kinda hard to explain if you haven't seen a normal one first... but there's an extra little board in the "hole" and a tiny RCA jumper from that board to the receiver. As far as "the best" preamp to use, that's very dependent on outside factors -- is your site quiet? Noisy? How much filtering are you doing? How much gain are you looking for? Example - at one local site, we run without a pre-amp because the noise floor from hundreds of transmitters and multiple broadcast stations there, is so bad -- that amplification just brings up more of the "crud". We've talked about getting aggressive with the filtering and adding a light amount of pre-amp at that site, but it's not a priority for us right now. At another similarly noisy site, on VHF the hams use a shared antenna for receive, and the noise is so high there that the "community" pre- amp has been removed forever, because it caused nothing but problems -- and we're all doing filtering and our own pre-amplification AFTER our filters and duplexers, because the repeaters are literally spread across the band, and band-pass filtering and pre-amplification of the community "feed" is ineffective... it ends up being too wide. One repeater is at the bottom of 145 with a 144 input, and the others are at the top of 146, with mid 146 inputs. It's better to target a specific usable receiver sensitivity number that you'd like to see (after knowing what the site noise-floor looks like) than trying to work backward into the design from the pre-amp, but with that said... pre-amps that have been successfully used by many people here include: - The stock GE pre-amp (not much gain, but also not too "unhappy" in high noise environments) - Advanced Receiver Research (my favorite, but can be a little too "hot" for the MASTR II receivers we use) - Hamtronics (I don't like them, but others report good luck and behavior from them, and they're cheaper than most) - Angle Linear (Chip's got some nice stuff there, and it won't be cheap, but he'll also custom build some pretty nice setups if you work with him and answer his questions about your setup and site. I keep meaning to try out one of his PHEMPT pre-amps on one of our systems to see if we find any reason to use them over the GaAsFET ARR's... but haven't had any time to do it yet.) Just popping a pre-amp in without measuring useable sensitivity first, sometimes works out... but it's far better to measure and know how much it helped. If you measure, you can then tell if you've over-done it in the pre- amp (common when using the ARR... it's pretty hot) and perhaps you may want to add a 3 or 6 dB pad behind it to keep from overloading the receiver if it's dragging in a lot of extra "stuff". You can measure the behavior of your specific receiver as you lower the signal (a set of different pads of different values or one of those accurate "DF'ing" switchable attenuators is nice during the testing). Remember, if you don't bandpass filter before a pre-amp, it's going to "stuff" a lot of off-channel extra signals (and noise) from other nearby transmitters -- or even far away ones! -- into your receiver. That off-frequency stuff, if strong enough is just going to make your receiver overload and may actually perform WORSE than without the pre- amplification. Another common problem is when people add pre-amplification and don't have enough isolation in the duplexer... now your transmitter is being "heard" by the receiver where it couldn't hear it previously... creating desense or just general "deafness". It's all about trade-offs when you start going for the "theoretical" receiver limits. Sensitivity versus selectivity, the same ol' game whether you're talking about repeaters or any other weak-signal station's receiver. Maybe some of the pros here can share some of their pre-amplifier "secrets". -- Nate Duehr, WY0X
