my second preamp just burnt out after six months use, What should the average life span be.
Can anyone recommend a preamp for my scanner that mite last longer. Or suggest what i mite be
doing wrong.
thank you
the preamps i've used have ac adapters. and are broad band preamps. An yes they are grounded
will any tv preamp work. And how would you connect to a scanner that has a bnc hookup on the scanner.
Many pre-amps are very senstive to having no load on them while powered up and can easily burn out as they go into oscillation and somewhat run away.
So a key point is ANYTIME you have the amp powered up, you MUST have the amp output terminated or connected to a radio.
Many times people disconnect the coax to the scanner and either swap scanners or take them with them when they travel. You MUST shut the amp power down before you disconnect the cable to the scanner.
I will say it again, you MUST never disconnect the output of the amp while the amp is still powered up.
ALWAYS remove the power to the amp when disconnecting the cables/scanner.
That's a bad design. You're right - it does happen (well, the unstable oscillation, anyway, if not the burn out) but it really shouldn't as a rule. Receive preamps SHOULD be designed so as to be either unconditionally stable or at least stable within normal usage parameters INCLUDING no load on the output. In many cases with low cost and low quality receive preamps they may start out as unconditionally stable but then degrade over time and go into oscillation - many times this causes localized RF interference that can bring unwanted attention from other radio users if not the FCC itself. Some bad TV preamps and FM radio preamps can cause unwanted radiation in critical bands such as the VHF air band so if near airports can cause problems.
Anyway, properly designed, they shouldn't do this. Also, I would think it unlikely that most low powered receive preamps made today would go into a self oscillation mode so bad that they would overheat and burn out but just because I have not heard of that happening does not mean it cannot - I can see where, under certain conditions and using some designs it could happen. Again, poor design - in this case REALLY poor design can lead to these problems and good design would prevent them.
-Mike
Bad design may or may not play into no load damage. Depends on a lot of factors and the first stage semi-conductor device.
See this cut sheet from a $275 Mini-Circuits Wideband LNA.
http://www.minicircuits.com/pdfs/ZFL-1000LN.pdf
See statement right below the spec box - "Open load is not recommended, can potentially cause damage."
Mini-Circuits devices are pretty top notch and not cheap. I run these in some of my applications.
Also keep in mind the OP is not likely running something as high quality and low noise as something like this?
Anyway, just good habit to shut off pre-amp power before disconnecting the coax cable from the load. One small extra step may save you some time and headaches.