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wobblyone

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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
 

davenlr

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I dont know if it covers the range you want, but I use Channel Master 7777 preamps (75 ohm) and have had them up for two years and counting with no issues.
 

N0UDG

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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
Some things that would be helpful are:

Where are you mounting your preamp?

Is it a broad spectrum or a narrow spectrum preamp?

How is it feed power?

Are you using any static discharge devices on your coaxial cable and how will grounded is the shield of your coaxial cable?
 

wobblyone

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preamp

the preamps i've used have ac adapters. and are broad band preamps. An yes they are grounded
 

N0UDG

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the preamps i've used have ac adapters. and are broad band preamps. An yes they are grounded
Hopefully some of this will be helpful:

The best Signal to Noise ratio is achieved when the preamp is mounted between the antenna and the coaxial cable (in comparison to mounting it between the coaxial cable and the radio), but you must be very careful to water proof all the connections if it is outside. Often in this situation the coaxial cable supplies the power to the Preamp from an ac adapter.

Some claim that broadband preamps often do more harm than good for reception but I think it depends on your location, what you are trying to hear and your radio. Since you have used them before you must find them useful. However, it is also claimed that a broadband preamp (as opposed to say a 800 MHz preamp) heats up with all of the FM broadcast, NOAA, Pagers, cell phones, etc. that it also amplifies and on a hot day in the sun it may overheat and fail early.

My experience has been that the most common cause of failure is Static Discharge either due to a near by lightning strike or static generated by strong winds and snow or dust. I always use an good quality 8 ft. cooper ground rod next to my antenna where I use static discharge devices like a "Blitz Bug", plasma discharge devices, etc.. They all fit on the coaxial cable just before they enter the house and on a windy day with a lot of dust or in a blizzard the Blitz bug can be seen in the dark discharging the build up of static.

Because I was told that the Blitz bug could not protect the newer radios that use ICs so I added the "plasma discharge tubes", especially for nearby lightning strikes.

Lastly, some wall warts do not supply a very good quality current and some houses do not have properly grounded AC outlets so lookout for those problems too.
 

wobblyone

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preamp

will any tv preamp work. And how would you connect to a scanner that has a bnc hookup on the scanner.
 

JamesO

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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.
 

Mike_G_D

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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
 

JamesO

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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.
 

Mike_G_D

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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.
Yep, I've used those or something(s) rather like them in the past. But they are more intended for lab use (or in well thought out self built systems intended more for professional environments and specialized usage). Generally speaking, in those cases the builder pays close attention to the specs and designs accordingly; like if I was building that into something that might present an open load to the output of that amp and I expected the input to possibly be at a level higher than the 20dB derating as called out in open output conditions I would build in an external protection mechanism - at least a 6dB to 10dB resistive pad between the output and the next stage or device in line (yes, I know a resistive pad degrades the noise figure accordingly as well as the system "gain" but I did say, "at least"; a better way would be to add in a reflected energy detection circuit of some form on the output and use the control of that to reduce the input level if the return loss falls below some minimal amount most likely by switching in input attenuation or completely cutting the input off depending on the needs of the overall design and prior stage or stages, etc.). The point being that that device is intended to be used in controlled environments by technically competent people who at least are decently versed in reading technical specifications and basic RF practices. In that type of usage that device is most certainly quite reliable and very capable; many transistors by themselves wouldn't fair well in certain situations so you design the overall "black box" within which they reside to take that into account. That low noise amp is intended more for being used as a "component" within a "black box" even if that "box" only contains no more than a power regulator and maybe some input and output protection or buffering.

In my last post, I was referring more to those fully independent final use devices intended for external use on consumer receivers such as radios and TV's. Most of those are designed NOT to go into oscillation when presented with no load as the designers know they will be used in many obscure ways by non-technical individuals and/or in uncontrolled or unpredictable operating conditions (I would think that open outputs would be a pretty likely thing to encounter for those devices and so would design them accordingly). Now, that doesn't mean that some are not designed this way (to cut costs at the expense of reliability and longevity) though I would also think that any company selling receiver preamps to consumer customers would suffer greatly if they sold devices that fried themselves quickly when they were powered up with no load on the output!

But hey...technically you're right, given the knowledge and the possibility, whether remote or otherwise, why take chances? But be aware that the "loads" presented by various receivers (and splitters and multicouplers, etc.) and in different stages of "on-off-ness" can vary quite a bit themselves, rarely purely resistive 50 or 75 ohms across the whole expected band limits (but, yes, of course, not a full open, at least, usually).

-Mike
 
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