Insight: Rx Preamps - A regular pit of vipers

We’ve all seen them advertised, those super duper ‘give your set bigger ears’ high gain receiver preamps.

Do you really believe the hype?

Hopefully not - you’ve really got to be scraping the barrel of desperation to even contemplate adding one into your receiver chain.

Note - don’t confuse this with the improved internal preamps used on such modules as the classic MuTek front end conversions for old radios and in some cases were around early in a radio’s life to offset design weaknesses.

No, I’m referring to those Rx amps that are to the receive chain what Tx ‘boots’ (aka crappy nonlinear garbage generating hellspawn so beloved in the CB world of old) were to QRM and unacceptably high out of band emissions.

In this case, instead of inflicting OOB emissions on others, you actually are hammering your own receiver by swamping with nndB of garbage and raising the noise floor of a receiver hideously.

If you’re out in the sticks, you’ll possibly find enough balance at a push to justify that ■■■■, but mostly - just don’t go there.

You see, that Rx Preamp you lovingly bought for its say 30db gain figure is often a wideband (usually) figure. So at the centre of it’s say 26-30Mhz there maybe 30dB boost around 28 MHz, potentially half that at 27mhz & 29mhz, and a quarter or less within 26 & 30 MHz if you’re lucky.

So if 30db falls at 28Mhz and your talking the top few frequencies of the UK CB segment, you’ll see some residual boost and also get an increase in adjacent 26/28mhz breakthrough (more from 28Mhz in that scenario). Likewise, same exact unit used on low-mid 27 MHz CEPT allocation will be of nil effective worth and about the same unwanted adjacent rejection worsening of the higher 27 and 26 MHz stuff.

So ask yourself, even if it’s putting that 30dB on gain where your radio is deaf or struggling frequencies wise, do you honestly believe that the consequential in-segment noise floor worsening which can be a quarter to half of the preamp gain figure worse (increased noise) and hideously awful birdies and other intermix nasties caused by swamping your receiver is a benefit, a gain?

I sure hope not. If you want clean gain, it’s passive gain you pursue and actual sharpening and sensitivity improvement to the receiver where it’s weak that you want, not the equiv of a size nine boot to the head impact on a struggling receiver which was not great to start with.

This is applicable across the board - the CB references reflect examples classically found but I’ve seen it ruin reception on some decent gear that owner was too lazy to sort out the antenna and chuck out the wet string substitute excuse they used.