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Gray 300 pre amp


Well-Known Member
Jun 27, 2021
Got a friend wanting me to try to improve the pre amp stage. He states the pre amp will only bring signals up .5db to 1db. I'm sure I could possibly upgrade the transistor in the pre amp circuit and possibly a few other things, but wanted to see if others had tried this with their pre amps with success.

Anyone have any recommendations on accomplishing this?

(Sound of crickets)...

I always recommend an external preamp if you really need one. The built-in preamps seldom perform well. As if that feature is there only because people think they want it.

I can't remember seeing an expert explain how to make the under designed internal preamp in mobile linears work better.

A preamp with too much gain will overload and exaggerate bleedover. A preamp with not enough gain will just add noise to the receiver. An external preamp with a gain knob gives you a shot at finding a sweet spot between the extremes.

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Your radio might have issues or need alignment. The Gray 300 has one of the better preamps compared to others I have used. Not much use on AM as the noise floor is almost always higher than a weak signal but excellent on SSB to make a weak signal loud enough to hear in a mobile.
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Pre-amps are a waste of time the way they are used on amplifiers. To be effective it would really need to be fairly complex. As executed on CB amplifiers they amplify the noise the same as the signal you want which is a problem today.

Get some sort of notch filter and learn how to use it or get DSP or both and learn to use them. What you want to do is pull out the signal you want with out amplifying the background noise and that just can not be done with a simple pre-amp.

I use EQ, Notch/Band Pass filter both manual and automatic, DSP and either headphones or home or car audio speaker's. Seldom is it a one and done situation.

If you designed a completely different circuit and looked at it like speech processing on TX but on receive the simplest thing you could do would be to select an IC and opamp that gives you eq, compression, gain control, agc, and notching, symmetry control etc.....There have been a lot of IC's in the 1980's and 1990's that where used for portable audio, home audio, car audio gear that had a lot of those functions but not all of them.

I have not tried to design my old gear I have just used off the shelf gear and repurposed it in some cases.

You really want selectivity or the ability to selectively nock down the noise and amplify the intelligent parts of voice data to hear what is going on. So the opposite of what people think they want on transmit audio. Darth Vader is hard to understand over CB in harsh conditions.

This is why a transistor swap is never enough you have to have some sort of filtering at the very least and more sophisticated and controllable the better!

Trying to put that into an amp will be hit and miss due to stray rf and other unknowns!
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