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preamp
- Thread starter axehandle
- Start date
That's a lot of work ! You'd have to not only build a pre-amp board, but you also have to add relay(s), and circuitry to make sure no output signal gets into the preamp circuit. Otherwise......POOF! Out comes the magic smoke........
You can buy an RM Italy amp with a preamp online for peanuts. If you value your time, it's a much cheaper option too......
Other consideration is that pre-amps just amplify everything, signal, static and whatever else.... You might as well just turn up the volume knob. You're not really gaining any kind of usefulness from the pre-amp. I have them on all my amps but never ever use them.
The preamp built into a lot of linears is poorly protected from the radio's transmit energy. Best way to "poof" the receiver preamp in your Palomar, Varmint or such is to flip the SSB switch. When you key the mike in AM, the radio's carrier will cause the linear's relay to switch over quickly, and only a very-brief burst of your transmit energy gets pumped up the anus of the preamp circuit. A diode or two usually provides protection in the preamp circuit. The key here is a "brief" burst of RF energy. Keeps the total energy pumped into that protection diode below the damage threshold.
When the sideband switch is selected, the relay will "hang" and delay dropping back to receive mode to prevent the relay from chattering, since there is no continuous carrier in sideband modes.
The problem is that the SSB delay circuit ALSO delays the relay switching over when you first speak up. The longer interval it takes to switch over from receive to transmit causes that burst of RF into the preamp take a LOT longer. Long enough to blow out the preamp circuit, most typically.
And if all you care about is AM, that simplifies things a bit. The simple untuned preamp circuits work best if you are not near any other source of RF energy. A police repeater down the street, a local AM station a couple of miles away won't bleed the average external preamp, since they usually contain tuned circuits for the band you're listening to. This suppresses bleedover from other frequencies. Preamps built into a linear almost never include any frequency-selective circuits at all.
An external preamp is just better.
73
When the sideband switch is selected, the relay will "hang" and delay dropping back to receive mode to prevent the relay from chattering, since there is no continuous carrier in sideband modes.
The problem is that the SSB delay circuit ALSO delays the relay switching over when you first speak up. The longer interval it takes to switch over from receive to transmit causes that burst of RF into the preamp take a LOT longer. Long enough to blow out the preamp circuit, most typically.
And if all you care about is AM, that simplifies things a bit. The simple untuned preamp circuits work best if you are not near any other source of RF energy. A police repeater down the street, a local AM station a couple of miles away won't bleed the average external preamp, since they usually contain tuned circuits for the band you're listening to. This suppresses bleedover from other frequencies. Preamps built into a linear almost never include any frequency-selective circuits at all.
An external preamp is just better.
73
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