A simple preamp and preselector

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Andrew (grayhat)
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Joined: Sat May 28, 2022 5:56 am
Location: JN63pn

A simple preamp and preselector

Post by Andrew (grayhat) »

This one is for the folks on the forum which feel ok with a soldering iron in their hands, this PDF document from George R. Steber (WB9LVI)

http://www.arrl.org/files/file/QEX_Next ... Steber.pdf

shows how to build a varactor tuned preamplifier which can also act, to some extent, as a preselector for your receiver, the circuit shown covers the range going from 6 to 23MHz but the document explains how to change the two inductors to cover other ranges, so it's possible to add a rotary switch (or some microrelays) and obtain a tunable preamp covering the whole SW bands range
steber_pre.png
steber_pre.png (180.88 KiB) Viewed 562 times
The resulting unit may then be used with whatever antenna, not just that, since the preamp is tuned using a varactor diode, that is, just changing the voltage to the diode, with a bit of modification, the unit may be turned into a remote tuned preamp having the preamp unit at the antenna and a control box in the shack to control band and tuning
Blubby
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Joined: Wed Jun 15, 2022 5:52 am

Re: A simple preamp and preselector

Post by Blubby »

Thanks for posting this. I am going to try it without the varactor and use a single turn loop
instead of the network and see what kind of silly active loop I come up with. I will tune with a
variable capacitor across the loop.
I will let you know how it works!
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Andrew (grayhat)
Posts: 210
Joined: Sat May 28, 2022 5:56 am
Location: JN63pn

Re: A simple preamp and preselector

Post by Andrew (grayhat) »

Blubby wrote: Thu Jun 16, 2022 12:17 am Thanks for posting this. I am going to try it without the varactor and use a single turn loop
instead of the network and see what kind of silly active loop I come up with. I will tune with a
variable capacitor across the loop.
I will let you know how it works!
Removing the tuned stage makes very little sense, since the preamp stage alone isn't special, what makes the whole unit interesting is the fact that it can be tuned to the desired slice of frequency, this means that the preamp input will only deal with the tuned signal (and will have much less problems with noise and overloading); plus, since the tuning is achieved through a varactor, the whole unit may be built as a "remote" preamp with the amplification unit at the antenna and a controlbox in the shack allowing to tune the preamp through a potentiometer

Again, removing the tuned stage would turn that unit into a wideband preamp, and there already are a lot of them around, so it makes no sense modifying such a circuit to work that way, it would be a better idea adding a bandswitch (or some relays) so that, by switching the inductances the unit would be able to cover (almost) the whole SW range
Blubby
Posts: 12
Joined: Wed Jun 15, 2022 5:52 am

Re: A simple preamp and preselector

Post by Blubby »

My point was to substitute a tuned loop for the input preselector. I built a 4 band preselector for 100kc to 30 mhz
for use with an old sw radio that had poor image rejection performance. I used an fet to act as a small amount of gain
but mostly as a high impedance load for the LC network and to drive the 50 ohm receiver input. It was used with a longwire
and worked well. I tried it with a whip but gain was not great. Also noisy indoors.
I also built a hairpin loop tuned with a 1000pf tuning cap and used the same single FET amp for that. Tuning was sharp and
it made a great indoor sw antenna with preselection. I think the Hairpin was about six feet tall. It really looked like a very acute
triangle. This circuit with a similar loop attached might work well. The LC of the loop and cap provides selectivity. I have found the
loops quieter indoors than active whips or ad hoc indoor random wires as well, hence my comment.
Also...I already have all the parts. Except for the varactor.
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