A Preamp for the SDR Hardware


Figure 1:  A pre-amp tossed together on a piece of proto-board. Click to enlarge.

Posted 09/27/2016:

The performance of the Soft66 Lite has been alright, given that the price of the little radio is about what a couple fast food dinners cost, so I’ve no complaints.  But, it isn’t playing like the $500 shortwave set at this point. I thought maybe some shoes could help it get a better footing, and bring those HF WX faxes down to earth with more clarity.  I thought it wouldn’t cost much to give it a shot.”

My thinking went along the lines that much of the trouble was selectivity, so a pre-amp could add a little of that, too – via the LC circuit on its front-end.  The only controls necessary are the 365 pf variable capacitor (not shown), and the RF gain potentiometer, both of which can be brought out to the front panel of the Hammond aluminum enclosure (I love those things!).  Internally, there’s a bias adjustment pot, and a coarse RF gain control.

Looking at the Hammond box, I decided that much room is wasted inside of it.  Then a thought struck me.  Instead of connecting the pre-amp to the SDR hardware: “Why not incorporate another SDR board right inside of the enclosure, since they’re so inexpensive at ~$20”

So, that’s the tack we’ll take (we’ll include another SDR board inside the pre-amp).  Then, it won’t be just a pre-amp.  I ordered the parts from BGMicro (plug here) – for about $17.   But, since that order contained far more parts than necessary (since things were just as cheap to buy in lots, where I could have stuff leftover when I was done) – I probably have about $7 in the preamp, excluding the case and the variable capacitor.  The latter was $13 (ouch).  But, you see, they don’t make them anymore.  Life is tough for an analog guy.

The T50-6 core is a little small for low HF, and I may be upgrading that.  I ran out of enameled wire, and temporarily used some thin telco stuff.  The enameled stuff is on order though.

The preamp is one that I mentioned before, which was designed by Phillip (KO6BB).  I’ve modded it a little.  It’s probably a little power hungry, but I haven’t yet measured that.  I need to do some quick DC circuit checking to make sure all those blobs on the back of the boards are not shorted to other blobs.  (Note I usually do NOT show the backs of my proto boards!)

The pre-amp schematic:


To be continued …


Has it been a month already?  We’re back, and we’ve put the preamp together:


Figure 2:  The pre-amp pieces and parts, sans interboard wiring.

Putting the parts in the box was easy.  I flipped the switch and heard a loud thump-thump-thump come from the speakers.  Looking at the spectral display, I saw dozens of images and other extraneous signals.  My guess was that the preamp was going into oscillation like its life depended on it. My workbench (what others may call a dryer/washing machine combo) – shows the unit, before I wired the boards together.  Below, the unit is shown with the lid screwed onto it.  You know how some guys shine their cars?  Well, with me it’s the Hammond aluminum enclosures that get the wax.


Figure 3: The pre-amp, with its lid in place.

The variable cap is on the left, the RF gain control is on the right.  About the thumping?  There was too much coupling between the Soft66 Lite board and the high gain pre-amp, and after trying to add shielding, with no result, I decided to pull the SDR.  Maybe it will go into the tablet enclosure itself now, instead of the preamp box.  With the SDR separated from the preamp, the amplifier works fine when it’s alone in the box, adding about 10 dB to my Quisk signal display.  Does it help with the WX faxes?  It’s hard to say, but my initial reaction to a single night’s worth of faxes, is that the improvement was slight – as the noise does rise with the additional signal.  I think more of the advantage is the extra preselection the LC circuit gives me – which lets me “tune” to the desired frequency about as well as the clunky old MJF tuner (that I had been using).  Much easier to carry this!


FIgure 4:   A snip from a WX chart, using the preamp. 

Figure 4 shows a snip from the first night’s testing.  Conditions were not optimal, IMO.  Nevertheless, the chart is readable (if you know how to read weather charts 🙂 .   Yet …

As time rolls on, I’m inclined to think that this setup will not be the WX station.  I’ll have a little more complex radio for that.  But, I have tons of other uses for this SDR + preamp combination.  Stay tuned …

Figure : Who need decals when a family member has good nail polish?


Note: the soft66 Lite hardware is a product that is sold on a Japanese website (http://zao.jp/radio) – and is not affiliated with this site or author in any way.


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