I hope you’ve had success with LULU so far. If you are building for CW use, then you’re done. However, this time around we will be a-modulating so we need to discuss some construction techniques before we start. Most builders aren’t as anal about RFI prevention as I am when building rigs yet they rarely have problems. I have experienced that too. However, I have also spent many hours trying to hunt down the source of hum, or whine, or RF hash that is stubbornly haunting my project. I have had rigs that worked fine for a time, then a bypass cap opens up… somewhere, and I’m off playing the feckless RFI hunter again. My philosophy is this: if you overbuild as you go along, the odds of having problems later on are greatly reduced. In that spirit, I give you some RFI-fighting tips to consider as you lay out your chassis pan. Best case would have the entire RF strip mounted inside a small metal box or RF shield inside the larger enclosure. With LULU being so compact and cool running (Class E), a large enclosure is not needed. And in this way, that sassy little hash generator is kept under wraps. Second, any signal wire runs use shielded wire that is grounded on ONE end only. Other long runs are wound through ferrites. And third: redundant bypass caps are used at most input/output ports in the chassis. Most of this is probably unnecessary, but I sleep better. Maybe you will too.
Purposely for this build I have IGNORED most of these “extras” as you can see in the photo. I have kept the RF hammerlock in the modulated voltage line, the shielded audio input lead, and most of the bypassing caps – you should too – but I have left out everything else including all ferrites. I have experienced no problems. But this build was to illustrate this article and will rarely be used. Any of my serious builds utilize all those tips I just gave you. It’s your choice.
In the past, little rigs in the 10 watt realm were modulated with a monolithic amp chip in a single-ended configuration. Unfortunately, the typical LM383 or TDA2003 starts gasping for air before the modulator’s peak power needs are met. Also, with a single-ended configuration, one side of the modulation transformer’s audio winding is grounded, which under some conditions can form a ground loop and cause a lot of hum. The better way to modulate is to use a pair of TDA2003s in a bridged amp configuration. This solves both problems. The bridged pair circuit is shown above.
I have probably built half a dozen of these and they work well. However, I have a better solution for you. Cana Kit now features the bridged pair in kit form for 20 bucks plus shipping http://www.canakit.com/20w-bridged-audio-amplifier-kit-ck193-uk193.html. That pre-made circuit board will save you a tremendous amount of time and cursing. I will never build one of these from scratch again – ever. Only you can determine what your time (and sanity) is worth. A second option: Parts Express has a Bridgeable Class D Amplifier kit for 20 bucks that uses the Texas Instruments TPA3122. I am just now experimenting with my first one of these. I don’t know if it will survive as a modulator over the long haul. Caveat Emptor…
Back in the day Boomer the Radio Animal rewound the venerable Radio Shack hash choke to use in his Grenades. Unfortunately, they are no longer available. Dave Martin uses a transformer pulled from a now defunct Taxi radio for his Commandos. They aren’t available Stateside, and now sources in the UK seem to be drying up too. Wind-able transformer iron in the small size range is tough to find. You can find a lot of cores with interleaved E and I sections, but none with the separate parts like the old RS hash choke had. There are a few websites that show how to separate interleaved cores. I’ve tried it. It’s about as do-able as shoving a wet noodle up a wildcat’s butt. I decided to sidestep this whole issue and develop a toroidal transformer that would modulate well. This is not a new idea. However, most folks shy away from toroids because they are hard to wind, should you need a lot of windings as with a modulation transformer. I’ll let you in on a little secret – they’re hard, but not THAT hard.
There are two downsides to using a toroid for this. One: good luck trying to find a T200-26 core locally. You’re gonna have to order one (or two?) from CWS Bytemark. At this time they’re only about 3 bucks apiece. You can stock up on your T50-2s and T80-2s on the same order. Two: They take up a lot of space. With the windings, it’s going to hog a footprint of 6 or 7 sq in. The upside is that the audio is decent and the coupling is good enough to allow full modulation of LULU’s carrier signal. Also, the #26 material has flat response across the entire audio range.
The “mod tranny” is basically an impedance matcher. Your tranny’s winding ratio will depend upon how good a tweaker you were when building LULU’s RF strip. If you feel unworthy because you could only get 8 or 10 watts out, then cheer up. Your winding task will be much easier than that of the overachiever. And you thought more was always better…
For up to 12 watts of RF output a 1:1 impedance ratio will work fine. The amplifier has plenty of power capability to compensate for any mismatch. Cut a couple of 7.5 foot lengths of 18 gauge magwire. Secure one end of each in the chuck of a variable speed electric drill. Secure the other pair of ends in a vise, or vice grips, or between your spouse’s teeth. Using short bursts of the drill, spin them together until they have a couple of turns to the inch or so. Then wind both windings at the same time. Keep the turns tight to each other on the INSIDE of the core with no overlapping. I was able to get 42 turns on the one in the photo. Less stress… Less pain in that arthritic wrist…
If you were able to tweak more output from LULU then you’ll need two separate windings. And this one really IS a tedious transformer to wind. Wrap the audio winding first. You should fill the INSIDE of the toroid, keeping the turns tight with no overwinds. If you use 18 gauge magwire you should be able to get about 80 turns on it. A single layer of masking tape wrapped around the outside (like a belt) will help hold the windings in place.
To calculate the DC winding, first get a decent measure of your RF output BEFORE the harmonic filter (at the C9/L2 junction). To find the approximate impedance at that point:
1) Square the input voltage (12 * 12 = 144) and then divide the result by TWICE your measured power. For example, if you measured your unfiltered output at 16 watts, your drain impedance would be approximately 144 / 32 = 4.5 ohms.
2) The output impedance of the bridged audio amplifier is 8 ohms. So the impedance ratio for our example would be: 4.5 / 8 = 0.5625. Then take the square root of that result (0.75). This is your required turns ratio.
3) If you have an 80 turn audio winding, multiply that by the turns ratio (80 * 0.75 = 60). You need 60 turns of wire overtop for your DC winding. I should tell you to spread it evenly around the toroid. However, it is mighty hard to do. Just wind it as best you can WITHOUT OVERWINDS. Admittedly this will take patience. But hey, you only have to do it once…
If you use LULU as is you will be a “wide Clyde”. For AM communications work you must neck the audio down. An outboard mic preamp or speech processor with a 3KHz audio cutoff filter is recommended. For program content, the audio input is set up to receive a Walkman or MP3 player type output. Pre-processing should be considered for increased readability of your signal. Most audio software will do a good job. Personally, I use Audacity to add compression and to roll off the highs above 8KHz with the EQ. If you are a live kind of guy or gal, processing should be done at line level following your mixer. A small line amp built from an LM 386 or similar can then boost the processed signal to LULU’s required drive level. Good luck with this, have fun, and be safe out there!