How would you like to have high fidelity audio without much work and still keep your treasured boatanchor stock? Sound too good to be true? Guess what? It's not. This idea is not new, it has been done by numerous people over the years. I finally got around to trying it out recently on a Viking II. It works great! Using an outboard solid state (or tube) hi-fi modulator on 100 watt or less class rigs like Vikings, DX-100's, 32V's and Rangers has several benefits. There is no need to modify any of the stock circuitry in your rig. All your audio is now outboard and will be capable of producing audio quality superior to the inboard circuitry - even with the most radical modifications. You'll need the following: 1) a hi-fi audio amplifier capable of producing at least one half the carrier input power of the rig you want to modulate. An amp capable of power equal to your final input power is preferable. You can use tube or solid state varieties. It's usually easier to find a solid state type when your looking for something that will modulate a 100 watt rig. Keep an eye open for PA type amps at hamfests. These can often be had cheaply and most have microphone inputs, a benefit that will be detailed later 2) An output transformer from a tube type amplifier that was rated at or near the power specified in 1, above. 3) An inductor of 30 to 50 Henries, capable of handling the current drawn by the final amplifier stage to be modulated. If you can't find one inductor to fit this requirement, several can be series or paralleled, as appropriate. Power supply type filter chokes work very well. 4) A 2-4 uF cap with a voltage rating of 3-4 times the B+ on the final. That's it. Now, all you need to do is hook it all up in the proper manner and interface it to your rig. I'll leave the interfacing to you rig put up to you and your particular tastes and ingenuity. Figure 1 shows how to connect the components together. Notice that no current flows through the secondary (the primary in its original use) of the transformer. This arrangement is called modified Heising because the modulating |
signal is developed across the inductance (or modulation reactor) that is is series with the B+, much the same as in Heising modulation. It is required here because the secondary of the transformer is not designed to carry unbalanced DC current and would saturate. This condition will produce severe distortion and is probably not good for the health of the transformer. So, don't get any wise ideas and think you can save some time or money by skipping the inductor(s) or capacitor when hooking things up! The only other consideration is how to interface your microphone to the audio amplifier. If you are using a PA type amp, you're probably ready to go, as most have microphone inputs. If your using a home stereo type amp, you'll need to use or build a microphone preamplifier. Although this extra piece of gear or circuitry may seem like a hassle, it has some advantages. Once your microphone output has been boosted to line level, it can drive equalizers, compressors and other audio processing gear, that will in turn drive the audio amplifier/modulator. The addition of this type of gear will allow you to tailor your sound and more fully modulate your rig. If you are lucky enough to have or obtain an audio amp and transformer that can handle more audio than you need, consider using them in conjunction with the Three Diode Negative-Peak Limiter circuit. With this arrangement, you'll be able to modulate in excess of 100 percent on positive modulation peaks yet not go over 100 percent on the negative peaks (no splatter). This greater level of modulation will give your signal more punch and can easily make your 100 watt rig sound as loud as several hundred watts! So, if your looking for better sounding audio, with more punch, but don't want to modify your rig because you'd like to keep it stock (or you're just lazy, like me), consider this quick and not very dirty method.
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