The Bean-Counter AM Transmitter

Posted on Feb 6, 2014

The Muntz one-tube design can be considerably improved by adding an audio preamplifier stage. This will give improved sensitivity, and thus give full modulation with lower-level signal sources (such as some older CD players, ceramic phono cartridge, etc. ). Furthermore, the use of negative feedback around the whole transmitter improves linearity

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and allows for deeper modulation levels without distortion. While the one-tube transmitter works well for such a simple circuit, it does have significant drawbacks. One problem is that the input sensitivity is a too low for some common input sources. Another is that linearity could be improved; the one-tube basic transmitter has a somewhat strident audio quality at higher modulations because of harmonics introduced by the non-linearity. Adding a second tube (as an audio preamp) solves the first problem, and improves the second. This is essentially "The Muntz" with a preamp stage, consisting of a 6AU6 pentode. This gives lots of gain, in fact enough for even high-output microphones. However, since this circuit is most useful to us as a line-level amplifier (in the range of 0. 1 to 2 volts RMS), we can readily afford to trade some of this gain for fidelity by adding an overall negative feedback loop. The network around the pair of germanium diodes full-wave rectifies the modulated RF output, just like the detector in a radio receiver. This recovered audio is fed back, out of phase, to the input of the preamp. While germanium diodes are less linear than vacuum diodes, this is mollified by the fact that they`re inside the feedback loop. We haven`t changed anything in the RF section, so the output power of this version is still only 3 milliwatts, but plenty for in-house broadcasting (or if your linear has enough gain). What`s...

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