Re: Another Pantry MW Thing
I found little or no hum when I kept the aerial wire away from the audio source (a cheap "Matsui" DAB tuner unit). The audio fidelity is remarkable given the simplicity of the circuit.
I mostly run mine from a 15V linear mains supply - toroidal mains transformer giving 15V AC, bridge rectifier (with 1n bypass capacitors across each diode), 1000µF 35V electrolytic smoothing (probably overkill) and a 7815 regulator IC (with 100n SM capacitors soldered to the legs - input and output to ground), then a 100µF 25V electrolytic across the output. The output 0V rail is coupled to mains earth and the whole PSU is in a small, earthed diecast box. I also have a "gel-cell" 15V rechargeable supply which I have used when away from home.
The transmitter itself lives in a diecast box, with LEDs for power, modulation peak and RF output. There are two controls on the outside of the box - mod level and power switch. The RF leaves through a BNC connector and the audio input is through a pair of "phono" sockets. Power enters through a 2.5mm power connector, allowing connection of the mains PSU or the battery pack.
I found that with a bit more wire attached to the aerial (about 8m more), an earth stake and fiddling with the match a bit, I could get quite wide coverage on a quiet medium wave frequency with the aerial wire thrown up into a tree! The little transmitter was used - with its battery pack - at a couple of open-air festivals last year. The best results were a perfectly usable signal at just about ½km! This might not be quite within the spirit of the "pantry" transmitter, but it was an interesting experiment nonetheless.
Incidentally - there's no hum whatsoever with the battery supply, as long as you keep the aerial wire away from the audio source.
I intend to put my simple audio processor details up here soon. It's just a quad op-amp and a few transistors, and provides audio bandwidth reduction and a measure of compression and limiting to the audio. In practice, it just makes the signal appear "louder" and has little effect on the treble content on most receivers.
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