Now that i understand FM transmitters better i built my own variation on the 1-transistor TX, and i need a power supply for it. Last time i fooled around with such things i would get HUM, HUM and MORE HUM, so i need a little help.
My transmitter is basically a spin-off of this design:
Code:
http://www.techlib.com/Karen.htm#FM Transmitter
but i skipped the pre-emphasis (i do it in software) and the tuned circuit has different values, i just used whatever i had around.
It works and sounds great, and has excellent stability for just one transistor, it's far better than the 3-transistor TX i used to have, that drifted frequency even if i just moved its antenna around (now i understand why, the antenna was a part of the tuned circuit in that tx). I wanted to add a buffer stage but it seems to do without, since i only need it to cover my house. I do have ONE problem with it - frequency drifts downwards about 0.05MHz every half hour, due to my cheap 9 volt battery draining. That is no problem for an analog radio but my Yamaha PLL tuner has to be readjusted when it happens.
So i want to make a power supply for this thing. Trouble is, if it gets within 50cm of anything plugged into the mains, it gets lots of hum. On my old TX i had a 10000uF cap followed by a LM317 and that wasn't enough, i would still have hum at about -20dB which is quite noticeable. I'm thinking a capacitance multiplier power supply would be better? Followed by a discrete regulator instead of a LM317 maybe? I noticed that the 317 does add a bit of hiss of its own.
I'd appreciate any insight.