Over the past few days I've been searching the internet for a decent FM Radio schematic, so far I've found tons of kits, transmitters, and those things they use to teach children about electronics (that couldn't pick up a radio station if it were coupled to the transmitter), but nothing really useful. I'm building a table radio, I have an excellent design for the cabinet but nothing to put in it. What I need is a transistor receiver covering the regular US FM band (88-108MHz) with relatively good sensitivity and selectivity, and the ability to drive one or two six inch (8 ohm) speakers. Stereo would be nice, but isn't necessary. I'd like to do this with discrete components, but ICs would be alright as long as the sensitivity and sound are good. Any help with this would be greatly appreciated.
Tuners aren't really as hard as you may think. I've been rebuilding old radios for years and have seen sets with as few as four tubes for AM or six or seven transistors for FM that can acheive relatively good fidelity with such a small speaker. The problem that I'm having so far is that all of the circuits I've found on the net are much too simple, one or two transistors, with the ability to possibly pick up one or two strong stations. The closest I've come so far is this circuit, which is interesting, but it uses an obselete IC, model TDA7000, that hasn't been manufactured for years, and none of my suppliers know of a cross-reference for it.
It doesn't have to be ultra hi-fi or super sophisticated, so a schematic from an older (1980s or so) portable could do nicely. I have a book with with about thirty or so designs for FM radios and even more for AM, if only I could find the blasted thing.
What would be ideal would be a small tuner circuit, around four transistors or an IC, which outputs a line-level audio signal, then I could use one of the many small amplifiers I've built over the years to drive the speaker.
I have an excellent AM/FM portable from the 70s (A little rectangular thing made by GE) that plays like a champ and uses eight transistors (and lots of "fun" little trimmers to spend hours playing with). I could probably duplicate it with some effort, but I'd much rather have a schematic to go by. In any case, I hope I can find the book soon.
It doesn't have to be ultra hi-fi or super sophisticated, so a schematic from an older (1980s or so) portable could do nicely. I have a book with with about thirty or so designs for FM radios and even more for AM, if only I could find the blasted thing.
What would be ideal would be a small tuner circuit, around four transistors or an IC, which outputs a line-level audio signal, then I could use one of the many small amplifiers I've built over the years to drive the speaker. I have an excellent AM/FM portable from the 70s (A little rectangular thing made by GE) that plays like a champ and uses eight transistors (and lots of "fun" little trimmers to spend hours playing with). I could probably duplicate it with some effort, but I'd much rather have a schematic to go by. In any case, I hope I can find the book soon.
I thought about it, but I enjoy torturing myself with problems like this. This is going to be a gift, and I really wanted it to be totally home made. If I get frustrated enough and can't find a proper design in the next week or so I may have to do just that. I have another nine-volt portable from the mid-60s with a damaged case that I could recap and build a power supply for, but it just seems a little too easy.
Many mp3 players now have FM radios built in. They must be using a single IC for the receiver circuit. If you can obtain the IC you can build your own radio. The easiest/cheapest way to obtain the IC is probably to buy an mp3 player and extend the user interface to the front panel of your box and add an audio amp to drive speaker(s).
I_F
I_F
AudioFox said:
http://www.cricklewoodelectronics.com/Cricklewood/search.php?mode=search&page=1
good luck
Andy
poynton said:
Thanks. They don't ship to the US, but I did find one
here for a small sum. What's really a shame is that I found one in an old car radio about four hours after I ordered it, but at least I'll have a spare. Apparently there were a lot of these things (link includes schematic) floating around back in the '80s and early '90s, a lot of them sold by Tandy for hobbyist use. Its a shame they don't make these any more, an entire 1.5μV sensitive superhet receiver on a single IC sounds pretty nice, no trimmers to mess with, and its tuned with a pot.
Old OEM car radios are virtually free, and should offer excellent RF performance since they had to operate under constantly varying reception conditions. Adding a 12V supply to one should make a great table radio.
Some also supported AM stereo; a friend had that in his '80s K-car, and it didn't sound bad at all.
Some also supported AM stereo; a friend had that in his '80s K-car, and it didn't sound bad at all.
The ZN414 chip was a real amazing one when it came out (late 70's, IIRC). It was a TRF type AM receiver in a 3 lead can that looked like a transistor. It was commonly found in behind-the-ear miniature radios that were powered by a hearing aid battery and tuned via a slug-tuned coil with a fixed capacitor. You could make SW radios out of them by tweaking the tuned circuit a little. Back then there was something to listen to on SW besides big-haired preachers begging for money so they can stay on the air and beg for more money.
That reminds me of another great chip. The Votrax speech synthesizer chip. It was the first (that I know of) phoneme based speech synthesizer that could be made to say anything, unlike the TI chips that were formant based and could only say prerecorded stuff. I built up a Votrax chip board and connected it to the printer port on my commodore 64 and had it talking up a storm.
I_F
That reminds me of another great chip. The Votrax speech synthesizer chip. It was the first (that I know of) phoneme based speech synthesizer that could be made to say anything, unlike the TI chips that were formant based and could only say prerecorded stuff. I built up a Votrax chip board and connected it to the printer port on my commodore 64 and had it talking up a storm.
I_F
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