Torben
Well-Known Member
Hi all,
Preface: this is a bit of a MacGyver problem. I have only a little information on it. I'm over 1000 km from the problem, the people giving me information on the problem know far less even than I do about electronics (what's a resistor?), and they have basically no resources available to them to help solve it. Anybody up for some jerry-rigging?
OK. So I got a call from my kid sister today. Her son decided to make a balloon speaker for his science project. For those who haven't heard of this (I hadn't until tonight, although I know of similar ideas), you can see the video he used as a basis here: How to Make a Speaker from a Balloon - Video
So my nephew took a length of speaker wire, ripped it in half lengthwise, and made a 25-turn coil about 2.5" in diameter. Stuck to the side of a balloon and plugged into a stereo, and with a magnet placed inside the coil, they heard sound. It got him through the local science fair and now he's going to the district science fair.
So far so good. The problem is that they've killed 1, maybe 2 out of the 3 stereos they've used it with. It works for a while, then becomes inaudible. One of the amplifiers worked with its original speakers afterward, and another amp didn't. The third one they're not sure.
My suspicion is that the impedance is too low for the amplifiers and the amp output stages are dying from trying to feed what is almost a short circuit. I expect they're rated for either 4Ω or 8Ω speakers but I doubt this thing is presenting that much impedance.
My first thought was to stick a couple of low-Ohm power resistors in series with the speaker. (Yes, I know that's adding resistance, not impedance, but my concern is protecting the amplifier, not the frequency response.) The problem there is that they stand almost no chance of finding such a thing in their town (a tiny northern logging town) before Thursday, when the district science fair is.
Next thought: make another coil like the one they have and put it in series. Problem: procuring another magnet in time, plus it might work and might not.
Current thought: put an incandescent light bulb in series with the speaker coil. It should reduce the volume somewhat and mess with the frequency response, but that's no problem. This is proof-of-concept only and really only has to last long enough to be judged. It would be cool if they could let it run the whole time but really it just has to prove to the judges that it will do what it's intended to do, which is act as a transducer.
I told my sister to hit the junk shops and try to pick up another stereo or two for the science fair as backups, but any thoughts on how to protect the amps would be great.
Again, it doesn't have to be loud, and doesn't have to sound good. It just has to make noise while the judges are there, and hopefully not kill the amp it's connected to.
So. Thoughts? I currently think the light bulb is the best bet. I was thinking a regular old truck taillight bulb ought to do the trick, and they could find one easily.
Thanks hugely for any ideas,
Torben
Preface: this is a bit of a MacGyver problem. I have only a little information on it. I'm over 1000 km from the problem, the people giving me information on the problem know far less even than I do about electronics (what's a resistor?), and they have basically no resources available to them to help solve it. Anybody up for some jerry-rigging?
OK. So I got a call from my kid sister today. Her son decided to make a balloon speaker for his science project. For those who haven't heard of this (I hadn't until tonight, although I know of similar ideas), you can see the video he used as a basis here: How to Make a Speaker from a Balloon - Video
So my nephew took a length of speaker wire, ripped it in half lengthwise, and made a 25-turn coil about 2.5" in diameter. Stuck to the side of a balloon and plugged into a stereo, and with a magnet placed inside the coil, they heard sound. It got him through the local science fair and now he's going to the district science fair.
So far so good. The problem is that they've killed 1, maybe 2 out of the 3 stereos they've used it with. It works for a while, then becomes inaudible. One of the amplifiers worked with its original speakers afterward, and another amp didn't. The third one they're not sure.
My suspicion is that the impedance is too low for the amplifiers and the amp output stages are dying from trying to feed what is almost a short circuit. I expect they're rated for either 4Ω or 8Ω speakers but I doubt this thing is presenting that much impedance.
My first thought was to stick a couple of low-Ohm power resistors in series with the speaker. (Yes, I know that's adding resistance, not impedance, but my concern is protecting the amplifier, not the frequency response.) The problem there is that they stand almost no chance of finding such a thing in their town (a tiny northern logging town) before Thursday, when the district science fair is.
Next thought: make another coil like the one they have and put it in series. Problem: procuring another magnet in time, plus it might work and might not.
Current thought: put an incandescent light bulb in series with the speaker coil. It should reduce the volume somewhat and mess with the frequency response, but that's no problem. This is proof-of-concept only and really only has to last long enough to be judged. It would be cool if they could let it run the whole time but really it just has to prove to the judges that it will do what it's intended to do, which is act as a transducer.
I told my sister to hit the junk shops and try to pick up another stereo or two for the science fair as backups, but any thoughts on how to protect the amps would be great.
Again, it doesn't have to be loud, and doesn't have to sound good. It just has to make noise while the judges are there, and hopefully not kill the amp it's connected to.
So. Thoughts? I currently think the light bulb is the best bet. I was thinking a regular old truck taillight bulb ought to do the trick, and they could find one easily.
Thanks hugely for any ideas,
Torben