Quest to demo ULTRA low frequencies into a woofer

I'd like to do demonstrations of the cone moving back and forth slowly, meaning like 1 Hz or actually more like 0.1 Hz.

This is not a trivial undertaking, especially since I want to spend as little as possible.
AND, because I am not going to start hand building anything-I don't have time and it is not a forte of mine.

I know there are sine generators that can go that low, maybe available used. There are also online signal generators, but I haven't checked if they will go that low because I'm imagining no laptop will actually output such a low frequency.

And THEN, you need essentially a DC coupled amp, or blocking capacitors inside won't output those frequencies either. DRA Labs (maker of MLSSA) used to recommend a Rolls amplifier but I don't see that on their site any more, and it was not inexpensive.

A kinda sorta alternative would be a variable DC supply, just turn the DC up and down and I guess the cone would move out and back, but not inward and back. A supply that could continuously adjust to negative as well would move the cone. But with this kind of setup you don't see the periodic motion.

Ideas?
 
HP made a number of DC amplifier/power supply models that show up at reasonable prices on eBay from time to time. The 6823A and 6824A are examples. Connect one of these to a function generator with subsonic capabilities (most of 'em) and you're set. Alternatively, if cost is the dominant issue, just solder up an LM1875 power amp chip in the split supply demo circuit. I've used these for DC motor drivers, so there's no doubt they can do the job.
 
Correction: The LM1875 split supply demo circuit in the datasheet is AC coupled. To make this a true DC amplifier, the input and feedback DC blocking caps have to be eliminated (shorted out). In my motor driver circuit, I reduced gain to 10X and added an input offset trimmer. The trimmer probably isn't necessary in your application. Let me know if you want to see the circuit drawing.

I see that LM1875 mono amplifier boards go for less than five bucks on eBay. They probably use the demo circuit verbatim.
 
Last edited: