Hey, thanks, Mike. That works a
lot better. I'm obviously getting a lot of gain out of that stage.
So here's what I ended up with:
**broken link removed**
Still some problems. (I should point out that a lot of my trouble is due to the funky construction method; I'm using a prototype board, and it's a snarled-up mess of components and wires.)
I tried cascading two stages like the one you suggested, but got absolutely no output out of the 2nd stage. I tried all the op amps separately and they all work fine individually.
Anyhow, I went ahead and constructed the low-pass filter stage as shown. It works, but at a greatly reduced output. This
should be a unity-gain stage, shouldn't it? But obviously there's a loss instead of any gain. It does, however, seem to be doing its job; the high-frequency component of the sound is greatly reduced. (I got this circuit from my current textbook, Malvino's
Electronic Principles). The formula he gives for the cutoff frequency is
which should be about 34 Hz with these values.
Questions:
1. When I connect the filter stage's output to the inverting input as shown, it goes dead. It works (sort of) without it. What gives?
2. The whole idea of this exercise, besides trying to learn some stuff, is to generate subsonic noise. Unfortunately, that doesn't seem possible with this method. I suspect that the spectrum of the noise generator doesn't include much below, say, 20 Hz. (What I'm hoping for is a low-frequency flicker.) Is this because of the noise generator? or are the lower frequencies disappearing somewhere else here?
3. In your suggested circuit, what's the function of the capacitor ground for the inverting input? I've seen lots of circuits without this. Is this to establish an AC ground? or to block DC?
By the way, I went through my parts box of transistors; some were obviously much louder (= more noise output) than others. The best one was some old TO-92 marked "7128" (god knows where that came from ...).
By the way, here's a link to another similar circuit I found on the web:
**broken link removed**