I've redone the circuit a little bit, this time using two batteries and setting the ground point between -9V and 9V. This way I don't have to put in the voltage dividers.
Here are the schematics of what I tried:
EDIT: I've reduce R3 from 6875 to 5k to get rid of the clipping distortion. So, R3 is now 5k rather than 6.9k.
EDIT2: Ok, I just realized that the gain for the darlington is MULTIPLIED by the second transistor XD I thought the Betas added together but apparently they are multiplied. That generates a whole lot more current than I expected! That explains why they are frying XD
I changed the op-amp... I'm not actually sure why. I think it was because I wasn't sure if the TL022 would get fried by the battery voltages. The new Op-Amp also has a higher short-circuit current. I guess that doesn't change all too much due to the transistor pair being placed right at it's output. I thought that since the Op-Amp will do about anything to cancel the voltage difference between the inputs, it would overcome the dead-zone of the transistors as described by the article, but the oscilloscope is clearly showing a malformed waveform close to 0V so I'm guessing it's not exactly succeeding. Is this because the Op-Amp is slow? Or can it simply not generate enough juice?
Also, I've attempted two ways of amplifying the output current. The first was feeding it through the same amplifier as the op-amp output but that wasn't really working for me, although I might have wired it wrong or used the wrong values for the resistors.
In the second schematic, I used a Darlington Pair which does ... ok amplifying the current compared to the other ways I tried but the juice running through the transistors will most certainly cook them after a short while.
I haven't been able to figure out how a non-inverting amplifier actually works. I'm using an inverting one because it's the one that I vaguely know how to wire. I'm also still having a little trouble visualizing how capacitors function and finding out the proper values I should use is a whole different story. It's ok when the caps are used alone or in some straightforward filters, but any kind of deviation makes me draw a blank as to what kind of values I should draw.
My questions are mostly the following:
What does it mean exactly to have low input impedance? Does it mean the thevenin equivalent impedance to the "left" of the feedback loop?
If I don't put a voltage divider to bias the input voltage from the phone and into the non-inverting input, won't the amplifier clip the negative portions of the amplified voltages? What I mean is, since in the first schematic ground is the most negative point in the circuit, then if the phone causes the op-amp to have to generate a negative voltage (between output and inverting input) , it seems to me like it wouldn't be able to do it. If the op-amp had a gain of 5, then if the input was 1V, would it be able to generate -5V?
Is it a good solution to use two batteries set up like the above (split voltage)? How does one deal if there is only one voltage source? It seems to me like making the ground point as the midpoint between the highest and lowest voltage would be unfeasible in that situation.
If I stick a resistor in series with the capacitor that couples the phone signal, how do I find the value of the capacitor I need?
And that'll be about all ^.^ I'm sure I'll have more questions as they arise, but for now, that's all I could think off. This is the first circuit I've ever made from scratch and I'm sure the clueless nature of the design shows very clearly. XD Well, one has to start from somewhere.