The key question is, what is the best or right way to modify input impedance?
I am a self-teaching novice at electronics. I'm using the LM386 as a learning platform. Signal input comes from a "headphone" type jack (tip).
I have tried to use line out from a small mp3 player as a test source, and it works, using the datasheet example minimal circuit. It was very tinny and distorted at 200x gain, somewhat better at 20x (default) gain, but not great. My goal from here is to learn to improve the circuit. I think I will want higher gain but I want higher fidelity at low gain before I worry about that. Fidelity is not great even at low gain and low output volume and I assume that's my fault and fixable due to the enduring popularity of this chip.
I have noticed when using the mp3 player the circuit seems more stable and less distorted. But when I plug in the guitar instead the circuit seems more distorted, less true to the frequency response across the guitar audio spectrum, less polyphonic, and I even think it drains the battery faster and makes it hotter.
This has led me to try to understand impedance matching. The guitar is said to be a very high impedance device. I have learned that modern impedance "matching" really is about making sure output impedance is as low as possible and input impedance is relatively high, because this is optimal for transference of signal via voltage, rather than power. I think I need to attenuate the input so the high impedance guitar output is handled better, basically so signal is not clipped or otherwise distorted.
Here is my big question: Many diagrams that describe impedance matching issues show "representative" resistors at the output of device "A" and at the input of device "B". Here is a link to such a diagram:
From containing page:
What puzzles me is the input representation shows the resistor basically in parallel with the input of device B, while the output representation shows the resistor in series with the output of device A. It seems to me that these are two very different things. Device A representation seems logical, as a resistance in series would increase the total resistance of the output. But the Device B representation is illogical to me. Adding a resistor in parallel should REDUCE the total resistance of this part of the circuit by adding a "new path" for current to follow. So it doesn't seem to represent what it means to. It means to represent the resistance to the input.... but it is not doing that, it's reducing that resistance if anything. I understand it is being compared to a voltage divider, so adding any second load would reduce the available voltage that goes down the first leg since they have to share. So I guess I can see that either approach could reduce the amplitude of the input signal. I'm just not sure why this representation is chosen to explain the theory, we are talking about input impedance so why not conceptualize it in series? I think I am missing something here, probably something important.
For my bench circuit to make my guitar sound better, I would think I'd want to add some resistance in series at the input of the LM386 which is MY Device "B" (the guitar is my Device A). I've read 1M ohm at the input is actually a reasonable estimate to attenuate for incoming electric guitar signal. My practical question is where do I put it? In parallel at the signal input pin as the theory diagram shows? In series at the signal input pin as seems more logical to me? What am I missing?
Please answer the theoretical question with sentences and metaphorical type answers (IE compare to water flow or such) if at all possible. I can look up the math anywhere and don't need to see it again. I'm asking here because I want some expert who understands this perfectly to provide the more metaphorical understanding. Thank you in advance for your assistance. I really want to understand in general how to combine multi-stage components. For example, I have a PT2399 I got in a sort of grab bag, and I want to add some "reverb" to the fun. I've actually tried that and I have gotten some results (not musical ones yet... too much high frequency in the echo). But that portion is put on hold while I get the LM386 to sound decent alone.
The example use of the LM386 is given as reference so you know what I'm attacking (as learning experience), and this question can be answered in direct practical terms. What should I do?. I also welcome comments about the LM386 in general if that's something you want to add. I already have found some good references on that such as...
but feel free to add your two cents. That's not my main question though.
For reference and clarity:
A) Why is the input impedance conceptualized as a resistor in parallel in the diagram? What is the important concept I'm missing here?
B) Where do I put the 1M resistor I want to experiment with? I know I can try both and I probably will. I just want the experts to give thoughts/guidance also.