A resonant filter passes only a narrow band of frequencies. Why use it?
It looks like you are using a lousy old LM324 opamp (you said 384 that is an LM384 little power amplifier). The LM324 has crossover distortion that sounds fuzzy.

You have pin 13 connected to the pin 14 output so the gain is 1 like a piece of wire. Your breadboard photo has no labels so I do not know where is the input signal. Please post a schematic of your circuit.

 

A resonant filter passes only a narrow band of frequencies. Why use it?

Because it sounds awesome - and I want to learn how to build it.

It looks like you are using a lousy old LM324 opamp (you said 384 that is an LM384 little power amplifier). The LM324 has crossover distortion that sounds fuzzy.
You have pin 13 connected to the pin 14 output so the gain is 1 like a piece of wire. Your breadboard photo has no labels so I do not know where is the input signal. Please post a schematic of your circuit.

Yes - I must have gotten a number wrong - it is a 324. I posted a labelled photo earlier on, that will have to do for a schematic - I'm constantly changing it anyway. I am aware the gain is 1 - that was my goal. Now I'm moving on to amplifying it.

The distortion is not an issue right now - signal is clear as the original at least casually listening to it.

 

Just added a 2.2k resistor inbetween output and (-) input, and a 5k pot from (-) input to 'ground' (halfway between voltages...is that 'virtual ground'?), and I am able to amplify the signal fine.

Still haven't used a cap and curious what I am screwing up with that.

Still sounds ok, though I think I'm overdriving the inputs on this guitar amp when I turn it up.

 

Your labelled photo in post #5 shows the GND and VCC labelled backwards.
With 2.2k and a 5k pot the maximum gain is 1+ (5/2.2)= 3.27 times which is not much but is fine for a guitar magnetic pickup.

 

Your labelled photo in post #5 shows the GND and VCC labelled backwards.

Good catch - at least I have it correct in it's current iteration.
Don't always trust the internet, kids!

 

I've got a new question on theory.
I solved my problem by using an equal-resistor voltage divider and using that as my audio ground reference.

The other suggestion was to bias my actual audio signal.

What are the advantages / disadvantages to either way, and why might I choose biasing the input vs splitting the rail and getting ground from there?

The one benefit I see, is by simply splitting the rail, I didn't have to use many more parts.

 

You want the input and output to be able to swing equally up and down.
Then you bias the input at half the supply voltage and use input and output coupling capacitors that pass AC but block DC, or use a plus and minus power supply and bias the input at 0V.

 

Then you bias the input at half the supply voltage ... or use a plus and minus power supply and bias the input at 0V

What is the difference here? When I have split the power supply using identical resistors, and used that reference as ground for my audio path, which of your methods am I using?
I ask because you say 'bias the input' in both cases, but I'm not doing anything to my input that I'm aware of, I'm simply using the divided voltage as ground.

I'm trying to educate myself so please forgive my lack of understanding.
Here's my circuit - I realise I don't have caps on there. I will add them and it seems to work ok without them - just providing the schematic to explain myself clearer.

Is the following what you are suggesting?

 

You can do it with one positive power supply or a positive power supply and a negative power supply like this:

 

Very clear - I'll have to try that.

Thank you!

Anything wrong with how I'm doing it now? Using the single power supply and splitting the rail for ground?
(not counting the missing caps, I know I need those)

 

You should never split the ground because many audio devices have the same ground which will short your split ground.

 

You should never split the ground because many audio devices have the same ground which will short your split ground.

I think I get it.
So what you're saying is, it works for me in this very basic circuit, but in a real-world application, biasing the input is the proper way to do this.

And I think I get it now - the voltages are additive, so the DC bias + my input AC = my biased voltage.

 

The input bias voltage is half the supply voltage so that the output can swing equally up and down. The input signal modulates the input bias voltage up and down.

 

The input bias voltage is half the supply voltage so that the output can swing equally up and down. The input signal modulates the input bias voltage up and down.

I think we're saying the same thing...thanks

So I've laid out your circuit in eagle, and I'm trying to use the built-in spice simulator (ngspice) to look at the circuit.

From what I can tell, I'm not blocking the DC again at the output. I thought just a coupling cap like I have would work.

Why is my DC still present in the output?
(It's hard to see, but the output signal closely follows the input-biased signal up around 6v)

 

Well it must have been a glitch in my layout - like the cap was acting as a jumper.
I've deleted and re-added the output filter cap, adjusted a few values, and now I'm getting the results I expect.
(I was using the 8ohm res to simulate a speaker, but I've since changed it to a 10k resistor and that really helped).

Thanks for all the help - and especially for taking the time to draw that circuit. Sincerely appreciated.

 

Your output coupling capacitor value of 1uF is far too small for audio into an 8 ohm speaker. It cuts frequencies below 20kHz but passes ultrasonic frequencies from bats. 1000uF cuts frequencies below 20Hz so it passes all audio frequencies.
Your 1uF capacitor feeding the new 10k "speaker" cuts frequencies below 16Hz but passes all audio frequencies.

 

Your output coupling capacitor value of 1uF is far too small for audio into an 8 ohm speaker. It cuts frequencies below 20kHz but passes ultrasonic frequencies from bats. 1000uF cuts frequencies below 20Hz so it passes all audio frequencies.
Your 1uF capacitor feeding the new 10k "speaker" cuts frequencies below 16Hz but passes all audio frequencies.

Yea that's about what I observed - figuring it out!

Really appreciate all the help - whether its confirming or denying what I work out. ;]