1. The power supply was the culprit in disabling all audio signal. (It also powers my processor/preamp)
Not sure theoretically how an AC audio signal would cancel the digital signal through the processor's power(maybe inverted phase cancellation?)
I had to run a separate power supply to get it to work.
Hmm, I didn't think of that but in retrospect perhaps I should have thought to check the power supply. However, I’m not sure about your proposed theory of inverted phase cancellation, instead my initial suspicion would that preamp was too much for the power supply to handle along with everything else. In any case this is progress.
2. I have only been able to get any sound with ceramic disc caps. I've tried electrolytic caps back to back(+ - +), metal film caps, and polyester film caps.
Welcome to the world of real, rather than theoretical, components.
I know it hasn’t been discussed in this thread, but material and construction can cause a lot of subtle and not-so-subtle differences between two parts with the same basic ratings. In this case the difference is probably due to the ripple current limitations
and higher ESR for film and electrolytic capacitors.
I probably should have brought this up on my own. However, I presumed that if you were using electrolytic capacitors they would be intended specifically for audio applications, not the general purpose ones. However, it seems you don’t mind learning through experimentation, which is a good thing.
3. The more resistance, the stronger the signal is. (Because its a bigger load?)
Yes, since we are mainly concerned with voltage levels and not power levels in this application. If you want to know more about this, read up on the concept of impedance bridging
4. The capacitance didn't seem to change sound level. I used as small as a 1nf, and as much as 0.5uf with no noticable difference.
Remember the intended purpose of the capacitor(s) in this application is to separate the variable signal from the constant DC and that will determine the target value. In this case the goal is to have capacitors that block the DC without attenuating any of the audio frequencies when combined with a given load. There should be a range of capacitances that will do this, especially with a load that can vary by thousands of Ohms.
5. Multiple caps in series seemed to increase the sound level just barely. While multiple in parallel greatly decreased it.
Given the differences capacitors combine in series versus parallel that’s to be expected.
Furthermore, even if you are using multiple capacitors configured in different ways to achieve the same total capacitance there could be some small but detectable differences in circuit behavior. Although capacitors can be combined to form an arbitrary effective value in the filter, there will be some differences between using one physical capacitor and multiple ones. This is because in reality they don’t all charge and discharge at exactly
the same rate. In series they will tend to charge and discharge one after the other, in parallel each individual capacitor can charge and discharge (mostly) independently of one another.
As for the variable resistor, it seemed to not make a difference. And the RLC method is for a low pass filter isn't it? I could try an LC method, but not sure how I would incorporate the resistance. Also, I've read places that a guitar's pickup is basically an inductor in series with a cap and resistor in parallel(RLC)
You can configure an RLC circuit as either a high-pass or a low-pass filter. Look Figure 9 & 10 from the link, the first is low-pass and the second is high-pass. Did you notice how the positions of the capacitor and inductor are switched?
Also, remember for an AC signal the load is impedance
, which is DC resistance plus the phase varying contributions of the load’s capacitance and inductance, known as reactance. This matters for the impedance bridging mentioned above. However, when designing the filter it’s the load’s resistance, not the impedance, that will be used in the formulas.
The op-amp circuit you describe.. by which you mean as a buffer to keep a specific resistance? That may be a good idea to try out, but would be sensitive to different pedal layouts(different IO impedance).
There also seems to not be any 20hz high pass filters available in a small package. I found this schematic however:http://www.eeweb.com/blog/circuit_projects/20hz-to-200hz-variable-high-pass-filter
May be useable, but the schematic states 15v input voltage, and the tl072 is rated ±15v.
Could I just swap for a lower voltage chip, or just run this with 9v?
This chip can be used at lower voltages and the datasheet
shows some testing done at as low as ±5 V. However, there will be modifications necessary to the values in this circuit, and proper setup of an active filter can be more a bit complicated than a passive one.
Thanks for the help Far-seeker, you have really helped me out so far.
No problem, but are you starting to see why I avoid overlaying AC signals on DC when I can?