As mentioned in another thread, I'm using this amp:
Coupled with this dac:
I have a 5v power source.
My goal is to maximize my power output, but I am working under the assumption this speaker will be used:
And that's rated for 2W, max of 2.5W, so I don't want to exeed that.
I've been looking at the Audio Shield to see how they filtered the output, as they're using the same dac:
http://www.ladyada.net/make/waveshield/design.html
And what I'm looking at right now is the voltage reference for the DAC, on pin 8.
We can see in the schematic above that LadyADA has used a 100K resistor paired with a .1uf capacitor to make a low pass filter with a 15.9hz corner frequency, to filter out noise from the microcontroller.
But as GrumpyMike told me a while back when I first looked at using this DAC, because of the 165K input impedance on the DAC, the resistor here will create a voltage divider, and instead of a 5v reference, I will get more like a 3.1v reference.
The result of which would be that the voltage output to my amp would swing from -3.1v to + 3.1v and that would in turn be output to my speaker because it has a gain of 1, and the result of that would be less wattage to my speaker and lower volume. And that's not good because I want to maximize my volume. But I also want to get rid of noise.
So, I looked for a way to change the voltage divider so I wouldn't lose almost 2v to my low pass filter and as GrumpyMike suggested originally, lowering my resistor value would do the trick... but at the cost of more noise.
Well I think I understand now that the reason for more noise is that if I lowered the 100K to a 10K, my corner frequency on my RC circuit would go up to 159hz. But I wondered what the effect of changing the capacitor would be.
So, I experimented with these RC filter and voltage divider calculators:
http://www.raltron.com/cust/tools/voltage_divider.asp
And I found that it was quite easy to see how different values of caps and resistors worked together for my filter, and what different values of resistor would do with my voltage divider.
The end result was I found that, if i wanted a 15.9hz cutoff I could use the following:
.1uf + 100K = 3.1v drop
1uf + 10K = 4.7v drop
10uf + 1K = 4.97v drop
Now, I still haven't done the math for it, but it looks like the filter caps I put on my amp are also going to give me a voltage drop. So it would probably be best to go with the lowest voltage drop I can on the DAC.
With that in mind, what I'm really wondering is... can I really choose any of those three combinations of capacitors and resistors and get the same result in regards to noise reduction, with varying levels of voltage drop? Is there any disadvantage to using a 10uf cap and a 1K resistor, aside from a 10uf cap being a little expensive compared to a .1uf cap?