Choosing resistors for audio circuits

[The Master]

For the RC filter. Would a low-pass filter in the configuration shown here be suitable with a 33Ohm resistor and a 100nF cap? I calculate that to allow 48,253.2Hz which is about as close as I can get to the 44,100Hz sampling frequency I'm using.

[The Master]

The 62.5KHz is only for the PWM. The actual sampling frequency is 44.1KHz. I assume that adding the low-pass filter will change those figures quite a lot.

[Charlie]

This amplifier does NOT have to pass your sampling frequency - in fact you really don't want it to. Focus on the audio spectrum (20 Hz to 20 KHz).
The values in the example are for stability of the amplifier, and I would not recommend changing them, or you are likely to have an oscillator rather than an amplifier.

[Charlie]

The filter link does not work.
Calculating the filter is also more complex than first meets the eye - you need to consider input and output impedance. As I said, a whole schematic would make it easier to help you. Feel free to send a private message if you don't want to post it publicly for some reason.

[The Master]

I've attached a schematic showing exactly how it's connected up right now.

[Charlie]

Thanks.
Just for clarification, the top "from PIC" is actually the PIC GND pin? The resistor we've been talking about is 2.2 ohms, not 2.2K as shown?

[The Master]

Yes, 1 of those lines goes to the PIC ground and the other goes directly to the output pin.

That should be 2.2Ohm sorry.

[Charlie]

It does not state if your AC supplies are Peak or RMS but a reasonable guess is that the maximum swing your output can make is going to be under 12V.
The input impedance of the circuit will be between 33K and about 50K depending on frequency. Overall gain of the circuit is about 20-ish It would help to know what the signal looks like coming out of the PIC, but a single pole filter is not really going to get rid of enough garbage for you, especially if there is a fair amount of 44 KHz stuff around. It would certainly be better than nothing. However, a couple RC stages should be able to knock it down enough to not be an issue for your amplifier and power dissipation at least.

But first, for your original question, redo your power calculation with a maximum of 12V and a maximum frequency of 20 KHz to see how much power will be in the resistor. Also, that's 12 V P-P. For sizing the resistor, it would make more sense to use RMS - call it 4.2V

[The Master]

OK, let's see how this goes.

Frequency = 20,000Hz
1/(2*PI*frequency*cap value) gives 36.2Ohms for the cap.
36.2 + 2.2 = 38.4Ohms total accross the resistor and capacitor.

30VA 12-0-12 supply gives 2.5A max.
I'm basing the current through the resistor on the difference between the resistor/cap and the speaker. I would say roughly 17% of the current should go through the resistor/cap.
2.5 * 0.17 = 0.425 through the resistor/cap.
0.425 * 0.425 * 2.2 = 0.4W.

0.4W is much better. If I used a 470nF as the datasheet suggests then I work it out to be 1.4W though which still seems a little high but it's still much better than before.