Choosing resistors for audio circuits

[The Master]

They are definately in series.

Let me check that I'm calculating this right so far.
Frequency = 62,500Hz
Using 1/(2*PI*frequency*cap value) I get roughly 5.4Ohms for the cap.
5.4 + 2.2 = 7.6Ohms total accross the resistor and capacitor.

I'm not 100% sure how to calculate how much current goes through the resistor/cap and how much goes through the speaker but since the resistor/cap is almost 8Ohms I would assume it's just a little more than half.
The supply is a 30VA 9-0-9 toroidal coil. If I understand transformers correctly then I think that would give me 3.3A max.
3.3 / 2 = 1.65A (the half going through the resistor/cap)
1.65 * 1.65 * 2.2 = 6W.

6W seems really high so I must have gone wrong somewhere.

[Charlie]

Without doing the calculations, that looks close the correct answer. Your logic is correct. The circuit you are using is for a DC powered audio amp. I think we need a whole schematic, not just the audio design reference. You certainly can not power a linear amp of that configuration from AC without rectification and filtering, which changes the max voltage.
Are you really pushing 62.5 KHz? If so, that is NOT an audio circuit, not even if you are a dog!!! What the heck are you building?

Note also that the power rating is for sustained power over time, not just instantaneous power. But an instantaneous power at a more reasonable worst case 20 KHz, max amplification (against the rails) is likely going to give you an answer expressed in Watts.
Since a more realistic audio application will see bigger powers at lower frequencies, where the cap impedance is a lot higher, and hence more of the power is in the speaker, and the fact that max volume is never sustained more than a small portion of the total time or it would be clipping, should let you get away with smaller power rated resistors. Again, without building a spreadsheet and doing the analysis properly, if this is an audio circuit, I'd guess a 1 Watt resistor would not be unreasonable, and I might use a 2W if I had one handy.

If it was for production though, I'd build that spreadsheet.

[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