Choosing resistors for audio circuits

[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.

[The Master]

I'm not quite sure how RMS works with current. Using 4.2V would give around 7A using my calculation. That doesn't seem right at all.

I just noticed one of your previous edits. Yes, designing analog circuits is fun but only because I get to see an explosion

[Charlie]

Treat RMS like DC. And don't forget to fix the frequency. If you are using 22nF, then that's about 330-ish ohms at 20 KHz. I think you should be seeing more like 1/2 amp than 7 amps.

[The Master]

I'm still not sure about the RMS. I'm Calculating the max current by dividing 30VA by the volts.

I just found my cable for connecting to the PC and I'm shocked. With the current setup I managed to get to the point where it started clipping and hold it there without anything blowing up.

I added a 10nF cap between the pot's wiper and ground which took a little of the sharpness off the high parts and it's sounding perfect. Now the problem is how to do that with the PIC. I will try connecting my scope up to see if I can get anything useful out of it.

How would multiple RC stages work? Couldn't the same effect be achieved using a single RC with different values?

[The Master]

I've attached the output from my scope. I can understand why the audio chip hates me. This signal is very... digital!

[The Master]

I've done a little playing around and it seems the solution was to simply add 1 extra capacitor to turn my extra resistor into an RC low-pass filter. Unfortunately it's started clipping again when the volume is on full but most of my circuits will use an internal preset to keep it just below the clipping point and the volume will be controlled digitally so that's not a major problem.

I swapped the RC filter and the pot to give the same effect but to suit my PCB layout better. I've uploaded an updated schematic and the scope readings for just below the clipping point and max volume.

I will be ordering some 470nF caps soon along with some 0.5W and 1W resistors. It's working so well now with the 0.25W that I think a 0.5W should be enough with the 470nF cap but I'll get the 1W ones as well just in case.

I know I still have pleanty to learn about audio circuits but I've learned a lot from this thread and gone from a fire hazard to a working audio amplifier so I'd like to say a big thanks to everyone who's helped me out