Choosing resistors for audio circuits

[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

[Charlie]

Sorry I was not clear - RMS is the AC equivalent of DC in terms of work or power. DC Watts is DC Volts * Amps. AC Watts is RMS Volts * Amps.

An RC filter with series R followed by C to gnd, follwed by series R followed by C to gnd is not the same as a single RC with appropriate values. A single RC will have an attenuation slope of 20 dB per decade; 2 stages will have a slope of 40 dB per decade - double the attenuation of unwanted signals.

You can clean up your signal a lot more, although you may not care.

[The Master]

I would like to clean up the signal as much as possible. What values do I use if I have 2 RC filters? If I calculate that I need a 10K resistor and 1uF cap for a single one then do I use 10K-1uF-10K-1uF or would it be 5K-0.5uF-5K-0.5uF for 2 of them?