Choosing resistors for audio circuits

[keithdoxey]

you shouldnt need a higher power resistor than that. With 470nF the resonant frequency of that network is around 150kHz, with a 220n it will rise to over 300kHz.

If you have another 220 then putting it in parallel with the existing one will take you to 440nf which is within 10% of the correct value.

You also say "when you turn the volume up full", depending on how hard you are driving it you could be going into severe clipping causing high frequency harmonics which are destroying your resistor as the impedance of the capacitor falls to a few ohms.

[The Master]

I tried adding a second capacitor in parallel and even a 3rd. It actually makes the problem worse.

For the volume control I'm using a 22K potentiometer with the wiper connected to Vi in the schematic, one side connected to ground and the other side connected to my PIC. Initially this did cause clipping around half way so I added an extra 22K resistor between the PIC and the pot. Now the max volume is just under where it starts to clip.

As a test I removed the ground from the left side of the pot. With the TDA2030 that would have caused a lot of noise through the speaker but on the TDA2050 it was fine until I ran the code in the PIC and that same resistor exploded violently.

As I have it now the resistor is fine until I turn the volume right up. Since this is about the limit of the chip's power before clipping and you recommend a 0.6W resistor then I think it's simply that my resistor can't cope rather than any destructive high frequencies. I will still check for those anyway just to be on the safe side but I think a tougher resistor should solve the problem.

Will a 0.6W resistor be suitable for running a 4Ohm speaker too?

[keithdoxey]

If there were high frequency harmonics caused by clipping or a "dirty" source signal then the resistor would be more likely to vapourise as it would be recieving lost of power at a lower frequency than with the original cap you had in there.

If you read the datasheet you will see that the footprint of the resistor on the PCB overlay is the same as for the other "signal" resistors so that to me implies that it isnt really taking a lot of power. The fact that you say it is worse with higher capacitor values would tend to rule out instability due to oscillation as the datasheet mentions instability as a likely side effect of too low a value for the capacitor.

You say you are driving the amp with a PIC. Do you have any RC filtering to shape the signal to more of a sine wave, if not you will be feeding it with a square wave which will cause the HF harmonics doing the damage.

What happens if you drive it with an MP3 player ? Does it sound OK and survive being played loud ?

[The Master]

I've just had a quick read of an RC circuit wikipedia page. The only thing I have like that is the 1uF capacitor at Vi and the resistors that connect to it.

It looks like I will need a series RC circuit. Do you know how I would start calculating the values for the resistor and capacitor? I found this RC calculator but I'm not really sure what kind of output I should be looking for.

I will do a few tests on the output from my PC and see what happens.

[keithdoxey]

What frequency range are you outputting from the PIC ?

[Charlie]

Adding caps will certainly make things worse, and a 1/4 watt resistor is way too small. The math can get a little hairy, but as a first order..

The capacitor will have an impedance based on frequency Z=1/(2*PI*frequency*cap value). Rough calculation says about 339 ohms an 1KHz, and about 34 ohms at 10K. To find out how much power you need, draw a little circuit with a 2.2 ohm resistor in series with a 34 ohm resistor, and an 8 ohm resistor across all of that (if 10KHz is the highest frequency you need to worry about having a lot of energy at). Resistor series / parallel calculations will find you the total load impedance. You power supply value will tell you the biggest peak to peak signal you can put through by accident or on purpose since you could swing close to the rails. Now Ohms law (V/R) will tell you how much current is flowing in your total load. Now work out how much current is flowing in the leg with the 2.2 ohm resistor. The I*I*R will tell you how much power will be in that resistor.

To really scare yourself, work out what happens if you disconnect the speaker while things are running.

If at this point you are thinking of being super lazy and just buying a 10W resistor and forgetting about it, you might want to look up calculating the impedance of inductors, since power resistors are usually wire wound.

Isn't analog design fun?

[The Master]

I'm using the PIC's hardware PWM (output compare) module. It uses Timer2 as a reference running at Fosc/2. The PIC has a 16MHz crystal with 2x PLL enabled so I believe the PWM is running at 62,500Hz with a duty cycle of 0-100% depending on the sample. The sample rate is 44,100Hz.

[kellyseye]

Check your circuit construction to make sure the resistor is actually in the circuit properly. I reckon you've got the capacitor/resistor in parallel, not in series.