Choosing resistors for audio circuits

[The Master]

Hi, I'm currently working on a circuit using a TDA2050 audio chip (TDA2050 datasheet). See "Figure 1: Split Supply Typical Application Circuit" for a schematic of my circuit.

The actual resistance value isn't much of a problem. I've started with the recommended values and adjusted them to suit my circuit. The problem I have is that the datasheet doesn't appear to say how many watts each resistor should handle. I'm using 0.25W carbon film resistors as I have those in every available value but when I turn the volume up to full the 2.2Ohm resistor connected to pin 4 goes up in smoke.

How would I know how many watts this resistor needs? It's a 32W chip but I would expect most of that power to go through the speaker and theres a series capacitor in there too so I'm really not sure how to calculate it. Should I be worried about the other resistors too? So far they all look fine but I've only managed about 10 seconds before having to cut the power.

[keithdoxey]

What impedance is the speaker ?

I normally use a 0.6W metal film resistor but it shouldnt really be taking any power under normal usage.

Are you sure you dont have some high frequency instability being introduced above the range of hearing ?

[The Master]

The speaker is 8Ohms but I would like this circuit to handle both 8Ohm and 4Ohm speakers.

It's very possible there is some noise. I didn't have a 470nF cap to put in series with this resistor so I used the highest non-polarized one I have which is only 220nF. The datasheet does say that a smaller value may cause oscillations. I will be buying some 470nF caps and a few other values on my next order. In the mean time I will try to safely connect my scope to check for high frequency oscillations.

Thanks for you're suggestion of a 0.6W metal film resistor. I will buy a few of those and some higher values just incase / for the future. I would still like to know if there's some kind of calculation I could use to find the exact value though. I want the components to cope easily but I don't want them to be unnecessarily large.

[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 ?