Resistor sizes in chip-amps

[pacificblue]

Originally posted by peranders
I think it's a bad idea to design parts which you easy can distroy.

Depends. I would trust National on their choice of resistor rating and assume a design issue, when it burns. Very much like a fuse. If it blows, something is wrong and needs fixing, not a bigger fuse. Usually the Zobel is there to prevent oscillations from starting, not to continuously convert them into heat. A normal music signal does not have enough HF content to affect the resistor.

It usually also strikes me as odd to see the size difference between a suitable 100nF capacitor e. g. X7R and a normal 2 W resistor, when the capacitor will have to dissipate most of the Zobel's load at audio frequencies from a much smaller case.

Originally posted by jackinnj
you can get the 2W Vishay PRO2 series which are about the size of a 1/2 W carbon comp resistor.

Higher power rating in a smaller package means
- steeper derating above the reference temperature and/or higher maximum temperature.
- higher temperature for a given dissipation -> higher value drift. Not an issue for a Zobel, yet must be kept in mind for certain applications.
- no space saved, because neighboring components need to be kept further away to keep them cool. Sometimes even the leads need to be left longer, because they have to help with dissipation.

[lhwidget]

Originally posted by pacificblue
The size of the Zobel resistor depends on the possible output swing. If 1/4 W burns on an LM3886, there is something severely wrong with the amplifier. Some people prefer to increase the resistor rating instead of looking for the fault and correct it.


An LM4702 cannot be compared to an LM3886 in that regard. 0R1 is also not a typical value for a Zobel network. Not really surprising that it didn't work. National shows 3R3 and 20 nF in their reference project. Shunning Zobels is the wrong conclusion from your experience.


.. if you use the amplifier to reproduce test signals.

I agree, an RC shunt, across the load, tuned to about 550kHz (2.7 ohm + 0.1 micro F) won't be conducting any electricity at 20 kHz. The load is dissapating the output. If high freq oscillations are present, the Zobel will bypass them to the speaker ground. If there is enough high frequency energy to burn up a 1/4 to 1/2 W resistor, fixing the oscillation would be my priority too...

I appreciate everybody's thoughts, it's very useful to know some of have failed Zobel resistors, while others have seriously consideder smaller ones. I think I'll some 1/2 W - 1/4 W Zobel components on the next chip-amp I build.