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What can make a resistor go noisy?


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G'day all, most of my DIY audio preamps are powered by DIY power supplies with many using the ESP P05 variety.  The P05's use two 10 ohm resistors for filtering purposes something that works very well.  However I have occasionally shorted those power supplies resulting in 'smoked' 10 ohm resistors although still measuring a nominal 10 ohms.  My question is whether these resistors can become damaged in this way, and become noisy?  I ask this because today I replaced several of these heat damaged  and discoloured resistors and to my ears playback noise has markedly reduced.  Is this possible?  Regards, Felix.     

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They might still measure the right value but they probably won't handle much current anymore.

As for the noise, heating these up too far interferes with the join from the leads to the resistive material resulting in minute arcing, hence noise.

Sometimes they can last a while but often will go O/C or HR.

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It depends mostly on the type of resistor.  In the ESP05 improved PSU the 10 ohm resistors shown are cracked carbon film 1W types.

When overloaded but not to the point of total destruction the thin layer of carbon can develop microfine fractures which can alter the resistance when the resistor is hot and dissipating heat.  This can potentially introduce minor changes in the resistance thus creating additional noise when current flows through it.

 

These carbon film resistors have brass end caps which are mechanically attached to to the carbon film which is bonded to the ceramic body of the resistor.  Heat stress can compromise the integrity of this connection and create noise when the resistor is passing current and dissipating heat.  If you dip one of these carbon film resistors in paint stripper you will discover that once the paint coating has been removed the end caps detach quite easily from the body of the resistor.

 

These days I steer well clear of carbon resistors for audio applications.  Compared to metal film, metal oxide and wirewound types they are noisier.  In the case of the ESP05 PSU, consider replacing the 10 ohm 1W carbon resistors with two 20 ohm 0.6W metal film types wired in parallel.  These can be easily accommodated on the PCB as they are physically smaller than 1W carbon types.  As with any resistor that is likely to operate close to its maximum dissipation, mount them slightly raised and separated to allow for air flow around them. IME metal film resistors tend to  survive temporary overload better than carbon types.

 

Cheers,

Alan R.

 

 

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