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What is the Max RMS System Noise before 'hiss' is can be heard in Headphones.

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thunderdantheman

thunderdantheman

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This question is a subjective and I understand it is dependent on other factors, however, IÂ’m hoping for some guidelines or experienced knowledge.

With an audio circuit designed to drive a typical set of off-the-shelf headphones, what should the RMS (system generated) noise be limited to at the headphone output to ensure 'hiss' isn't noticed during silence periods? I understand all headphones will have different sensitivity, but is there a typical noise figure one should aim for at full gain to avoid the 'hiss'?

The audio is voice and the amplifier has approx. 16kHz bandwidth (10Hz - 16kHz).
 

You already admitted that this is pretty subjective, so there really is no answer. But, nominally, the minimum detectable level is 0 dBm SPL. This is related to air pressure. How this relates to your electrical signals TOTALLY dependent on your transducer.
 

I'd say you want any noise to be at least 60dB below the maximum peak audio signal level, with 75dB or more desirable.
Old analog audio tape recorders had a S/N of only about 45dB if I remember correctly, and the tape hiss was often audible during quiet passages.
Using signal to noise ratio as the defining factor makes it independent of the amplifier gain or sensitivity of the earphones.
 

A fraction of a volt is audible in headphones.
A microphone signal has amplitude in the area of 10mV, and needs to be amplified.
I suppose 50-100 mV of hiss amplitude is strong enough to hear in headphones.

To reduce hiss, avoid making too much gain (voltage gain) in your amplifier. If you apply excessive gain then you must attenuate your input signal, yet this does not reduce the level of hiss generated in your amplifier. There are popular audio IC's which can be configured to deliver 20 gain or 200. In many cases your headphones mainly need current gain rather than voltage gain since many devices produce signals at line level, nominally 1V.

There are IC's which dynamically reduce noise. Example, LM1894.
 

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