What, Exactly IS hiss?

  • Thread starter Thread starter Doctor Varney
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Just turning up the preamp gain with nothing plugged in can give a bit of skewed view of the actual self-noise of the preamp. There are all kinds of factors involved, but source impedance can influence it, and as is it's infinite - worst case scenario and nothing like real operating conditions. Plug in a short cable with the two signal wires shorted together (or better with a 600R resistor between them) for a bit better idea. It might not make a difference, but theoretically could.
 
ashcat_lt said:
Just turning up the preamp gain with nothing plugged in can give a bit of skewed view of the actual self-noise of the preamp. There are all kinds of factors involved, but source impedance can influence it, and as is it's infinite - worst case scenario and nothing like real operating conditions. Plug in a short cable with the two signal wires shorted together (or better with a 600R resistor between them) for a bit better idea. It might not make a difference, but theoretically could.
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Most pres have an internal input impedance prior to any electronics: a resistor to ground (2K2 seems typical), or the one side of a transformer, or the two 6k8 resistors on the phantom power. So it won't actually be infinite. Just higher than it would be with a mic attached. /pedantry


As for hiss, as ashcat says, the various gains you are using is perhaps the first thing to examine.
 
Oooo! BIG subject!
Many years ago there was a series of articles in the hallowed pages of Wireless World entitled "Noise: Confusion in more ways than One". Ran to 3 issues IIRC!

But yes, there is a limit as to how quiet we can get. The thermal (Johnson) noise of a 200 Ohm (mic) resistor is about -130dBu and all mic amps are going to add to that. HOW much they add is complex depending on factors such as the gain at which you measure the noise, the bandwidth over which you measure it. Peak, rms, average, (Avo!) weighted ? If so which curve?!
All that can be reasonably said is that manufacturers will find the best figure to hand on to the adpuff dept!

But in the real world you will struggle to find an AI with a better rec/play noise performance than -120dBFS and most of the best of "prosumer" gear gets down to about -100dBFS. That is academic tho because few of us can ever get a room that quiet and any other source, keys, guitar amps (awful, noisy fekkin things!) will be way noisier than even a budget soundcard (the Behringer UCA202 gets to about -80dBFS but is still good enough for most purposes noisewise, it is the 16bits and lack of headroom that limits it. Perfectly fine for tape or black disc dubbing tho but).

There are things that are "counter intuitive" about noise. For instance, a modest UN balanced input will be MUCH quieter than a balanced one!

If anyone wants the dog's whatsits about noise and amps buy Duggy Self's book Small Signal Amplifiers. Very little math in it. (which suits me Sir!).

Dave.
 
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