What's the difference between Hiss and Hum??

[playonit]

After posting about "Chasing noise" a couple days ago... it struck me that what I have is more of a Hiss then a Hum?? Difference as far as what to look for as the cause??

[jaysg]

Hiss is white noise and is sensed as high frequency since there's so much of it. It comes from the tubes. Hum in the amp building context, has a dominant single frequency -- most typically 120Hz in 60Hz countries, 100Hz in 50Hz countries. It comes from the power supply. hth

[TheGimp]

Hum can also be casued by bad (old) filter caps in the power supply, insufficcient filter caps in the power supply, poor wiring technique, poor shielding (especially high gain stages), and not shielding the enclosure (no bottom on the chassis).

Then there are ground loops in external wiring and I'm sure other items that can cause it.

Hiss can be generated by resistors (1/f noise) as well. Shot noise is generally tube generated. Then there is partition noise in Tetrodes and Pentodes.

It all comes together as hiss.

[Aurora]

Hum with twice the line freq - 120 or 100 Hz for 60 or 50 Hz line, is usually power supply related - bad caps, too little capacity in the filters, too much current drawn.
60 or 50 Hz hum is usually a grounding problem - ground loops etc.

[VacuumVoodoo]

Both terms are onomatopeic:
hummmmmmmm
hisssssssssssss

[jjman]

Hiss = DC related. e.g. plate resistor

Hum = AC related e.g. PS ripple, heater current bleeding

[Structo]

Sometimes it can even be hard to figure out what frequency the hum is!
I've had to look up the note that corresponds to the Hz before just to identify it.
Hum problems are the worst and can be the hardest to nail down.
That and a pesky parasitic oscillation can be a real pain in the rear.