Hum and Hiss from Sonic Frontier Preamp

I agree completely with Sgmlaw. I have a line three and there is a slight hiss that is audible if you place your ears next to one of the speakers with no active source. Increasing the volume control does not change the hiss amplitude appreciably which indicates that the hiss is part of the noise floor. I used to own an ARC LS22 and it acted the same way. I have no experience with solid state preamps but my experience with SS amps indicates that their noise floor should be inaudible. I am curious as to how the sound quality of a good quality SS preamp compares to my Line 3 (which sounds great).

I forgot to mention that I have never used headphones with the Line 3.

It is my understanding that the SF Preamps use a solid state op-amp type headphone amplifier made by Headroom

See http://headroom.headphone.com/RootNavigation/home.asp

This is a feature of the SF preamps (see their web site) as Headroom is known for very high quality headphone amps. I don't think they use tubes for the headphone functions, but the tubes might buffer the signal prior to the hitting the headphone amp. I have a Line 2 and it does not have the headphone problems you are experencing. My SF used with the BAT VK-200 amp has very little "hiss" when the volume is all the way down, by the way. I would say that the headphone output is very quiet, quieter than the line output. I would also offer this: I had one bad tube in the Line 2 which caused excessive hissing out of the right speaker but had absolutley no effect on the right channel of the headphone (or left channel for that matter). This would appear to indicate that at least some tubes are not part of the headphone circuit, and that tubes may not be the root of the problem. I am guessing but you may have a problem with the headroom amp part of the preamp. If you are using low-impedance headphones, perhaps the current drive from the circuit is not sufficient? Have you tried different headphones? Anyway, that's my input I hope it helps you out.