Ssssh, is your tube preamp really that quiet?

Why would a volume control (attenuator) after the gain stage affect the level of noise? Wouldn't that be true only of a volume control that is before the gain stage? Or have I somehow got this backwards in my head? I sense that I still have much to learn!

You do have this backwards. Think about the volume control as a valve of sorts that lets the sound through the circuit. The circuit is going to make a fixed amount of noise. So if the volume control precedes the line stage it will have little effect on the noise of the line section. If it occurs after the line section the noise will be reduced as you reduce the setting of the control.

The reason the latter is not done more often is that there is a price paid for having the volume control be the last thing the signal sees in the preamp- and that is that it can raise the output impedance substantially, and make the preamp highly susceptible to artifacts in the interconnect cable.

So to allow the preamp to drive longer cables without artifact, and to be able to make bass easier into transistor amplifiers, most designers put the volume control at the input of the line section (which has the additional advantage of preventing overload of the line section by a strong input signal).

Does this help your understanding?

I had the opportunity to listen to a Nagra Jazz pre-amp last summer. Dead quiet and fantastic sounding hooked up to the ARC Ref 250s and Vandersteen 7s.

Grimace, so that we are all on the same page...how did you "listen" to the Nagra jazz pre-amp? For example...at a distance of...fill in the blank here. With music playing, without music playing, etc,.

Thanks Atmasphere. Yes, I do see now what you were saying about a post-gain volume control affecting noise level. It attenuates the signal level, including the level of any noise in the signal.