Does the phase switch allow you to reverse the phase of the signal?
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I suspect the gain control allows you to reduce the gain of a source(s) before it reaches the attenuator on your amp - this is convenient when you have a high gain source. Without it you would only be able to get miniumum use of the VC before you were overdriving you amp.
The phase control probability allow for reversal of absolute polarity. The change is subtle, works on only certain recordings, and is not heard at all by many.
Yes, Newbee pretty much explained it right on.
For years, I had an ARC LS5 II which had a 30/12 db switch. For LP as my source, the full 30db gain in the line stage was needed with lower output cartridges as my phono stage at the time only had 48db gain. This put my volume control at about 2 o'clock for the loudest listening levels. With CD as source, the volume control setting was much lower for the same output - more like 9:30 or so. Switching to the 12db setting brought the level back to about 1:00 but the soundstage was audibly compressed at the 12db setting - so I always used the 30db setting with CD even though I had a smaller volume control range. This only goes to show that nothing ever comes for free.
Concerning phase, I have such a switch on my BAT line stage and also on my DAC and the few times I have flipped the switch, I could not hear any difference. So maybe it is only audible on certain music....or I am simply deaf!
And one thing nice about the BAT is that you can add an offset for each source to match with one "primary" source. This is very handy with component comparison tests.
The "gain" controls how much yr pre increases the voltage of the incoming (source) signal. So, you can match it to yr amplifier (in the case of very low level phono signal you may have to increase the gain so the auditory level matches line-level sources -- as mentioned above).
The phase switch reverts absolute polarity, as some recordings or components, have reversed polarity.
Jafox: "So maybe it is only audible on certain music....or I am simply deaf!"
No, you're not deaf. Probably your listening skills are similar to mine--I can hear lots of differences but I definitely have a limit. Last fall, in a lengthy audition of interconnects, I could distinguish lots from others, but after some culling, those that passed all sounded the same--GREAT!
Each of us has a limit to our listeneing skills, and they're all different. Keep trying that signal-polarity switch; one of these days and recordings, you'll hear it.
One more point about preamp gain--linestages with high gain can create audible tube noise with high-gain poweramps. Understand that line stages amplify AFTER the volume control; that is, the volume control decreases the input voltage, not the output voltage, so the preamp's output noise is constant--unless a switch decreases it.