Switching preamps in and out

Good summary Al. I overlooked your mention of cheater plugs and two-prong power plugs, so you had already covered that aspect. At the risk of beating an already dead horse, I would add that the concerns are probably still more theoretical than real even with an amp that is not referenced to the AC mains ground. In the old, old days, all of my amps were configured that way (Dyna ST-70, Mk. IV, and homemade tube and solid state amps) and on each one I wired an input shorting switch so I could mute the input in order to swap interconnects. I never had any problems with any of my amps in doing this. Of course, your point is there may have been a momentary spike when I removed the i/c cable and lost the ground reference, and I can't say that this didn't occur, only that I never had any problems.

Wolf's responses are right on the money. Since your amps have volume controls on their inputs, you can turn them down to zero and then safely disconnect and connect i/c cables. This will leave the power amps powered up continuously and avoid stressing them through repeated off/on cycles. It also means that you avoid any changes in sound due to the power amps being turned on and off.

As far as the preamps are concerned, you can certainly leave them all powered up so each one is ready to go when you want to change from one to the other. This will leave each one in a good-to-go status without worrying about changes during warmup. You will be switching the input connections for each preamp as well as the outputs, so that only one preamp is actually connected to the source at a time. The idea of running all 3 preamps in parallel and using a switching box between the preamps and the power amps makes no sense at all. You not only have the sonic colorations of the switch itself but also the unnecessary i/c cables. It is far better to take advantage of the amps' volume controls and then move all of the input and output connections from one preamp to the next.

Two final comments. First, leaving the unused 3 preamps powered up while you are listening will change the sound compared to just having the one being tested powered up. There is nothing wrong with leaving all 3 on for some initial comparisons, but eventually you need to listen while just one is powered up. The true sound of the preamp will only be demonstrated when the others are turned off and the power cords unplugged from the outlets.

Second, the point about needing to do longterm listening is valid. There are aspects of the sound that become apparent only through longer listening. Sometimes differences are masked when you change quickly from one component to another. That doesn't mean that you shouldn't ever change quickly from one to another, only that you shouldn't use this as the sole means for evaluating components.

Al, I think we are still not communicating. When the power amp's volume control is reduced to zero, its input connection is shorted to ground. Yes, it's true that the input jack is no longer connected to the preamp's ground once the i/c is disconnected. However, you are overlooking that in any modern amplifier, the power amp is still connected to an electrical ground reference due to the ground wire for the AC power connection. The amp is sitll referenced to ground when the input cable is disconnected. I agree with you that if the gentleman was using a vintage amp from the 1950s that used a simple 2-prong AC power connection, the amp would lose its outside ground reference if the input cable was disconnected and there might possibly be a momentary spike even with the inputs shorted. But not with a VAC or any other modern amp.

Very nice of Kevin to respond so thoroughly. If only all mfrs were as accomodating.....