Switching preamps in and out

Salectric and I live as brave rebels unafraid to dare the electrons around us...go ahead obscure grounding issue...make my day.

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.

Al and Salmetric,

Here are Kevin's responces to some of your assertions. See below and following posts.

Yes, you can get transient snaps, hum, or (in some cases) oscillations when 'hot swapping' single ended interconnects. We do it all the time at the factory, but generally speaking it's a bad practice. Some solid state amplifiers have been known to self destruct.

On 2/5/2013 9:43 PM, Mike Bauer wrote:
Kevin,

Just in case it matters: the source will be Playback Designs MPS5 SACD player with 11 ohm output impedance. The IC's are single ended. Someone said something about a grounding issue with the single ended connector.
see below. Is this a concern?

The problem with RCA connections is that during insertion the signal on the center pin is applied before the ground connection on the shell is made, and during removal the ground connection is removed before the center pin connection is broken. So there will be moments during insertion and removal when the zero volts that should be present on the center pin when no music is being played will not have a defined ground reference, and therefore may be seen as a very large and conceivably damaging voltage.

Thanks for your thoughts

More from Kevin,

The first sentence is true, and the core of the procedure I've suggested to you. The rest of this poster's theory has some flaws, and I would not rely upon his assumptions or assertions.

On 2/5/2013 11:10 PM, Mike Bauer wrote:
Kevin,

So here is the other counter argument about RCA connectors. I didn't want to bother you so I put the question on Audiogon and seemed to have started an argument. See below.

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.

Final Word from Kevin,

If you switch it to the variable volume mode and turn the volume controls all the way down, yes.

With the volume controls at minimum, the "direct/level adjust" switch will act like a mute switch.

On 2/6/2013 10:01 AM, Mike Bauer wrote:
OK. Thanks Kevin. So just to be redundant: I can "hot swap" the IC's between the preamps with the Phi 300.1a if I turn the volume all the way down.

--- On Wed, 2/6/13, VAC wrote:

From: VAC
Subject: Re: Phi 300.1
To: "Mike Bauer"
Date: Wednesday, February 6, 2013, 5:28 AM

The version of the Phi 300.1 with a front panel meter have a mute switch at the right of the front panel; this may be used to mute the amplifier.

The Phi 300.1a (no meter) does not have a mute switch, but you can effectively mute the unit by switching the attenuators in with the back panel switch and turning the front panel volume controls all the way down; return to the 'direct' mode when you're ready to listen.


On 2/5/2013 7:51 PM, Mike Bauer wrote:
Kevin,

Hello. I'm very much enjoying my VAC Phi 300.1a and Sig. MKIIa preamp combo. I have some audiophile buddies that want to try out their preamp in my system to compare with the Sig. Can I just power up amp,SACD player and three different preamps and switch IC's between preamps without powering off anything if I turn the level controls on the Phi 300.1a down to "0" and have the mute button on the preamps engaged?

I'm hoping "yes" because powering everything off and on and going back and forth for a couple of hours between the three preamps seems like a lot of wear and tear on tubes and such.

Connections are all single ended.

Thanks,

Mike