Switching preamps in and out


I have three reference preamps in house. I need to decide on which one I like best. My question is: Do I need to power preamp,source and amp off before switching IC's to the next preamp? Is there a more efficient/faster way without putting any components in danger?
mikeba316
I would definitely make sure power amp is off before making and breaking any connections.
So the power amp has dual level controls. What about just turning down to "0"?
Can I leave the preamps on when switching IC's? I can press the mute button-would that work? They are all tube preamps so constsantly turning off and on is not ideal.
If the interconnections are RCA I don't think it would be a good idea even if the level controls on the power amp are turned all the way down. 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.

XLR connectors, on the other hand, make the ground connection first and break it last, so they do not have that issue. I suspect that it would be safe doing what you are proposing with XLRs, provided that you make absolutely certain to turn down the amp's level controls when changing connections. Personally, though, I still wouldn't do it.

Other approaches: If the output impedance of your source component is low enough in relation to the input impedances of the three preamps, you could use splitters or y-adapters to drive all three preamps at once, and a manual switchbox to select which preamp's outputs are routed to the power amp. If the impedance relationships in that situation would be unsatisfactory for good sonics, you could use a second switchbox to select which preamp the output of the source component is routed to.

If you want to consider those possibilities, and want further analysis of the impedance issue, post back with the specific models that are involved, and the preamp input impedances and source component output impedance if you know them.

The DB Systems DBP-2JAU/5 would be a suitable RCA switchbox. Click on the "photo" and "review" links for further info on it.

Regards,
-- Al
Utter nonesense. Amazing...If a power amp has level controls you can turn it down and switch input cables all day, same goes with the preamp.
Hey, Wolfie, don't you think that is a bit of a bold statement, considering that you don't even know what components are being used, what the designs of their input stages are, what effect a brief overvoltage might have on their long-term reliability, what their grounding configuration is, whether or not they provide a ground lift switch, whether or not he's using cheater plugs on any of his components, whether or not any of his components have two-prong power plugs, and whether the connections are RCA or XLR?

Note also that I referred to a "CONCEIVABLY damaging voltage."

Have you ever connected the probe of an oscilloscope to a low or zero voltage circuit point on a piece of equipment, without connecting its ground lead, and looked at the voltage waveform that is displayed on the scope under that condition?

Regards,
-- Al
A completely different point:
Reads like the op wants to do quick repeated changes to compare the preamps.
BAD IDEA. Better to listen to one for a few days. Then the next and afterwards think if you liked one bettter than the others that way.
The part of you brain you want to have deciding is not the same one you use in short conscious attempts to 'listen'.
So chill out, and just listen to them for a few days each. (You could take notes if you must.)
I completely agree with Al, (as usual). My preamp, (Modwright LS100) and my amp, (Sanders ESL) BOTH MUST be turned off and the amp allowed to drain off.

If I try to plug or unplug the RCA cable from preamp to amp with either on, it makes a gut wrenching, heart stopping, horrible noise and will usually blow the fuses in the amp.

IMHO, always turn off any component(s) that you are changing cables to.
Turn the amp off before switching.
And I agree with Elizabeth on this one. You really need to take time to listen to each pre to fully understand what you are hearing. Too often the piece that sounds best in short term comparisons don't hold up as better over time. They just stand out during the test. At least that's my experience.
If you do make the hot swaps, please post the results.
Elizabeth got it right. I find it difficult to switch from albums to CD's (and vice versa) in one listening session. The first source always sounds better than the second one.
Thanks everyone for input. Here are components. Everything purchased here on Audiogon. I agree with long term listening but I want to enlist some other ears too, so we will be doing switching in and out on that night.

Amp is -VAC Phi 300.1a

Source is- Playback Designs MPS5

Preamps are:
-VAC Sig MKIIa
-Messenger
-Allnic L-3000

Cables are all single ended
Mike, the output impedance of the MPS-5 is extremely low, as indicated in the third paragraph of JA's measurements in Stereophile, so I don't think that it would have any problem driving all three preamps at once, via splitters or y-adapters, even though the input impedance of the Allnic is only 10K. The input impedances of the other two preamps being much higher, 250K for the Messenger, and probably something like 67.5K for the VAC, which is what JA measured for the Mk II (non-A). The parallel combination of those three input impedances is 8.4K, which is hundreds of times higher than the output impedance of the MPS-5.

So IMO a good approach, assuming that the cost is worth it to you, would be to drive all of the preamps simultaneously, via splitters or y-adapters, and purchase the DB Systems switchbox to select which of their outputs is routed to the power amp.

FWIW, I used that switchbox extensively some years ago and I did not perceive it as having any sonic effects of its own when it was inserted into the system. Although my system then was not quite as resolving as it is now.

Regarding Wolfie's post, upon re-reading what preceded it I'm not sure that he was understanding that what I was talking about, given that the level controls on the amp would be turned down, was NOT the possibility of damage to the speakers (although I can envision ways in which even that might be possible). What I have been talking about is the possibility of either damage or degradation of long-term reliability of the input and/or output stages of the electronic components. If that wasn't clear, I hope he will re-read everything I said from that perspective.

Regards,
-- Al
I have only practical experience and my comment applies only to amps with gain knobs (like the OP's amp) where no...and I mean zero...signal gets anywhere in the amp when the gain is all the way down (rare in home audio, common in pro audio), thereby rendering the amp safe for changing input sources, which I've done thousands of times without issue (I am extremely lucky however)...wear and tear on the jacks are also a non issue unless you're pretty clumsy or really obsessed with constant switching out and in. Grounding issues will raise their ugly head even if you turned the amp off first, so there's that. Also, regarding listening for differences...it's possible to switch things around and hear differences immediately and decide if it's good or not so good...although living with things for subtle in depth consideration is always a good idea unless the auditioned item sucks too badly as life is short...all a matter of degree. Have fun in your frenetic input damaging preamp testing voltage mismatch explosion session!
Mike - I will be very interested in your findings. I currently have a VAC Renaissance 2 (modified by VAC) and had an Allnic L3000 for a while.
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.
Salectric, I would pose to you the question I posed to Wolf:
Have you ever connected the probe of an oscilloscope to a low or zero voltage circuit point on a piece of equipment, without connecting its ground lead, and looked at the voltage waveform that is displayed on the scope under that condition?
I assume that your answer, and his, would be "no." I have done that. Depending on the grounding configuration of the particular equipment, a 60 Hz sine wave measuring well upwards of 60 volts will sometimes be seen in that situation. And depending on the grounding configuration of the components in the audio system, a similarly huge sine wave might end up being applied to the inputs and/or outputs of all of the audio components that are involved in the interconnect swaps (including the source component), during the fraction of a second or so that the RCA plugs are being inserted or removed, due to the absence of a ground connection during that fraction of a second.

Do you really want to risk applying upwards of 60 volts to the inputs or outputs of components like these, at least some of which cost well over $10K, even for a fraction of a second? Inputs and outputs that are designed to generate or receive voltages that are around 2 volts or less? If so, be my guest. I would encourage others not to do so, even if the approach I suggested compromises the resolution of the comparison that will be made.

I am NOT saying that damage WILL occur as a result of the approach that you and Wolf are espousing. Most likely it will not. But I am saying that it creates a risk, especially given that RCA's are involved, and that it is therefore a very poor practice that should not be encouraged.

Regards,
-- Al
Peter,
Check this thread in three or four weeks. I will post which preamp I like the most and why. I have some initial thoughts, but I want to do more listening with a wider variety of music before I post any thoughts.

Salectric,
I hope you are right. I'm going to put an email in to Kevin Hayes at VAC, and see what he thinks.
I'm going to put an email in to Kevin Hayes at VAC, and see what he thinks.
Mention to him that you are using the unbalanced RCA output of a source component that is solid state, for which the output impedance is 11 ohms.

I would expect that in general solid state components will have greater susceptibility to damage or degradation of long term reliability in this kind of situation than tube components.

Regards,
-- Al
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.
I agree, Salectric, that the chances of there being a problem in this particular situation appear to be slim.

I would note, however, that my own VAC Renaissance 70/70 Mk III amplifier has a 3-position switch controlling its ground configuration. One of those positions isolates circuit ground from chassis ground (except at RF frequencies), and hence from the AC safety ground that as you indicate would otherwise prevent circuit ground from floating to voltage levels that are significantly different than those of the upstream components, when the IC's are disconnected. I don't know if any sort of ground lift provision is included in any of Mike's components.

I would also note that two-prong power plugs were not just creatures of the 1950’s and earlier. The STAX and Tandberg components in my system, from the mid and early 1980’s, respectively, all have two-prong power plugs. And some people have cheater plugs on some of their components.

I would also re-emphasize that the front end situation has to be carefully considered, as well as the preamp-to-amp interface. Especially given that the source component is solid state, per my comment to Mike just above.

One of my basic motivations in this thread has been to dispel the impression, which threatened to be created early on, that in a situation where no signal can be propagated through the power amp one should necessarily feel free to do anything and everything with interconnects while power is on. As you appear to realize, but others may not, there are a whole lot of variables that can make that assumption risky. Including those I mentioned in my initial response to Wolf:
… what components are being used, what the designs of their input stages are, what effect a brief overvoltage might have on their long-term reliability, what their grounding configuration is, whether or not they provide a ground lift switch, whether or not he's using cheater plugs on any of his components, whether or not any of his components have two-prong power plugs, and whether the connections are RCA or XLR….
Regards,
-- Al
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
Kevin's responsiveness never fails to amaze.

Enjoy!

-- Al
Very nice of Kevin to respond so thoroughly. If only all mfrs were as accomodating.....
Agreed. I thanked him for all of us.
You are joking right? If you have to swap them out so fast to tell the difference then there is probably not much difference. You need to play music that you are familiar with or that highlights different instruments or vocals and listen for a while before swapping them out. I agree with what Elizabeth said:

BAD IDEA. Better to listen to one for a few days. Then the next and afterwards think if you liked one better than the others that way.

Happy Listening!
I agree with longer term listening, but swapping in and out is helpful too, and the only way when you have friends over listening with you!

My wife gave me the house this Saturday. She taking the kids to a birthday party so that's when the testing begins!